A closer look into close packing: pentacoordinated silicon in a high-pressure polymorph of danburite

Pakhomova, A.; Bykova, E.; Bykov, M.; Glazyrin, K.; Gasharova, B.; Liermann, H.-P.; Mezouar, M.; Gorelova, L.; Krivovichev, S.; Dubrovinsky, L.

doi:10.1107/S2052252517010612

research papers

IUCrJ

Volume 4| Part 5| September 2017| Pages 671-677

ISSN: 2052-2525

https://doi.org/10.1107/S2052252517010612

CHEMISTRY | CRYSTENG

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A closer look into close packing: pentacoordinated silicon in a high-pressure polymorph of danburite

Anna Pakhomova,^a ^* Elena Bykova,^a Maxim Bykov,^b Konstantin Glazyrin,^a Biliana Gasharova,^c Hanns-Peter Liermann,^a Mohamed Mezouar,^d Liudmila Gorelova,^e Sergey Krivovichev ^e and Leonid Dubrovinsky ^b

^aDeutsches Elektronen-Synchrotron (DESY), Hamburg 22607, Germany, ^bBayerisches Geoinstitut, University of Bayreuth, Bayreuth 95440, Germany, ^cInstitut für Beschleunigerphysik und Technologie (IBPT), Karlsruhe Institute of Technology, Karlsruhe 76021, Germany, ^dEuropean Synchrotron Radiation Facility, Grenoble Cedex 38043, France, and ^eInstitute of Earth Sciences, Saint Petersburg State University, Saint Petersburg 199155, Russian Federation
^*Correspondence e-mail: anna.pakhomova@desy.de

Edited by P. Lightfoot, University of St Andrews, Scotland (Received 31 May 2017; accepted 18 July 2017; online 10 August 2017)

Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB₂Si₂O₈, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si₂O₇ groups of danburite first transform into chains of vertice-sharing SiO₅ trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO₆ octahedra (danburite-III). It is suggested that the unusual formation of an SiO₅ configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II.

Keywords: phase transitions; polymorphism; five-coordinate silicon; danburite; silicates.

CCDC references: 1562973; 1562974; 1562975; 1562976

1. Introduction

Silicates are widely used in different chemical processes ranging from catalysis to cement production. As the most common minerals in the Earth's crust and mantle, silicates have attracted significant attention since the beginning of structural crystallography. Due to the high geological relevance of silicates, numerous X-ray diffraction studies have been performed in order to understand their crystal structures as a function of chemical composition, temperature and pressure.

Modern crystal chemistry of silicates is based on the idea of nearly exclusive fourfold coordination of silicon under ambient conditions. Polymerization of SiO₄ tetrahedra results in the formation of a very high number of different silicate anions that constitute the basis of silicate crystal structures. While Pauling (1929 ) gave rules to describe the principles of structural organization of ionic compounds under ambient conditions, several `rules of thumb' were outlined by Prewitt & Downs (1998 ) to address the high-pressure evolution of silicates. `Increasing pressure increases coordination number' is one of them, reflecting the observed tendency of silicon to change its coordination from tetrahedral to octahedral upon compression (Finger & Hazen, 2000 ). Indeed, the crystal chemistry of rock-forming silicates with four- and six-coordinated silicon is well established, whereas only a very few examples of structures with five-coordinated Si are known. These examples are limited to cases where SiO₅ groups co-exist with SiO₄ tetrahedra and/or SiO₆ octahedra. Distorted SiO₅ square pyramids along with SiO₆ octahedra were found in the crystal structure of the titanite-like silicate CaSi₂O₅ (Angel et al., 1996 ). Recently, high-pressure polymorphs of inosilicate pyroxenes have been considered as possible carriers of SiO₅ structural units. Thus, Finkelstein et al. (2015 ) reported two new phases displaying silicon in a nearly fivefold coordination. In that work, Si was not described as fully pentacoordinated because of the high level of distortion of the SiO₅ polyhedra: the four conventional `tetrahedral' bond lengths varied in the range 1.58–1.69 Å, while the fifth exceeded 1.92 Å. Pentacoordinated silicon with highly distorted coordination polyhedra was also mentioned in the structure of dehydrated brewsterite (Alberti et al., 1999 ). Here, we report a high-pressure single-crystal X-ray diffraction study of danburite, CaB₂Si₂O₈, the first compound showing a step-wise transition in Si coordination from tetrahedral to octahedral through a trigonal bipyramid.

2. Materials and methods

The sample of danburite originated from the Dalnegorsk boron deposit (Primorsky Krai, Russian Federation) (Karas et al., 2008 ). BX90 diamond anvil cells (DACs) were used for pressure generation (Kantor et al., 2012 ). The sample chamber was obtained by drilling a preindented rhenium gasket. Danburite crystals were placed inside the sample chambers, along with ruby spheres to enable pressure estimation (Mao et al., 1986 ). The DACs were loaded with a neon pressure-transmitting medium (Kurnosov et al., 2008 ). Monochromatic single-crystal X-ray diffraction experiments were performed on beamline P02.2 at PETRA III (DESY; experiment 1) and on beamline ID27 at the European Synchrotron Radiation Facility (ESRF; experiment 2). At each pressure both a wide scan and a stepped ω scan were collected for each crystal. Wide scans consisted of 40 s exposure during rotations of ± 20° of the DAC. Step scans were performed with individual exposures taken over 0.5° intervals to constrain the ω angle of maximum intensity of each peak. Collected diffraction images were analysed using the program CrysAlisPRO (Agilent, 2012 ). The SHELXL program package was used for all structural determinations (Sheldrick, 2015 ).

The structure refinements performed at 25.4 (1) GPa for danburite-II and danburite-III in experiment 1 are of the best quality and therefore they are used for the following crystal chemical discussion. Equation-of-state fits were performed with the EoSFit7c program using the Murnaghan equation of state (Angel et al., 2014 ). For full details of the structure refinement, see the supporting information.

High-pressure Raman and IR spectroscopy measurements were performed on beamline IR2 of the ANKA synchrotron facility at Karlsruhe Institute of Technology. The same strategy of DAC preparation was applied, with the only difference being the usage of type IIa diamonds to ensure high transparency for the IR experiment. The IR spectra were collected with a Vertex80v FT–IR spectrometer coupled to an IRscopeII microscope in transmitted-light mode using Schwarzschild objectives (15×, 0.5 N.A.), an aperture of 30 µm and a liquid-N₂-cooled MCT detector. The Raman spectra were collected in the range 100–1220 rel. cm⁻¹ in back-scattering geometry using a WITec alpha300 confocal Raman system. A 532 nm frequency-doubled Nd-YAG laser was used, coupled via a single-mode fibre to a microscope with a 50× long working distance objective (N.A. = 0.5). The scattered light was sent to the spectrometer via a multi-mode fibre and recorded on a CCD detector. The collected spectra are given in the supporting information.

It is important to note that the high-pressure transformation route of danburite is sensitive to the presence of deviatoric stresses within the pressure chamber. In the course of experiment 1, danburite-II and danburite-III were found simultaneously at 25.4 (1) GPa, while in experiment 2 we clearly observed a step-wise character of these transformations (Fig. S1). In addition, the transition of danburite-II into a P2₁/c phase with the β angle deviating slightly from 90° (danburite-IV) was detected above ∼32 GPa in the course of experiment 1. We assume that the observed differences in transformation routes originated from the large thickness of the crystal (>20 µm) that bridged between the diamonds at the transition pressures in experiment 1. This feature makes it difficult to define the assignment of the vibrational bands to either five- or eight-coordinated silicon above the danburite → danburite-II transition pressure. Thus, the characteristic Raman band for T—O—T stretching disappears at 25 GPa, and instead a new band appears at 640 cm⁻¹ at 25.5 GPa and moves to 655 cm⁻¹ at 34 GPa, which most probably reflects the formation of SiO₆ octahedra (Goryainov, 2016 ) and/or SiO₅ trigonal bipyramids. The abrupt change in the IR spectrum above ∼23 GPa also reflects the increase in Si coordination, which is manifested by the appearance of many new bands in the region 600–900 cm⁻¹.

3. Results

3.1. Crystal structure of danburite under ambient conditions

Under ambient conditions danburite, CaB₂Si₂O₈, is orthorhombic in the space group Pnam and the unit cell is a = 8.038 (3), b = 8.752 (5) and c = 7.730 (3) Å (Phillips et al., 1974 ). The asymmetric unit of danburite contains two tetrahedrally coordinated cations T (Si and B) and five O atoms (Fig. 1). Polymerization of Si₂O₇ and B₂O₇ structural units by common sharing of O atoms results in the formation of a tetrahedral framework, with channels outlined by four- and eight-membered rings along the c axis (Fig. 1). The channels with eight-membered rings are occupied by Ca atoms in either seven-fold (for Ca—O bonds shorter than 3 Å) or nine-fold coordination (taking into account two Ca—O bonds of 3.02 Å). The eight-membered rings consisting of Ca atoms are elliptically elongated. The elongation can be measured in terms of the L/S ratio between the longest (L) and shortest (S) diagonals, and it is equal to 1.33 under ambient conditions.

Figure 1
The crystal structure of danburite under ambient conditions, viewed along the c axis. Green and blue tetrahedra represent BO₄ and SiO₄, respectively. The Ca atoms are shown in dark blue. The inset shows the coordination of a Ca atom, where the dotted lines correspond to bond lengths exceeding 3 Å.

3.2. High-pressure phase transitions of danburite

The high-pressure behaviour of danburite was investigated up to 36 GPa by synchrotron-based single-crystal X-ray diffraction (experiments 1 and 2) and by IR and Raman spectroscopy. The transformation route was found to be sensitive to hydrostatic conditions within the sample chamber (see Materials and methods section for details and Fig. S1). The structural refinements performed in experiment 1 (Tables S1 and S2) are used for the following crystal chemical discussion, while the outlined transformation route was followed in experiment 2.

Below 6.5 GPa, continuous contraction of the unit-cell parameters (Fig. 2) and atomic bonds (Fig. S2) is observed, in agreement with the previous high-pressure diffraction study of danburite up to 4.6 GPa (Hackwell & Angel, 1992 ). The unit cell undergoes anisotropic compression, with the a axis displaying the highest compressibility and the c axis the lowest (Fig. S3). The stiffness of the c axis is governed by the low compressibility of the Si—O bonds through bridging O atoms in the Si₂O₇ ditetrahedral groups. In agreement with the earlier report, the compression of the material up to ∼6.5 GPa is controlled by changes in the T—O—T angles of the tetrahedral framework (Hackwell & Angel, 1992).

Figure 2
The evolution of the unit-cell parameters of danburite along the compression. The inset shows the dependence of the volume of the $[P{\overline 1}]$ phase (danburite-III) on the pressure increase. Filled and open symbols correspond to experiments 1 and 2, respectively. The errors are smaller than the size of the symbols.

Unexpected behaviour of the crystal structure is observed above 6.5 GPa. While the a and b axes continue to decrease, the c axis reveals an anomalous increase (Figs. 2 and S3). Bulk moduli obtained by fitting of the P–V data with the Murnaghan equation of state below 6.5 GPa and between 6.5 and 23 GPa are 94.1 (4) and 33 (1) GPa, respectively (Table S3) (Angel, 2000 ). The difference in bulk modulus of almost three times indicates significant softening of the structure above ∼6.5 GPa. Anomalous behaviour is also observed for certain bond lengths (Fig. S2). Si and B atoms are displaced out of the centres of the TO₄ tetrahedra. The deviation of the SiO₄ and BO₄ units from the ideal tetrahedral geometry at ∼10 GPa is clearly visible on plots showing the quadratic elongation (QE) and bond angular variation (BAV) parameters as a function of pressure (Fig. S4) (Robinson et al., 1971 ).

Under compression to about 23 GPa the danburite framework undergoes severe distortion: the eight-membered rings become strongly elliptically distorted (the L/S ratio increases by almost three times from 1.33 to 3.27) and the T—O—T angles decrease from ∼126–138° to ∼118–126°. This last feature is also reflected in the evolution of the Raman band assigned for the Si—O—Si symmetric bending mode of the Si₂O₇ units (Garbev et al., 2007 ): it shifts continuously from ∼615 cm⁻¹ at 1.4 (1) GPa to ∼663 cm⁻¹ at 22.7 (1) GPa (Fig. S5). Framework distortion is particularly pronounced in the rotation of tetrahedra in the T₂O₇ groups (Fig. 3a) which is responsible for the anomalous increase in the c parameter with pressure. Up to ∼23 GPa, neither discontinuity in volume nor changes in symmetry of the structure and Wyckoff positions of individual atoms are observed. Still, the Raman and IR spectra in this pressure range are significantly affected by the progressive distortion of the coordination environment of the cations, indicating that not only does the compressive mechanism change with accompanying structure softening and the material become softer, but the vibrational properties of danburite are very different below and above ∼10 GPa. Thus, the IR band at ∼917 cm⁻¹ at 2.5 (1) GPa, which gradually shifts to 940 cm⁻¹ at 9.2 (1) GPa and then successively moves back to lower frequencies [∼917 cm⁻¹ at ∼21.2 (1) GPa], is most probably due to vibrations of the B—O5 and/or B—O3 bonds (Fig. S6).

Figure 3
The evolution of silicon-based structural units along the high-pressure treatment of danburite. (a) Distortion of an Si₂O₇ ditetrahedral group from 1.1 (1) to 22.6 (1) GPa. Note the respective increase in the O3—O4—O3 angle from 148.81 (2) to 174.57 (2)° that causes the anomalous enlargement of the c axis along the compression. (b) A chain built of SiO₅ trigonal bipyramids in the structure of danburite-II (Pnam) at 25.4 (1) GPa. (c) A chain built of edge-sharing SiO₆ octahedra in danburite-III ( $[P{\overline 1}]$ ) at 25.4 (1) GPa. The Si—O bond distances are given in ångströms.

Above ∼23 GPa the crystal structure of danburite undergoes a phase transition that is reflected in the abrupt change in its vibrational spectra (Figs. S5 and S6) and unit-cell parameters (Figs. 2 and S3). The high-pressure phase (danburite-II) preserves the orthorhombic (Pnam) symmetry while the structural change is displacive and is induced by a shift of the Si atoms along the a axis in such a way that an extra O1* atom of an adjacent tetrahedron, across the eight-membered ring, joins its coordination sphere (Fig. S7a), so that the Si coordination becomes fivefold. The Si—O—Si symmetric bending mode of the Si₂O₇ ditetrahedral unit of danburite is not visible in the Raman spectrum above 25 GPa, in agreement with the absence of tetrahedral silicon above this pressure. At 25.4 (1) GPa, the coordination polyhedron of Si has trigonal–bipyramidal geometry (Fig. 3b) with two long (1.81–1.85 Å) apical and three short (1.60–1.62 Å; Table S2) equatorial Si—O bonds. The O—Si—O apical bond angle is 7.3° away from the 180° angle required for a regular trigonal bipyramid. More detailed inspection of the SiO₅ geometry can be performed through the Liebau approach (Liebau, 1984 ), where the mean 〈O_eq—Si—O_eq〉, 〈O_eq—Si—O_ap〉 and 〈O_eq—Si—O_ap′〉 angles are considered (O_eq are equatorial O atoms, while O_ap and O_ap′ are the two apical O atoms with shorter and longer Si—O bonds, respectively). These angles in the structure of danburite-II are 119.9, 88.6 and 91.4° respectively, indicating that the SiO₅ polyhedron is exceptionally close to the values of 120° and 90° characteristic for an ideal trigonal bipyramid. The SiO₅ trigonal bipyramids share common vertices to form chains running along the a axis. The connection along the c axis through bridging O atoms is preserved and leads to the formation of double [Si₂O₇]_∞ chains (Fig. 3b). To the best of our knowledge, danburite-II is the first example of a high-pressure phase possessing silicon in a fivefold coordination only. Moreover, the geometry of a trigonal bipyramid has not previously been reported before for silicates, although it is known in metal–organic compounds (Rappoport & Apeloig, 1998 ; Baus et al., 2013 ; Wagler et al., 2014 ; Mück et al., 2016 ).

Upon compression above 32 GPa, the appearance of a new high-pressure phase, co-existing with danburite-II (Fig. S1), is observed. The structure of this new phase, named danburite-III, was solved and refined in the $[P{\overline 1}]$ space group (Table S1, and Figs. 3c and S7b). The crystal structure contains two crystallographically independent Si, two B, one Ca and eight O sites. The Pnam → $[P{\overline 1}]$ transition is reconstructive in character and involves breaking of chemical bonds and re-formation of the bonding network within Si-based chains. Across the transition, the chain composed of corner-sharing SiO₅ trigonal bipyramids transforms into chains of edge-sharing SiO₆ octahedra (Fig. 3c). The bond-length distributions are similar for both Si sites and are in the range 1.62–1.86 Å (Table S2). We are not aware of any other example of this kind of topology consisting of octahedral chains with six-coordinated Si atoms in silicates.

4. Discussion

Pentacoordinated silicon is extremely rare in inorganic compounds, and the reason why SiO₄ tetrahedra transform directly into SiO₆ octahedra without transitional SiO₅ coordination is not clear. Liebau (1984) explained the absence of [SiO₅] groups in terms of the ionic character of chemical bonding in silicates. The difference in electronegativities between Si and O ions (Δχ) is relatively high at 1.76; for comparison, the Δχ values for Si—N and Si—C pairs are 1.33 and 0.76, respectively, enabling the occurrence of SiX₅ groups in organosilicon compounds. Ionic oxide compounds are known for their tendency of forming a close-packed arrangement of oxygen atoms, with cations filling up the voids between the anions. Many silicates under ambient conditions can be described as distorted derivatives of cubic or hexagonal close-packed (c.c.p. and h.c.p., respectively) layer structures (Liebau, 1985 ; Lima-de-Faria, 1994 ; Krivovichev, 2009 ; Thompson & Downs, 2003 ). The application of high pressure forces a rearrangement of the structural units so as to occupy the space most efficiently, i.e. the oxygen sublattice evolves towards an ideal close packing. In close packings there are only tetrahedral and octahedral voids, which explains the common presence of four- and six-coordinated silicon (Liebau, 1984). However, h.c.p. arrays may be considered as containing pentacoordinated voids with a trigonal–bipyramidal geometry. These voids are formed by three spheres in a layer and two spheres above and below the layer, i.e. they are formed by merging two tetrahedral voids (Figs. 4a and S8). However, the size of the void within the plane of the layer is so small that no cations can fit inside. For example, assuming an ideal close packing of O²⁻ anions with an ionic radius of 1.32 Å, the cation would have to have a radius of ∼0.2 Å. It is noteworthy that filling of pentacoordinated voids has been repeatedly reported for metallic and intermetallic compounds (Isaeva et al., 2010 ; Larsson et al., 1995 ), as well as for hexagonal M-type ferrites (Townes et al., 1967 ; Kreber & Gonser, 1976 ; Albanese et al., 1981 ). The possible presence of Si in a trigonal–bipyramidal void has also been discussed for the CaAl₄Si₂O₁₁ phase with a close-packed structure based upon the hexagonal barium ferrite model (Gautron et al., 1999 ). Our work demonstrates that densification of crystal structures may involve an intermediate stage of atomic arrangement with occupied trigonal–bipyramidal voids.

Figure 4
Drawing parallels between the danburite-II crystal structure and an ideal h.c.p. structure. (a) A view of an h.c.p. structure based on hexagonal cubooctahedra (shown in green). A void with trigonal–bipyramidal geometry is shown in yellow. (b) A representation of the crystal structure of danburite-II based on a distorted h.c.p. structure made of Ca and O atoms. CaO₁₁ polyhedra (defective hexagonal cubooctahedra) are shown in green. The void with trigonal–bipyramidal geometry is occupied by an Si atom (shown in black). The bright-green sphere towards the top centre of panel (b) represents the B atom occupying a tetrahedral void.

Under ambient conditions danburite possesses a relatively open structure, which has no direct relation to the close-packed arrangements of the O atoms. In contrast, danburite-III is dense and its structure is based upon the c.c.p. arrangement formed by both Ca and O atoms (Fig. S9). The close-packed layers are coplanar with the ( $[1{\overline 1}1]$ ) plane, with Si occupying 2/9 of all octahedral voids and B occupying 2/18 of all tetrahedral voids. The structure of danburite-II can be described as having elements of distorted c.c.p and h.c.p. close packings. It contains ribbons of close-packed O and Ca atoms parallel to the (010) plane and extended along the c axis. Fig. 5(a) shows the arrangement of the octahedral voids, organized into chains running along [001] in the structure, whereas Figs. 5(b) and 5(c) specify the deviations of the packing from both h.c.p. and c.c.p. The double chain of edge-sharing octahedral voids (shown in grey in Fig. 5c) is covered by two adjacent chains of octahedral voids, one according to c.c.p. stacking (blue octahedra) and the other according to h.c.p. stacking (green octahedra). As a result, a square contact appears on the contact of the blue and green octahedra, which is not allowed in an ideal close-packed structure. Deviation from a close-packed geometry is manifested in the coordination number of Ca (11, instead of 12 in the ideal situation; Belov, 1941 ; Giacovazzo et al., 2002 ). Nevertheless, the presence of close-packed arrangements in danburite-II compared with danburite-I shows that, under increasing pressure, the structure evolves towards a close-packed architecture, which is realised in danburite-III. In danburite-II, trigonal–bipyramidal voids within close-packed structure regions are occupied by Si, whereas tetrahedral voids are occupied by B (Fig. 4b). The danburite-II → danburite-III transition involves [h.c.p. + c.c.p.] → c.c.p. transformation and therefore has a distinct reconstructive character.

Figure 5
Representation of the crystal structure of danburite-II on the basis of the close-packing approach. (a) The arrangement of octahedral voids in the packing of O (red) and Ca (blue) atoms in danburite-II, featuring chains of edge-sharing octahedral voids parallel to the c axis. The area outlined by the dashed line is magnified in panel (b). (b) Four chains of edge-sharing octahedral voids making up adjacent c.c.p. and h.c.p. blocks. (c) A scheme showing the origin of the `square contact' between the c.c.p. and h.c.p. blocks through the cover of underlying double chains of octahedra by octahedra in either a c.c.p. (lilac octahedron) or h.c.p. (green octahedron) fashion.

The mechanism of the pressure-induced phase transition invoked by our experimental work on danburite can be applied to explain the observation of silicon in a (4+1) coordination in the high-pressure modification of Mg–Fe orthoenstatite (OEn) with the chemical formula (Mg_0.900Fe_0.088Ca_0.003Mn_0.003)(Al_0.004Si_0.999)O₃ (Finkelstein et al., 2015). Under ambient conditions, the oxygen sublattice of OEn is described by a distorted ABCBACBC sequence (Thompson & Downs, 2003). Compression to ∼30 GPa results in a transition to α-postOEn, with the O atoms forming a nearly ideal ABCBACBC pattern; further compression leads to the ∼40 GPa transition to β-postOEn with the …AB… stacking sequence (Fig. S10) (Isaeva et al., 2010; Larsson et al., 1995). The shifts in the packing of the O atoms towards an h.c.p. structure result in the transformation of the tetrahedral voids into octahedral ones. The fivefold coordination of Si forms due to the asynchronization of two processes: formation of octahedral voids and movement of Si atoms into the centres of these voids. Therefore, Si moves from the equatorial plane of the octahedron and its coordination polyhedron can be considered as a distorted square pyramid (Fig. S10b). The same mechanism may be responsible for the formation of the very first known silicate with an SiO₅ group, triclinic CaSi₂O₅: the phase is found only in a very limited pressure range and, upon the transition at 0.2 GPa, SiO₅ square pyramids transform into octahedra (Angel et al., 1996; Angel, 1997 ). By analogy with CaSi₂O₅, the filling of trigonal voids in CaAl₄Si₂O₁₁ was proposed to be pressure-dependent and the transition from four- to five-coordinated silicon was expected to occur upon compression (Gautron et al., 1999).

Our results on the phase transitions of danburite and comparisons with other known examples of silicates with pentacoordinated silicon suggest that SiO₅ groups may be not as rare as previously thought and may form as intermediate configurations upon (a) transformation of silicates with relatively open structures into phases based upon close packing of oxygen (or oxygen and large cations), and (b) transformations of close-packed structures that involve transition between octahedral and tetrahedral voids due to shifts of close-packed layers. It is very likely that the presence of boron as the second framework-forming cation plays an important role in the discovered transformation route of danburite. Further studies are needed to illuminate how the interplay between the two cations influences the high-pressure structural behaviour of silicates.

Supporting information

CCDC references: 1562973; 1562974; 1562975; 1562976

Crystal structure: contains datablocks danburite_1.1GPa, danburite_22.6GPa, danburite_25.4GPa, danburite-III_25.4GPa, danburite-IV_32.3GPa. DOI: https://doi.org/10.1107/S2052252517010612/lt5006sup1.cif

Structure factors: contains datablock danburite_22.6Gpa. DOI: https://doi.org/10.1107/S2052252517010612/lt5006danburite_22.6Gpasup2.hkl

Structure factors: contains datablock danburite_25.4GPa. DOI: https://doi.org/10.1107/S2052252517010612/lt5006danburite_25.4GPasup3.hkl

Structure factors: contains datablock danburite-III_25.4GPa. DOI: https://doi.org/10.1107/S2052252517010612/lt5006danburite-III_25.4GPasup4.hkl

Structure factors: contains datablock danbburite-IV_32.3GPa. DOI: https://doi.org/10.1107/S2052252517010612/lt5006danburite-IV_32.3GPasup5.hkl

Supplementary tables and figures. DOI: https://doi.org/10.1107/S2052252517010612/lt5006sup6.pdf

Computing details top

For all structures, data collection: CrysAlis PRO, Agilent Technologies, Version 1.171.36.28 (release 01-02-2013 CrysAlis171 .NET) (compiled Feb 1 2013,16:14:44); cell refinement: CrysAlis PRO, Agilent Technologies, Version 1.171.36.28 (release 01-02-2013 CrysAlis171 .NET) (compiled Feb 1 2013,16:14:44); data reduction: CrysAlis PRO, Agilent Technologies, Version 1.171.36.28 (release 01-02-2013 CrysAlis171 .NET) (compiled Feb 1 2013,16:14:44); program(s) used to solve structure: SHELXL2014/7 (Sheldrick, 2014); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2014).

danburite (danburite_22.6GPa) top

Crystal data top

B₂CaO₈Si₂	D_x = 3.771 Mg m⁻³
M_r = 245.88	Synchrotron radiation, λ = 0.29024 Å
Orthorhombic, Pnam	Cell parameters from 1462 reflections
a = 6.8576 (3) Å	θ = 1.9–16.8°
b = 8.1526 (3) Å	µ = 0.20 mm⁻¹
c = 7.7457 (2) Å	T = 293 K
V = 433.04 (3) Å³	Plate crystal, colorless
Z = 4	0.02 × 0.02 × 0.01 mm
F(000) = 488

Data collection top

Esperanto-CrysAlisPro-abstract goniometer imported esperanto images diffractometer	R_int = 0.019
Radiation source: synchrotron, PETRA III, Beamline P02.2	θ_max = 16.9°, θ_min = 1.9°
ω scans	h = −10→11
2397 measured reflections	k = −13→12
977 independent reflections	l = −13→15
926 reflections with I > 2σ(I)

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0392P)² + 0.1215P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.026	(Δ/σ)_max < 0.001
wR(F²) = 0.072	Δρ_max = 0.64 e Å⁻³
S = 1.19	Δρ_min = −0.75 e Å⁻³
977 reflections	Extinction correction: SHELXL-2014/7 (Sheldrick 2014, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
65 parameters	Extinction coefficient: 0.31 (3)

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Ca1	0.29419 (6)	0.10190 (4)	0.2500	0.00719 (9)
Si1	0.03614 (5)	0.20673 (4)	−0.07259 (4)	0.00489 (10)
O1	0.24215 (15)	0.11070 (9)	−0.06377 (11)	0.00669 (15)
O2	0.11319 (15)	0.39013 (9)	−0.05885 (10)	0.00566 (15)
O3	0.41806 (15)	0.35396 (10)	0.09171 (10)	0.00630 (15)
O4	0.5693 (2)	0.65715 (14)	0.2500	0.00591 (19)
O5	0.1327 (2)	0.39500 (13)	0.2500	0.0059 (2)
B1	0.2286 (2)	0.43902 (14)	0.40961 (13)	0.00560 (19)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.01083 (19)	0.00418 (16)	0.00657 (12)	0.00043 (7)	0.000	0.000
Si1	0.0071 (2)	0.00241 (16)	0.00514 (13)	−0.00039 (7)	−0.00012 (9)	0.00010 (8)
O1	0.0089 (5)	0.0022 (3)	0.0090 (3)	0.0007 (2)	−0.0005 (3)	−0.0003 (2)
O2	0.0081 (5)	0.0022 (3)	0.0067 (3)	−0.00042 (19)	−0.0005 (3)	−0.00006 (19)
O3	0.0076 (4)	0.0051 (3)	0.0062 (3)	0.0015 (2)	0.0006 (3)	−0.0005 (2)
O4	0.0085 (6)	0.0045 (5)	0.0047 (3)	0.0008 (3)	0.000	0.000
O5	0.0080 (7)	0.0046 (5)	0.0052 (3)	−0.0010 (3)	0.000	0.000
B1	0.0080 (6)	0.0033 (5)	0.0054 (3)	0.0000 (3)	−0.0001 (4)	0.0000 (3)

Geometric parameters (Å, º) top

Ca1—O5ⁱ	2.3218 (15)	O1—B1^vii	1.4289 (14)
Ca1—O2ⁱⁱ	2.3614 (8)	O2—B1^iv	1.4566 (15)
Ca1—O2ⁱⁱⁱ	2.3614 (8)	O2—Ca1^ix	2.3614 (8)
Ca1—O1^iv	2.4574 (8)	O3—B1^iv	1.4727 (17)
Ca1—O1	2.4575 (8)	O3—Si1ⁱ	1.5875 (9)
Ca1—O3^iv	2.5392 (9)	O3—Ca1ⁱ	2.8785 (11)
Ca1—O3	2.5392 (9)	O4—Si1^ix	1.6044 (7)
Ca1—O4^v	2.6152 (12)	O4—Si1^x	1.6044 (7)
Ca1—O5	2.6336 (12)	O4—Ca1ⁱ	2.6152 (12)
Ca1—O3^v	2.8786 (11)	O5—B1^iv	1.4454 (14)
Ca1—O3^vi	2.8786 (11)	O5—B1	1.4455 (14)
Ca1—B1^vii	2.9562 (11)	O5—Ca1^v	2.3218 (15)
Si1—O3^v	1.5875 (9)	B1—O1^x	1.4289 (14)
Si1—O2	1.5894 (8)	B1—O2^iv	1.4566 (15)
Si1—O4ⁱⁱ	1.6044 (7)	B1—O3^iv	1.4727 (17)
Si1—O1	1.6166 (10)	B1—Ca1^xi	2.9562 (11)
Si1—Si1^viii	2.7483 (6)	B1—Ca1^v	3.2426 (14)
Si1—Ca1^v	3.3809 (4)

O5ⁱ—Ca1—O2ⁱⁱ	74.87 (3)	O3^vi—Ca1—B1^vii	112.87 (3)
O5ⁱ—Ca1—O2ⁱⁱⁱ	74.87 (3)	O3^v—Si1—O2	114.15 (4)
O2ⁱⁱ—Ca1—O2ⁱⁱⁱ	77.66 (4)	O3^v—Si1—O4ⁱⁱ	112.23 (6)
O5ⁱ—Ca1—O1^iv	98.33 (3)	O2—Si1—O4ⁱⁱ	116.37 (5)
O2ⁱⁱ—Ca1—O1^iv	132.88 (3)	O3^v—Si1—O1	105.13 (5)
O2ⁱⁱⁱ—Ca1—O1^iv	55.97 (3)	O2—Si1—O1	99.34 (5)
O5ⁱ—Ca1—O1	98.33 (3)	O4ⁱⁱ—Si1—O1	107.94 (6)
O2ⁱⁱ—Ca1—O1	55.97 (3)	O3^v—Si1—Si1^viii	143.29 (3)
O2ⁱⁱⁱ—Ca1—O1	132.88 (3)	O2—Si1—Si1^viii	93.84 (3)
O1^iv—Ca1—O1	162.97 (5)	O4ⁱⁱ—Si1—Si1^viii	31.08 (4)
O5ⁱ—Ca1—O3^iv	69.92 (3)	O1—Si1—Si1^viii	92.42 (3)
O2ⁱⁱ—Ca1—O3^iv	143.56 (4)	O3^v—Si1—Ca1	64.54 (4)
O2ⁱⁱⁱ—Ca1—O3^iv	101.48 (3)	O2—Si1—Ca1	90.87 (3)
O1^iv—Ca1—O3^iv	63.09 (3)	O4ⁱⁱ—Si1—Ca1	148.90 (4)
O1—Ca1—O3^iv	120.16 (3)	O1—Si1—Ca1	49.47 (3)
O5ⁱ—Ca1—O3	69.92 (3)	Si1^viii—Si1—Ca1	141.816 (8)
O2ⁱⁱ—Ca1—O3	101.48 (3)	O3^v—Si1—Ca1^v	45.65 (3)
O2ⁱⁱⁱ—Ca1—O3	143.56 (4)	O2—Si1—Ca1^v	71.31 (3)
O1^iv—Ca1—O3	120.16 (3)	O4ⁱⁱ—Si1—Ca1^v	121.88 (5)
O1—Ca1—O3	63.09 (3)	O1—Si1—Ca1^v	128.40 (3)
O3^iv—Ca1—O3	57.74 (4)	Si1^viii—Si1—Ca1^v	137.651 (7)
O5ⁱ—Ca1—O4^v	126.77 (4)	Ca1—Si1—Ca1^v	79.416 (8)
O2ⁱⁱ—Ca1—O4^v	64.42 (3)	B1^vii—O1—Si1	126.22 (9)
O2ⁱⁱⁱ—Ca1—O4^v	64.42 (3)	B1^vii—O1—Ca1	95.38 (6)
O1^iv—Ca1—O4^v	86.44 (2)	Si1—O1—Ca1	100.54 (4)
O1—Ca1—O4^v	86.44 (2)	B1^iv—O2—Si1	119.42 (7)
O3^iv—Ca1—O4^v	148.30 (2)	B1^iv—O2—Ca1^ix	98.71 (6)
O3—Ca1—O4^v	148.30 (2)	Si1—O2—Ca1^ix	137.33 (4)
O5ⁱ—Ca1—O5	114.24 (4)	B1^iv—O3—Si1ⁱ	126.24 (7)
O2ⁱⁱ—Ca1—O5	140.94 (2)	B1^iv—O3—Ca1	95.12 (7)
O2ⁱⁱⁱ—Ca1—O5	140.94 (2)	Si1ⁱ—O3—Ca1	107.79 (4)
O1^iv—Ca1—O5	84.98 (2)	B1^iv—O3—Ca1ⁱ	137.27 (6)
O1—Ca1—O5	84.98 (2)	Si1ⁱ—O3—Ca1ⁱ	85.60 (4)
O3^iv—Ca1—O5	53.58 (3)	Ca1—O3—Ca1ⁱ	101.26 (3)
O3—Ca1—O5	53.58 (3)	Si1^ix—O4—Si1^x	117.85 (8)
O4^v—Ca1—O5	119.00 (4)	Si1^ix—O4—Ca1ⁱ	117.98 (4)
O5ⁱ—Ca1—O3^v	153.464 (19)	Si1^x—O4—Ca1ⁱ	117.98 (4)
O2ⁱⁱ—Ca1—O3^v	93.75 (3)	B1^iv—O5—B1	117.58 (14)
O2ⁱⁱⁱ—Ca1—O3^v	126.83 (3)	B1^iv—O5—Ca1^v	116.88 (8)
O1^iv—Ca1—O3^v	106.71 (3)	B1—O5—Ca1^v	116.88 (8)
O1—Ca1—O3^v	56.29 (3)	B1^iv—O5—Ca1	91.95 (7)
O3^iv—Ca1—O3^v	113.84 (3)	B1—O5—Ca1	91.95 (7)
O3—Ca1—O3^v	89.59 (2)	Ca1^v—O5—Ca1	115.48 (5)
O4^v—Ca1—O3^v	64.69 (3)	O1^x—B1—O5	115.50 (9)
O5—Ca1—O3^v	60.65 (3)	O1^x—B1—O2^iv	103.28 (8)
O5ⁱ—Ca1—O3^vi	153.464 (19)	O5—B1—O2^iv	111.33 (11)
O2ⁱⁱ—Ca1—O3^vi	126.83 (3)	O1^x—B1—O3^iv	109.78 (10)
O2ⁱⁱⁱ—Ca1—O3^vi	93.75 (3)	O5—B1—O3^iv	106.16 (9)
O1^iv—Ca1—O3^vi	56.29 (3)	O2^iv—B1—O3^iv	110.85 (9)
O1—Ca1—O3^vi	106.71 (3)	O1^x—B1—Ca1^xi	55.86 (5)
O3^iv—Ca1—O3^vi	89.59 (2)	O5—B1—Ca1^xi	148.24 (9)
O3—Ca1—O3^vi	113.84 (3)	O2^iv—B1—Ca1^xi	52.15 (5)
O4^v—Ca1—O3^vi	64.69 (3)	O3^iv—B1—Ca1^xi	105.33 (6)
O5—Ca1—O3^vi	60.65 (3)	O1^x—B1—Ca1	157.13 (8)
O3^v—Ca1—O3^vi	50.42 (3)	O5—B1—Ca1	59.75 (6)
O5ⁱ—Ca1—B1^vii	93.31 (3)	O2^iv—B1—Ca1	98.94 (6)
O2ⁱⁱ—Ca1—B1^vii	29.15 (3)	O3^iv—B1—Ca1	56.10 (5)
O2ⁱⁱⁱ—Ca1—B1^vii	104.18 (3)	Ca1^xi—B1—Ca1	140.79 (4)
O1^iv—Ca1—B1^vii	152.56 (3)	O1^x—B1—Ca1^v	106.54 (8)
O1—Ca1—B1^vii	28.76 (3)	O5—B1—Ca1^v	39.69 (7)
O3^iv—Ca1—B1^vii	144.28 (3)	O2^iv—B1—Ca1^v	77.01 (7)
O3—Ca1—B1^vii	87.17 (3)	O3^iv—B1—Ca1^v	139.41 (7)
O4^v—Ca1—B1^vii	66.80 (3)	Ca1^xi—B1—Ca1^v	109.74 (4)
O5—Ca1—B1^vii	112.66 (3)	Ca1—B1—Ca1^v	83.58 (3)
O3^v—Ca1—B1^vii	68.22 (3)

Symmetry codes: (i) x+1/2, −y+1/2, z; (ii) −x+1/2, y−1/2, −z; (iii) −x+1/2, y−1/2, z+1/2; (iv) x, y, −z+1/2; (v) x−1/2, −y+1/2, z; (vi) x−1/2, −y+1/2, −z+1/2; (vii) −x+1/2, y−1/2, z−1/2; (viii) x, y, −z−1/2; (ix) −x+1/2, y+1/2, −z; (x) −x+1/2, y+1/2, z+1/2; (xi) −x+1/2, y+1/2, −z+1.

danburite (danburite_25.4GPa) top

Crystal data top

B₂CaO₈Si₂	D_x = 4.035 Mg m⁻³
M_r = 245.88	Synchrotron radiation, λ = 0.29056 Å
Orthorhombic, Pnam	Cell parameters from 977 reflections
a = 6.3537 (5) Å	θ = 1.7–16.6°
b = 7.9518 (3) Å	µ = 0.22 mm⁻¹
c = 8.0112 (3) Å	T = 293 K
V = 404.75 (4) Å³	Plate crystal, colorless
Z = 4	0.02 × 0.02 × 0.01 mm
F(000) = 488

Data collection top

Esperanto-CrysAlisPro-abstract goniometer imported esperanto images diffractometer	R_int = 0.024
Radiation source: synchrotron, PETRA III, Beamline P02.2	θ_max = 16.7°, θ_min = 1.7°
ω scans	h = −10→9
2163 measured reflections	k = −13→12
891 independent reflections	l = −15→14
742 reflections with I > 2σ(I)

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0923P)² + 0.4473P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.042	(Δ/σ)_max < 0.001
wR(F²) = 0.178	Δρ_max = 1.05 e Å⁻³
S = 1.32	Δρ_min = −1.44 e Å⁻³
891 reflections	Extinction correction: SHELXL-2014/7 (Sheldrick 2014, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
65 parameters	Extinction coefficient: 0.18 (5)

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Ca1	0.25404 (13)	0.10929 (9)	0.2500	0.0112 (2)
Si1	−0.00581 (14)	0.23537 (9)	−0.07365 (10)	0.0103 (3)
O1	0.2513 (3)	0.1368 (2)	−0.0738 (3)	0.0111 (4)
O2	0.1243 (4)	0.4068 (2)	−0.0572 (2)	0.0099 (4)
O3	0.4325 (4)	0.3664 (2)	0.0939 (2)	0.0110 (4)
O4	0.5630 (5)	0.6518 (3)	0.2500	0.0114 (5)
O5	0.1238 (5)	0.4081 (3)	0.2500	0.0111 (5)
B1	0.2292 (5)	0.4549 (3)	0.4046 (3)	0.0093 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.0157 (5)	0.0069 (4)	0.0111 (3)	0.00054 (18)	0.000	0.000
Si1	0.0155 (6)	0.0061 (4)	0.0092 (3)	−0.0008 (2)	0.0000 (2)	−0.00001 (18)
O1	0.0153 (13)	0.0061 (9)	0.0118 (8)	−0.0004 (5)	0.0003 (6)	0.0002 (5)
O2	0.0147 (11)	0.0036 (8)	0.0113 (6)	−0.0009 (5)	−0.0006 (7)	0.0001 (5)
O3	0.0135 (12)	0.0092 (8)	0.0101 (7)	0.0020 (5)	0.0006 (7)	−0.0010 (5)
O4	0.0169 (17)	0.0078 (11)	0.0095 (9)	0.0022 (8)	0.000	0.000
O5	0.0154 (17)	0.0087 (11)	0.0092 (9)	−0.0010 (7)	0.000	0.000
B1	0.0118 (15)	0.0053 (11)	0.0108 (9)	0.0000 (7)	−0.0004 (9)	0.0001 (7)

Geometric parameters (Å, º) top

Ca1—O5ⁱ	2.354 (3)	Si1—Ca1^viii	3.4633 (10)
Ca1—O2ⁱⁱ	2.3614 (19)	O1—B1^ix	1.461 (3)
Ca1—O2ⁱⁱⁱ	2.3614 (19)	O1—Si1ⁱ	1.848 (2)
Ca1—O3^iv	2.403 (2)	O2—B1^vi	1.444 (3)
Ca1—O3^v	2.403 (2)	O2—Ca1^x	2.3614 (19)
Ca1—O4^iv	2.405 (3)	O3—B1^vi	1.471 (4)
Ca1—O5	2.516 (3)	O3—Si1ⁱ	1.6157 (19)
Ca1—O1	2.603 (2)	O3—Ca1ⁱ	2.403 (2)
Ca1—O1^vi	2.603 (2)	O4—Si1^xi	1.6030 (15)
Ca1—O3^vi	2.651 (2)	O4—Si1^x	1.6030 (15)
Ca1—O3	2.651 (2)	O4—Ca1ⁱ	2.405 (3)
Ca1—B1	3.019 (3)	O5—B1	1.456 (3)
Si1—O2	1.600 (2)	O5—B1^vi	1.456 (3)
Si1—O4ⁱⁱ	1.6030 (15)	O5—Ca1^iv	2.354 (3)
Si1—O3^iv	1.6157 (19)	B1—O2^vi	1.444 (3)
Si1—O1	1.812 (2)	B1—O1^xi	1.461 (3)
Si1—O1^iv	1.848 (2)	B1—O3^vi	1.471 (4)
Si1—Si1^vii	2.8256 (15)	B1—Ca1^xii	3.029 (3)
Si1—Ca1^iv	3.2522 (9)

O5ⁱ—Ca1—O2ⁱⁱ	68.48 (7)	O3^iv—Si1—O1	90.14 (11)
O5ⁱ—Ca1—O2ⁱⁱⁱ	68.48 (7)	O2—Si1—O1^iv	87.88 (10)
O2ⁱⁱ—Ca1—O2ⁱⁱⁱ	81.70 (9)	O4ⁱⁱ—Si1—O1^iv	92.20 (14)
O5ⁱ—Ca1—O3^iv	148.59 (5)	O3^iv—Si1—O1^iv	94.22 (12)
O2ⁱⁱ—Ca1—O3^iv	89.58 (7)	O1—Si1—O1^iv	172.27 (8)
O2ⁱⁱⁱ—Ca1—O3^iv	132.34 (8)	O2—Si1—Si1^vii	94.73 (7)
O5ⁱ—Ca1—O3^v	148.59 (5)	O4ⁱⁱ—Si1—Si1^vii	28.20 (9)
O2ⁱⁱ—Ca1—O3^v	132.34 (8)	O3^iv—Si1—Si1^vii	146.19 (7)
O2ⁱⁱⁱ—Ca1—O3^v	89.58 (7)	O1—Si1—Si1^vii	89.96 (7)
O3^iv—Ca1—O3^v	62.71 (10)	O1^iv—Si1—Si1^vii	89.96 (7)
O5ⁱ—Ca1—O4^iv	116.94 (10)	O2—Si1—Ca1	86.23 (8)
O2ⁱⁱ—Ca1—O4^iv	64.95 (7)	O4ⁱⁱ—Si1—Ca1	133.94 (11)
O2ⁱⁱⁱ—Ca1—O4^iv	64.95 (7)	O3^iv—Si1—Ca1	45.75 (9)
O3^iv—Ca1—O4^iv	68.92 (8)	O1—Si1—Ca1	53.56 (7)
O3^v—Ca1—O4^iv	68.92 (8)	O1^iv—Si1—Ca1	126.69 (7)
O5ⁱ—Ca1—O5	112.57 (9)	Si1^vii—Si1—Ca1	143.315 (18)
O2ⁱⁱ—Ca1—O5	138.73 (5)	O2—Si1—Ca1^iv	81.56 (8)
O2ⁱⁱⁱ—Ca1—O5	138.73 (5)	O4ⁱⁱ—Si1—Ca1^iv	138.93 (13)
O3^iv—Ca1—O5	69.18 (8)	O3^iv—Si1—Ca1^iv	54.13 (8)
O3^v—Ca1—O5	69.18 (8)	O1—Si1—Ca1^iv	126.01 (8)
O4^iv—Ca1—O5	130.49 (11)	O1^iv—Si1—Ca1^iv	53.11 (7)
O5ⁱ—Ca1—O1	90.67 (5)	Si1^vii—Si1—Ca1^iv	142.869 (18)
O2ⁱⁱ—Ca1—O1	53.68 (6)	Ca1—Si1—Ca1^iv	73.62 (2)
O2ⁱⁱⁱ—Ca1—O1	135.34 (7)	O2—Si1—Ca1^viii	160.61 (8)
O3^iv—Ca1—O1	57.92 (6)	O4ⁱⁱ—Si1—Ca1^viii	37.74 (9)
O3^v—Ca1—O1	120.40 (7)	O3^iv—Si1—Ca1^viii	80.34 (8)
O4^iv—Ca1—O1	93.96 (5)	O1—Si1—Ca1^viii	93.89 (7)
O5—Ca1—O1	85.33 (5)	O1^iv—Si1—Ca1^viii	93.14 (7)
O5ⁱ—Ca1—O1^vi	90.67 (5)	Si1^vii—Si1—Ca1^viii	65.925 (14)
O2ⁱⁱ—Ca1—O1^vi	135.34 (7)	Ca1—Si1—Ca1^viii	108.32 (2)
O2ⁱⁱⁱ—Ca1—O1^vi	53.68 (6)	Ca1^iv—Si1—Ca1^viii	114.34 (3)
O3^iv—Ca1—O1^vi	120.40 (7)	B1^ix—O1—Si1	120.32 (18)
O3^v—Ca1—O1^vi	57.92 (6)	B1^ix—O1—Si1ⁱ	118.25 (18)
O4^iv—Ca1—O1^vi	93.96 (5)	Si1—O1—Si1ⁱ	120.98 (11)
O5—Ca1—O1^vi	85.33 (5)	B1^ix—O1—Ca1	91.96 (14)
O1—Ca1—O1^vi	170.34 (9)	Si1—O1—Ca1	92.39 (9)
O5ⁱ—Ca1—O3^vi	67.57 (7)	Si1ⁱ—O1—Ca1	92.28 (9)
O2ⁱⁱ—Ca1—O3^vi	133.98 (8)	B1^vi—O2—Si1	122.29 (16)
O2ⁱⁱⁱ—Ca1—O3^vi	94.44 (6)	B1^vi—O2—Ca1^x	102.82 (15)
O3^iv—Ca1—O3^vi	123.16 (6)	Si1—O2—Ca1^x	134.13 (11)
O3^v—Ca1—O3^vi	93.22 (6)	B1^vi—O3—Si1ⁱ	117.34 (17)
O4^iv—Ca1—O3^vi	151.83 (4)	B1^vi—O3—Ca1ⁱ	134.73 (15)
O5—Ca1—O3^vi	54.01 (8)	Si1ⁱ—O3—Ca1ⁱ	105.45 (12)
O1—Ca1—O3^vi	114.09 (7)	B1^vi—O3—Ca1	89.41 (14)
O1^vi—Ca1—O3^vi	57.87 (6)	Si1ⁱ—O3—Ca1	96.28 (9)
O5ⁱ—Ca1—O3	67.57 (7)	Ca1ⁱ—O3—Ca1	100.38 (7)
O2ⁱⁱ—Ca1—O3	94.44 (6)	Si1^xi—O4—Si1^x	123.61 (18)
O2ⁱⁱⁱ—Ca1—O3	133.98 (8)	Si1^xi—O4—Ca1ⁱ	118.18 (9)
O3^iv—Ca1—O3	93.22 (6)	Si1^x—O4—Ca1ⁱ	118.18 (9)
O3^v—Ca1—O3	123.16 (6)	B1—O5—B1^vi	116.5 (3)
O4^iv—Ca1—O3	151.83 (4)	B1—O5—Ca1^iv	118.29 (17)
O5—Ca1—O3	54.01 (8)	B1^vi—O5—Ca1^iv	118.29 (17)
O1—Ca1—O3	57.88 (6)	B1—O5—Ca1	95.17 (17)
O1^vi—Ca1—O3	114.09 (7)	B1^vi—O5—Ca1	95.17 (17)
O3^vi—Ca1—O3	56.28 (8)	Ca1^iv—O5—Ca1	105.81 (11)
O5ⁱ—Ca1—B1	96.08 (9)	O2^vi—B1—O5	116.1 (3)
O2ⁱⁱ—Ca1—B1	155.01 (8)	O2^vi—B1—O1^xi	101.6 (2)
O2ⁱⁱⁱ—Ca1—B1	111.69 (7)	O5—B1—O1^xi	113.1 (2)
O3^iv—Ca1—B1	95.50 (8)	O2^vi—B1—O3^vi	105.8 (2)
O3^v—Ca1—B1	70.66 (8)	O5—B1—O3^vi	106.7 (2)
O4^iv—Ca1—B1	139.42 (9)	O1^xi—B1—O3^vi	113.4 (2)
O5—Ca1—B1	28.71 (7)	O2^vi—B1—Ca1	97.47 (14)
O1—Ca1—B1	109.40 (7)	O5—B1—Ca1	56.11 (14)
O1^vi—Ca1—B1	60.95 (7)	O1^xi—B1—Ca1	160.88 (18)
O3^vi—Ca1—B1	29.17 (8)	O3^vi—B1—Ca1	61.42 (12)
O3—Ca1—B1	60.91 (7)	O2^vi—B1—Ca1^xii	49.48 (12)
O2—Si1—O4ⁱⁱ	122.89 (12)	O5—B1—Ca1^xii	153.7 (2)
O2—Si1—O3^iv	118.93 (11)	O1^xi—B1—Ca1^xii	59.20 (12)
O4ⁱⁱ—Si1—O3^iv	118.01 (13)	O3^vi—B1—Ca1^xii	98.96 (15)
O2—Si1—O1	84.42 (11)	Ca1—B1—Ca1^xii	137.90 (10)
O4ⁱⁱ—Si1—O1	91.39 (14)

Symmetry codes: (i) x+1/2, −y+1/2, z; (ii) −x+1/2, y−1/2, −z; (iii) −x+1/2, y−1/2, z+1/2; (iv) x−1/2, −y+1/2, z; (v) x−1/2, −y+1/2, −z+1/2; (vi) x, y, −z+1/2; (vii) x, y, −z−1/2; (viii) −x, −y, −z; (ix) −x+1/2, y−1/2, z−1/2; (x) −x+1/2, y+1/2, −z; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x+1/2, y+1/2, −z+1.

danburite (danburite-III_25.4GPa) top

Crystal data top

B₂CaO₈Si₂	Z = 2
M_r = 245.88	F(000) = 244
Triclinic, P1	D_x = 4.193 Mg m⁻³
a = 5.479 (5) Å	Synchrotron radiation, λ = 0.29024 Å
b = 5.532 (2) Å	Cell parameters from 150 reflections
c = 6.681 (6) Å	θ = 2.1–12.8°
α = 91.74 (5)°	µ = 0.22 mm⁻¹
β = 104.57 (8)°	T = 296 K
γ = 95.59 (5)°	Plate crystal, colorless
V = 194.7 (3) Å³	0.02 × 0.02 × 0.01 mm

Data collection top

Esperanto-CrysAlisPro-abstract goniometer imported esperanto images diffractometer	R_int = 0.055
Radiation source: synchrotron, PETRA III, Beamline P02.2	θ_max = 11.2°, θ_min = 1.8°
ω scans	h = −6→6
1042 measured reflections	k = −7→7
591 independent reflections	l = −8→7
397 reflections with I > 2σ(I)

Refinement top

Refinement on F²	68 parameters
Least-squares matrix: full	0 restraints
R[F² > 2σ(F²)] = 0.089	w = 1/[σ²(F_o²) + (0.1393P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.271	(Δ/σ)_max = 0.488
S = 1.12	Δρ_max = 0.76 e Å⁻³
591 reflections	Δρ_min = −0.81 e Å⁻³

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Ca1	0.1796 (4)	0.9158 (4)	0.7626 (4)	0.0238 (9)
Si1	0.3213 (5)	0.3165 (5)	0.5071 (5)	0.0203 (10)
Si2	0.5902 (5)	0.5701 (5)	0.8447 (5)	0.0202 (10)
O1	0.2648 (12)	0.0351 (12)	0.4132 (11)	0.0160 (15)*
O2	0.5163 (13)	0.2513 (12)	0.7444 (12)	0.0166 (16)*
O3	0.0664 (13)	0.3002 (13)	0.5906 (12)	0.0199 (17)*
O4	0.4153 (13)	0.6186 (13)	0.5989 (12)	0.0205 (17)*
O5	0.0598 (12)	0.8038 (12)	0.1066 (12)	0.0176 (16)*
O6	0.3841 (13)	0.1260 (13)	0.0992 (12)	0.0208 (17)*
O7	0.7126 (13)	0.4847 (13)	0.0838 (13)	0.0235 (18)*
O8	−0.1464 (13)	0.5792 (12)	0.7461 (12)	0.0193 (16)*
B1	−0.027 (2)	0.612 (2)	0.223 (2)	0.022 (2)*
B2	0.702 (2)	0.063 (2)	0.777 (2)	0.023 (2)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.0143 (12)	0.0223 (12)	0.0342 (17)	0.0071 (8)	0.0034 (9)	0.0021 (9)
Si1	0.0137 (14)	0.0191 (14)	0.029 (2)	0.0055 (11)	0.0044 (12)	0.0043 (12)
Si2	0.0118 (14)	0.0217 (15)	0.029 (2)	0.0075 (11)	0.0055 (12)	0.0047 (12)

Geometric parameters (Å, º) top

Ca1—O6ⁱ	2.373 (7)	O1—Ca1^vii	2.585 (8)
Ca1—O5ⁱⁱ	2.404 (7)	O2—B2	1.509 (14)
Ca1—O8	2.431 (7)	O2—Ca1^vii	2.514 (7)
Ca1—O6ⁱⁱⁱ	2.446 (8)	O3—B1^iv	1.398 (16)
Ca1—O1^iv	2.468 (7)	O3—Ca1^vii	2.505 (8)
Ca1—O3^v	2.505 (8)	O3—Ca1^iv	2.594 (7)
Ca1—O2^v	2.515 (7)	O4—Si1ⁱ	1.773 (8)
Ca1—O7ⁱ	2.530 (8)	O5—B1	1.446 (16)
Ca1—O4	2.566 (8)	O5—B2ⁱ	1.455 (13)
Ca1—O1^v	2.585 (8)	O5—Ca1ⁱⁱ	2.404 (7)
Ca1—O3^iv	2.594 (7)	O5—Ca1^xi	2.620 (9)
Ca1—O5^vi	2.620 (9)	O6—B2^x	1.468 (17)
Si1—O3	1.625 (9)	O6—Si2ⁱ	1.696 (8)
Si1—O1	1.631 (7)	O6—Ca1ⁱ	2.373 (7)
Si1—O2	1.744 (9)	O6—Ca1^xii	2.446 (8)
Si1—O4	1.745 (7)	O7—B1^viii	1.577 (13)
Si1—O4ⁱ	1.773 (8)	O7—Si2^xi	1.671 (9)
Si1—O8^iv	1.861 (9)	O7—Si2ⁱ	1.839 (9)
Si1—Si2ⁱ	2.599 (6)	O7—Ca1ⁱ	2.530 (8)
Si1—Si2	2.639 (5)	O8—B1^iv	1.477 (13)
Si1—Si1ⁱ	2.702 (6)	O8—Si2^xiii	1.728 (8)
Si1—Ca1^vii	2.998 (5)	O8—Si1^iv	1.861 (9)
Si1—Ca1^iv	3.019 (5)	B1—O3^iv	1.398 (16)
Si2—O7^vi	1.671 (9)	B1—O8^iv	1.477 (13)
Si2—O6ⁱ	1.696 (8)	B1—O7^xiii	1.577 (13)
Si2—O4	1.724 (9)	B1—Ca1ⁱⁱ	2.824 (12)
Si2—O8^viii	1.728 (8)	B1—Ca1^iv	2.970 (12)
Si2—O2	1.837 (7)	B2—O1^x	1.427 (15)
Si2—O7ⁱ	1.839 (9)	B2—O5ⁱ	1.455 (13)
Si2—Si1ⁱ	2.599 (6)	B2—O6^x	1.468 (17)
Si2—Si2^ix	2.620 (7)	B2—Ca1^xiv	2.838 (13)
O1—B2^x	1.427 (15)	B2—Ca1^vii	2.876 (13)
O1—Ca1^iv	2.468 (7)	B2—Ca1^ix	2.977 (14)

O6ⁱ—Ca1—O5ⁱⁱ	125.8 (3)	O6ⁱ—Si2—O8^viii	95.4 (4)
O6ⁱ—Ca1—O8	121.5 (3)	O4—Si2—O8^viii	87.6 (4)
O5ⁱⁱ—Ca1—O8	91.9 (3)	O7^vi—Si2—O2	91.1 (4)
O6ⁱ—Ca1—O6ⁱⁱⁱ	63.3 (3)	O6ⁱ—Si2—O2	169.8 (4)
O5ⁱⁱ—Ca1—O6ⁱⁱⁱ	63.4 (2)	O4—Si2—O2	81.8 (4)
O8—Ca1—O6ⁱⁱⁱ	118.5 (3)	O8^viii—Si2—O2	87.1 (3)
O6ⁱ—Ca1—O1^iv	174.5 (3)	O7^vi—Si2—O7ⁱ	83.5 (4)
O5ⁱⁱ—Ca1—O1^iv	57.5 (2)	O6ⁱ—Si2—O7ⁱ	93.1 (4)
O8—Ca1—O1^iv	60.9 (2)	O4—Si2—O7ⁱ	87.3 (4)
O6ⁱⁱⁱ—Ca1—O1^iv	120.6 (2)	O8^viii—Si2—O7ⁱ	169.9 (3)
O6ⁱ—Ca1—O3^v	117.9 (3)	O2—Si2—O7ⁱ	83.6 (3)
O5ⁱⁱ—Ca1—O3^v	57.6 (3)	O7^vi—Si2—Si1ⁱ	146.1 (3)
O8—Ca1—O3^v	120.2 (2)	O6ⁱ—Si2—Si1ⁱ	86.0 (3)
O6ⁱⁱⁱ—Ca1—O3^v	93.7 (3)	O4—Si2—Si1ⁱ	42.7 (3)
O1^iv—Ca1—O3^v	59.3 (2)	O8^viii—Si2—Si1ⁱ	45.7 (3)
O6ⁱ—Ca1—O2^v	59.0 (3)	O2—Si2—Si1ⁱ	88.7 (3)
O5ⁱⁱ—Ca1—O2^v	91.7 (2)	O7ⁱ—Si2—Si1ⁱ	130.0 (3)
O8—Ca1—O2^v	174.5 (3)	O7^vi—Si2—Si2^ix	44.2 (3)
O6ⁱⁱⁱ—Ca1—O2^v	66.9 (3)	O6ⁱ—Si2—Si2^ix	97.4 (4)
O1^iv—Ca1—O2^v	118.0 (3)	O4—Si2—Si2^ix	126.4 (3)
O3^v—Ca1—O2^v	58.8 (2)	O8^viii—Si2—Si2^ix	143.8 (3)
O6ⁱ—Ca1—O7ⁱ	63.1 (3)	O2—Si2—Si2^ix	86.2 (3)
O5ⁱⁱ—Ca1—O7ⁱ	124.8 (3)	O7ⁱ—Si2—Si2^ix	39.3 (3)
O8—Ca1—O7ⁱ	58.6 (2)	Si1ⁱ—Si2—Si2^ix	168.69 (19)
O6ⁱⁱⁱ—Ca1—O7ⁱ	89.4 (3)	O7^vi—Si2—Si1	131.1 (3)
O1^iv—Ca1—O7ⁱ	119.5 (3)	O6ⁱ—Si2—Si1	128.8 (3)
O3^v—Ca1—O7ⁱ	176.8 (3)	O4—Si2—Si1	40.8 (3)
O2^v—Ca1—O7ⁱ	122.1 (2)	O8^viii—Si2—Si1	89.5 (3)
O6ⁱ—Ca1—O4	57.6 (2)	O2—Si2—Si1	41.2 (3)
O5ⁱⁱ—Ca1—O4	176.2 (3)	O7ⁱ—Si2—Si1	81.0 (3)
O8—Ca1—O4	87.2 (3)	Si1ⁱ—Si2—Si1	62.09 (15)
O6ⁱⁱⁱ—Ca1—O4	120.3 (2)	Si2^ix—Si2—Si1	108.01 (18)
O1^iv—Ca1—O4	119.0 (2)	O7^vi—Si2—Ca1	120.9 (3)
O3^v—Ca1—O4	119.8 (3)	O6ⁱ—Si2—Ca1	50.7 (2)
O2^v—Ca1—O4	88.9 (2)	O4—Si2—Ca1	57.2 (3)
O7ⁱ—Ca1—O4	57.7 (3)	O8^viii—Si2—Ca1	127.1 (3)
O6ⁱ—Ca1—O1^v	90.7 (3)	O2—Si2—Ca1	120.3 (2)
O5ⁱⁱ—Ca1—O1^v	112.7 (3)	O7ⁱ—Si2—Ca1	55.9 (3)
O8—Ca1—O1^v	116.2 (2)	Si1ⁱ—Si2—Ca1	87.62 (14)
O6ⁱⁱⁱ—Ca1—O1^v	125.3 (3)	Si2^ix—Si2—Ca1	86.26 (14)
O1^iv—Ca1—O1^v	83.9 (3)	Si1—Si2—Ca1	86.70 (13)
O3^v—Ca1—O1^v	55.3 (2)	B2^x—O1—Si1	128.7 (8)
O2^v—Ca1—O1^v	58.5 (2)	B2^x—O1—Ca1^iv	89.4 (6)
O7ⁱ—Ca1—O1^v	122.1 (3)	Si1—O1—Ca1^iv	92.6 (3)
O4—Ca1—O1^v	64.5 (3)	B2^x—O1—Ca1^vii	143.0 (7)
O6ⁱ—Ca1—O3^iv	120.5 (3)	Si1—O1—Ca1^vii	87.6 (3)
O5ⁱⁱ—Ca1—O3^iv	113.0 (2)	Ca1^iv—O1—Ca1^vii	96.1 (3)
O8—Ca1—O3^iv	59.5 (3)	B2—O2—Si1	125.8 (7)
O6ⁱⁱⁱ—Ca1—O3^iv	176.1 (2)	B2—O2—Si2	125.2 (6)
O1^iv—Ca1—O3^iv	55.6 (2)	Si1—O2—Si2	94.9 (4)
O3^v—Ca1—O3^iv	85.0 (2)	B2—O2—Ca1^vii	87.5 (6)
O2^v—Ca1—O3^iv	115.2 (3)	Si1—O2—Ca1^vii	87.5 (3)
O7ⁱ—Ca1—O3^iv	91.9 (2)	Si2—O2—Ca1^vii	134.2 (4)
O4—Ca1—O3^iv	63.4 (2)	B1^iv—O3—Si1	130.6 (7)
O1^v—Ca1—O3^iv	56.7 (2)	B1^iv—O3—Ca1^vii	87.9 (6)
O6ⁱ—Ca1—O5^vi	93.4 (3)	Si1—O3—Ca1^vii	90.5 (3)
O5ⁱⁱ—Ca1—O5^vi	64.2 (3)	B1^iv—O3—Ca1^iv	141.1 (7)
O8—Ca1—O5^vi	62.0 (2)	Si1—O3—Ca1^iv	88.3 (3)
O6ⁱⁱⁱ—Ca1—O5^vi	56.5 (2)	Ca1^vii—O3—Ca1^iv	95.0 (2)
O1^iv—Ca1—O5^vi	92.0 (3)	Si2—O4—Si1	99.0 (4)
O3^v—Ca1—O5^vi	121.8 (2)	Si2—O4—Si1ⁱ	96.0 (4)
O2^v—Ca1—O5^vi	123.4 (2)	Si1—O4—Si1ⁱ	100.3 (4)
O7ⁱ—Ca1—O5^vi	60.7 (3)	Si2—O4—Ca1	88.4 (3)
O4—Ca1—O5^vi	118.5 (3)	Si1—O4—Ca1	129.6 (4)
O1^v—Ca1—O5^vi	175.8 (2)	Si1ⁱ—O4—Ca1	128.6 (4)
O3^iv—Ca1—O5^vi	121.3 (3)	B1—O5—B2ⁱ	111.9 (9)
O3—Si1—O1	93.1 (4)	B1—O5—Ca1ⁱⁱ	90.8 (6)
O3—Si1—O2	94.0 (4)	B2ⁱ—O5—Ca1ⁱⁱ	91.3 (6)
O1—Si1—O2	95.1 (4)	B1—O5—Ca1^xi	146.2 (6)
O3—Si1—O4	93.6 (4)	B2ⁱ—O5—Ca1^xi	89.1 (7)
O1—Si1—O4	173.3 (4)	Ca1ⁱⁱ—O5—Ca1^xi	115.8 (3)
O2—Si1—O4	83.9 (4)	B2^x—O6—Si2ⁱ	125.3 (7)
O3—Si1—O4ⁱ	170.5 (4)	B2^x—O6—Ca1ⁱ	94.0 (6)
O1—Si1—O4ⁱ	93.7 (4)	Si2ⁱ—O6—Ca1ⁱ	95.7 (3)
O2—Si1—O4ⁱ	92.0 (4)	B2^x—O6—Ca1^xii	95.8 (6)
O4—Si1—O4ⁱ	79.7 (4)	Si2ⁱ—O6—Ca1^xii	126.1 (4)
O3—Si1—O8^iv	91.2 (4)	Ca1ⁱ—O6—Ca1^xii	116.7 (3)
O1—Si1—O8^iv	90.4 (4)	B1^viii—O7—Si2^xi	121.4 (8)
O2—Si1—O8^iv	172.2 (4)	B1^viii—O7—Si2ⁱ	127.5 (7)
O4—Si1—O8^iv	90.0 (4)	Si2^xi—O7—Si2ⁱ	96.5 (4)
O4ⁱ—Si1—O8^iv	82.2 (4)	B1^viii—O7—Ca1ⁱ	89.5 (6)
O3—Si1—Si2ⁱ	132.7 (3)	Si2^xi—O7—Ca1ⁱ	134.5 (4)
O1—Si1—Si2ⁱ	86.8 (3)	Si2ⁱ—O7—Ca1ⁱ	87.0 (3)
O2—Si1—Si2ⁱ	133.1 (3)	B1^iv—O8—Si2^xiii	123.0 (7)
O4—Si1—Si2ⁱ	89.0 (3)	B1^iv—O8—Si1^iv	126.4 (7)
O4ⁱ—Si1—Si2ⁱ	41.3 (3)	Si2^xiii—O8—Si1^iv	92.7 (4)
O8^iv—Si1—Si2ⁱ	41.6 (3)	B1^iv—O8—Ca1	95.8 (6)
O3—Si1—Si2	92.0 (3)	Si2^xiii—O8—Ca1	128.6 (4)
O1—Si1—Si2	139.0 (3)	Si1^iv—O8—Ca1	88.3 (3)
O2—Si1—Si2	43.9 (2)	O3^iv—B1—O5	112.6 (9)
O4—Si1—Si2	40.2 (3)	O3^iv—B1—O8^iv	112.0 (9)
O4ⁱ—Si1—Si2	87.1 (3)	O5—B1—O8^iv	110.5 (10)
O8^iv—Si1—Si2	130.2 (3)	O3^iv—B1—O7^xiii	108.6 (10)
Si2ⁱ—Si1—Si2	117.91 (15)	O5—B1—O7^xiii	107.4 (9)
O3—Si1—Si1ⁱ	133.5 (3)	O8^iv—B1—O7^xiii	105.3 (7)
O1—Si1—Si1ⁱ	133.2 (4)	O3^iv—B1—Ca1ⁱⁱ	62.4 (5)
O2—Si1—Si1ⁱ	87.4 (3)	O5—B1—Ca1ⁱⁱ	58.4 (5)
O4—Si1—Si1ⁱ	40.2 (3)	O8^iv—B1—Ca1ⁱⁱ	157.8 (7)
O4ⁱ—Si1—Si1ⁱ	39.5 (2)	O7^xiii—B1—Ca1ⁱⁱ	96.7 (6)
O8^iv—Si1—Si1ⁱ	84.9 (3)	O3^iv—B1—Ca1^iv	100.6 (7)
Si2ⁱ—Si1—Si1ⁱ	59.67 (15)	O5—B1—Ca1^iv	146.8 (8)
Si2—Si1—Si1ⁱ	58.24 (14)	O8^iv—B1—Ca1^iv	54.5 (5)
O3—Si1—Ca1^vii	56.7 (3)	O7^xiii—B1—Ca1^iv	58.4 (5)
O1—Si1—Ca1^vii	59.5 (3)	Ca1ⁱⁱ—B1—Ca1^iv	145.4 (5)
O2—Si1—Ca1^vii	56.9 (3)	O1^x—B2—O5ⁱ	108.9 (9)
O4—Si1—Ca1^vii	124.5 (3)	O1^x—B2—O6^x	112.2 (9)
O4ⁱ—Si1—Ca1^vii	132.8 (3)	O5ⁱ—B2—O6^x	110.5 (10)
O8^iv—Si1—Ca1^vii	130.8 (3)	O1^x—B2—O2	112.6 (9)
Si2ⁱ—Si1—Ca1^vii	146.31 (13)	O5ⁱ—B2—O2	104.2 (8)
Si2—Si1—Ca1^vii	90.67 (14)	O6^x—B2—O2	108.2 (9)
Si1ⁱ—Si1—Ca1^vii	144.29 (19)	O1^x—B2—Ca1^xiv	60.4 (5)
O3—Si1—Ca1^iv	59.2 (3)	O5ⁱ—B2—Ca1^xiv	57.9 (5)
O1—Si1—Ca1^iv	54.8 (3)	O6^x—B2—Ca1^xiv	100.7 (7)
O2—Si1—Ca1^iv	134.1 (3)	O2—B2—Ca1^xiv	150.3 (8)
O4—Si1—Ca1^iv	130.0 (3)	O1^x—B2—Ca1^vii	104.6 (6)
O4ⁱ—Si1—Ca1^iv	120.5 (3)	O5ⁱ—B2—Ca1^vii	146.4 (8)
O8^iv—Si1—Ca1^iv	53.6 (2)	O6^x—B2—Ca1^vii	55.4 (5)
Si2ⁱ—Si1—Ca1^iv	83.67 (14)	O2—B2—Ca1^vii	60.9 (5)
Si2—Si1—Ca1^iv	150.93 (15)	Ca1^xiv—B2—Ca1^vii	147.0 (5)
Si1ⁱ—Si1—Ca1^iv	138.4 (2)	O1^x—B2—Ca1^ix	150.6 (8)
Ca1^vii—Si1—Ca1^iv	77.32 (12)	O5ⁱ—B2—Ca1^ix	61.6 (6)
O7^vi—Si2—O6ⁱ	98.1 (4)	O6^x—B2—Ca1^ix	54.9 (5)
O7^vi—Si2—O4	169.0 (4)	O2—B2—Ca1^ix	96.8 (6)
O6ⁱ—Si2—O4	88.4 (4)	Ca1^xiv—B2—Ca1^ix	94.1 (4)
O7^vi—Si2—O8^viii	100.5 (4)	Ca1^vii—B2—Ca1^ix	89.0 (4)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x, −y+2, −z+1; (iii) x, y+1, z+1; (iv) −x, −y+1, −z+1; (v) x, y+1, z; (vi) x, y, z+1; (vii) x, y−1, z; (viii) x+1, y, z; (ix) −x+1, −y+1, −z+2; (x) −x+1, −y, −z+1; (xi) x, y, z−1; (xii) x, y−1, z−1; (xiii) x−1, y, z; (xiv) x+1, y−1, z.

danburite (danburite-IV_32.3GPa) top

Crystal data top

B₂CaO₈Si₂	F(000) = 488
M_r = 491.76	D_x = 4.152 Mg m⁻³
Monoclinic, P2₁/c	Synchrotron radiation, λ = 0.29024 Å
a = 7.9989 (17) Å	Cell parameters from 326 reflections
b = 7.8697 (14) Å	θ = 1.7–10.4°
c = 6.249 (3) Å	µ = 0.22 mm⁻¹
β = 89.75 (3)°	T = 293 K
V = 393.4 (2) Å³	Plate crystal, colorless
Z = 2	0.02 × 0.02 × 0.01 mm

Data collection top

Esperanto-CrysAlisPro-abstract goniometer imported esperanto images diffractometer	R_int = 0.054
Radiation source: synchrotron, PETRA III, Beamline P02.2	θ_max = 10.5°, θ_min = 1.7°
ω scans	h = −9→9
940 measured reflections	k = −9→9
524 independent reflections	l = −6→6
408 reflections with I > 2σ(I)

Refinement top

Refinement on F²	68 parameters
Least-squares matrix: full	0 restraints
R[F² > 2σ(F²)] = 0.098	w = 1/[σ²(F_o²) + (0.1924P)² + 3.6413P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.310	(Δ/σ)_max < 0.001
S = 1.18	Δρ_max = 1.19 e Å⁻³
524 reflections	Δρ_min = −0.98 e Å⁻³

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Ca1	0.7504 (3)	0.1112 (3)	0.7465 (4)	0.0245 (11)
Si1	0.0746 (4)	0.2356 (4)	0.0072 (6)	0.0231 (12)
Si2	0.4261 (4)	0.2350 (4)	0.0072 (6)	0.0232 (12)
O1	0.5962 (11)	0.1336 (10)	0.0670 (16)	0.0249 (19)*
O2	−0.0958 (11)	0.1343 (10)	0.0663 (16)	0.026 (2)*
O3	0.7492 (12)	0.0931 (10)	0.3777 (17)	0.029 (2)*
O4	0.2490 (11)	0.1500 (10)	0.0633 (16)	0.0246 (18)*
O5	0.4245 (11)	0.3625 (11)	0.2490 (17)	0.027 (2)*
O6	0.0746 (11)	0.1369 (11)	−0.2515 (17)	0.0258 (19)*
O7	0.4417 (10)	0.4091 (10)	−0.1258 (16)	0.024 (2)*
O8	0.0587 (10)	0.4087 (10)	−0.1263 (16)	0.025 (2)*
B1	0.5951 (16)	0.0457 (17)	0.269 (2)	0.026 (3)*
B2	−0.0939 (16)	0.0468 (18)	0.272 (3)	0.029 (3)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.0351 (16)	0.0111 (15)	0.027 (2)	−0.0001 (7)	−0.0059 (13)	−0.0009 (7)
Si1	0.036 (2)	0.0104 (18)	0.023 (3)	−0.0023 (10)	−0.0061 (16)	−0.0004 (10)
Si2	0.036 (2)	0.0072 (17)	0.027 (3)	−0.0010 (10)	−0.0065 (16)	−0.0005 (10)

Geometric parameters (Å, º) top

Ca1—O3	2.309 (11)	O1—B1	1.438 (17)
Ca1—O8ⁱ	2.329 (8)	O1—Ca1^xi	2.354 (9)
Ca1—O7ⁱ	2.339 (9)	O1—Ca1^x	2.613 (9)
Ca1—O1ⁱⁱ	2.354 (9)	O2—B2	1.457 (18)
Ca1—O2ⁱⁱⁱ	2.358 (11)	O2—Ca1^vii	2.358 (11)
Ca1—O4^iv	2.375 (9)	O2—Ca1^viii	2.604 (8)
Ca1—O3^v	2.468 (9)	O3—B1	1.459 (18)
Ca1—O6ⁱⁱⁱ	2.601 (9)	O3—B2^xii	1.463 (15)
Ca1—O2^vi	2.604 (8)	O3—Ca1^x	2.468 (9)
Ca1—O1^v	2.613 (9)	O4—Ca1^iv	2.375 (9)
Ca1—O5^v	2.615 (9)	O5—B1^ix	1.454 (15)
Ca1—B2^vi	2.971 (14)	O5—Si2^v	1.787 (11)
Si1—O4	1.590 (10)	O5—Ca1^x	2.615 (9)
Si1—O8	1.603 (9)	O6—B2^xiii	1.459 (16)
Si1—O2	1.621 (8)	O6—Si1^x	1.811 (11)
Si1—O6	1.794 (11)	O6—Ca1^vii	2.601 (9)
Si1—O6^v	1.811 (11)	O7—B1^x	1.435 (14)
Si1—Si2	2.811 (5)	O7—Ca1^ix	2.339 (9)
Si1—Ca1^vii	3.221 (5)	O8—B2^x	1.423 (18)
Si1—Ca1^viii	3.222 (4)	O8—Ca1^ix	2.329 (8)
Si1—Ca1^iv	3.435 (4)	B1—O7^v	1.435 (14)
Si1—Ca1^ix	3.631 (4)	B1—O5ⁱ	1.454 (15)
Si2—O4	1.603 (9)	B1—Ca1^x	2.976 (14)
Si2—O7	1.607 (9)	B1—Ca1^iv	3.029 (13)
Si2—O1	1.623 (9)	B2—O8^v	1.423 (18)
Si2—O5^x	1.787 (11)	B2—O6^xiii	1.459 (16)
Si2—O5	1.813 (11)	B2—O3^xiv	1.463 (15)
Si2—Ca1^xi	3.209 (4)	B2—Ca1^viii	2.971 (14)
Si2—Ca1^x	3.235 (4)	B2—Ca1^iv	3.017 (14)
Si2—Ca1^iv	3.430 (4)	B2—Ca1^xiv	3.253 (15)

O3—Ca1—O8ⁱ	69.0 (3)	O4—Si2—O5^x	90.5 (5)
O3—Ca1—O7ⁱ	68.3 (3)	O7—Si2—O5^x	84.3 (5)
O8ⁱ—Ca1—O7ⁱ	82.0 (3)	O1—Si2—O5^x	90.3 (5)
O3—Ca1—O1ⁱⁱ	148.1 (3)	O4—Si2—O5	92.6 (5)
O8ⁱ—Ca1—O1ⁱⁱ	131.7 (3)	O7—Si2—O5	87.7 (5)
O7ⁱ—Ca1—O1ⁱⁱ	89.0 (3)	O1—Si2—O5	94.8 (5)
O3—Ca1—O2ⁱⁱⁱ	148.8 (3)	O5^x—Si2—O5	171.8 (4)
O8ⁱ—Ca1—O2ⁱⁱⁱ	89.0 (3)	O4—Si2—Si1	28.2 (3)
O7ⁱ—Ca1—O2ⁱⁱⁱ	132.2 (3)	O7—Si2—Si1	94.3 (3)
O1ⁱⁱ—Ca1—O2ⁱⁱⁱ	63.0 (3)	O1—Si2—Si1	147.2 (3)
O3—Ca1—O4^iv	116.5 (3)	O5^x—Si2—Si1	89.4 (3)
O8ⁱ—Ca1—O4^iv	64.3 (3)	O5—Si2—Si1	89.8 (3)
O7ⁱ—Ca1—O4^iv	64.9 (3)	O4—Si2—Ca1^xi	134.0 (3)
O1ⁱⁱ—Ca1—O4^iv	69.0 (3)	O7—Si2—Ca1^xi	86.3 (3)
O2ⁱⁱⁱ—Ca1—O4^iv	68.9 (3)	O1—Si2—Ca1^xi	44.8 (3)
O3—Ca1—O3^v	112.9 (3)	O5^x—Si2—Ca1^xi	54.5 (3)
O8ⁱ—Ca1—O3^v	138.9 (3)	O5—Si2—Ca1^xi	126.4 (3)
O7ⁱ—Ca1—O3^v	138.3 (3)	Si1—Si2—Ca1^xi	143.77 (16)
O1ⁱⁱ—Ca1—O3^v	69.2 (3)	O4—Si2—Ca1^x	139.9 (4)
O2ⁱⁱⁱ—Ca1—O3^v	69.3 (3)	O7—Si2—Ca1^x	81.9 (4)
O4^iv—Ca1—O3^v	130.5 (4)	O1—Si2—Ca1^x	53.3 (3)
O3—Ca1—O6ⁱⁱⁱ	91.0 (3)	O5^x—Si2—Ca1^x	125.9 (3)
O8ⁱ—Ca1—O6ⁱⁱⁱ	53.3 (3)	O5—Si2—Ca1^x	53.9 (3)
O7ⁱ—Ca1—O6ⁱⁱⁱ	135.3 (3)	Si1—Si2—Ca1^x	143.46 (15)
O1ⁱⁱ—Ca1—O6ⁱⁱⁱ	120.6 (3)	Ca1^xi—Si2—Ca1^x	72.55 (10)
O2ⁱⁱⁱ—Ca1—O6ⁱⁱⁱ	57.7 (3)	O4—Si2—Ca1^iv	37.7 (3)
O4^iv—Ca1—O6ⁱⁱⁱ	93.5 (3)	O7—Si2—Ca1^iv	160.1 (3)
O3^v—Ca1—O6ⁱⁱⁱ	85.8 (3)	O1—Si2—Ca1^iv	81.4 (3)
O3—Ca1—O2^vi	67.7 (3)	O5^x—Si2—Ca1^iv	93.4 (3)
O8ⁱ—Ca1—O2^vi	94.5 (3)	O5—Si2—Ca1^iv	93.7 (3)
O7ⁱ—Ca1—O2^vi	133.9 (4)	Si1—Si2—Ca1^iv	65.91 (10)
O1ⁱⁱ—Ca1—O2^vi	123.7 (3)	Ca1^xi—Si2—Ca1^iv	108.47 (10)
O2ⁱⁱⁱ—Ca1—O2^vi	93.4 (3)	Ca1^x—Si2—Ca1^iv	114.78 (12)
O4^iv—Ca1—O2^vi	151.7 (3)	B1—O1—Si2	115.9 (7)
O3^v—Ca1—O2^vi	54.5 (3)	B1—O1—Ca1^xi	135.3 (7)
O6ⁱⁱⁱ—Ca1—O2^vi	58.2 (3)	Si2—O1—Ca1^xi	106.1 (5)
O3—Ca1—O1^v	67.4 (3)	B1—O1—Ca1^x	89.7 (7)
O8ⁱ—Ca1—O1^v	134.2 (3)	Si2—O1—Ca1^x	96.8 (4)
O7ⁱ—Ca1—O1^v	94.2 (3)	Ca1^xi—O1—Ca1^x	100.2 (3)
O1ⁱⁱ—Ca1—O1^v	93.5 (2)	B2—O2—Si1	114.9 (9)
O2ⁱⁱⁱ—Ca1—O1^v	123.6 (3)	B2—O2—Ca1^vii	135.9 (8)
O4^iv—Ca1—O1^v	151.9 (3)	Si1—O2—Ca1^vii	106.6 (5)
O3^v—Ca1—O1^v	54.3 (3)	B2—O2—Ca1^viii	89.4 (7)
O6ⁱⁱⁱ—Ca1—O1^v	114.5 (3)	Si1—O2—Ca1^viii	96.6 (4)
O2^vi—Ca1—O1^v	56.4 (2)	Ca1^vii—O2—Ca1^viii	100.3 (3)
O3—Ca1—O5^v	90.1 (3)	B1—O3—B2^xii	116.8 (11)
O8ⁱ—Ca1—O5^v	135.1 (3)	B1—O3—Ca1	119.2 (8)
O7ⁱ—Ca1—O5^v	53.1 (3)	B2^xii—O3—Ca1	117.4 (9)
O1ⁱⁱ—Ca1—O5^v	58.0 (3)	B1—O3—Ca1^x	95.1 (7)
O2ⁱⁱⁱ—Ca1—O5^v	120.8 (3)	B2^xii—O3—Ca1^x	94.7 (7)
O4^iv—Ca1—O5^v	94.0 (3)	Ca1—O3—Ca1^x	105.9 (3)
O3^v—Ca1—O5^v	85.4 (3)	Si1—O4—Si2	123.4 (6)
O6ⁱⁱⁱ—Ca1—O5^v	171.0 (3)	Si1—O4—Ca1^iv	118.7 (4)
O2^vi—Ca1—O5^v	114.2 (3)	Si2—O4—Ca1^iv	117.8 (5)
O1^v—Ca1—O5^v	57.9 (3)	B1^ix—O5—Si2^v	119.7 (9)
O3—Ca1—B2^vi	96.5 (4)	B1^ix—O5—Si2	119.0 (9)
O8ⁱ—Ca1—B2^vi	111.3 (3)	Si2^v—O5—Si2	121.0 (5)
O7ⁱ—Ca1—B2^vi	155.4 (4)	B1^ix—O5—Ca1^x	91.7 (6)
O1ⁱⁱ—Ca1—B2^vi	96.0 (3)	Si2^v—O5—Ca1^x	91.6 (4)
O2ⁱⁱⁱ—Ca1—B2^vi	70.4 (4)	Si2—O5—Ca1^x	92.0 (3)
O4^iv—Ca1—B2^vi	139.1 (4)	B2^xiii—O6—Si1	120.4 (9)
O3^v—Ca1—B2^vi	29.4 (3)	B2^xiii—O6—Si1^x	118.5 (9)
O6ⁱⁱⁱ—Ca1—B2^vi	60.7 (3)	Si1—O6—Si1^x	120.7 (5)
O2^vi—Ca1—B2^vi	29.4 (4)	B2^xiii—O6—Ca1^vii	91.6 (7)
O1^v—Ca1—B2^vi	61.6 (4)	Si1—O6—Ca1^vii	92.4 (3)
O5^v—Ca1—B2^vi	110.3 (3)	Si1^x—O6—Ca1^vii	92.0 (4)
O4—Si1—O8	123.1 (5)	B1^x—O7—Si2	120.9 (8)
O4—Si1—O2	118.6 (5)	B1^x—O7—Ca1^ix	104.2 (7)
O8—Si1—O2	117.9 (5)	Si2—O7—Ca1^ix	134.2 (4)
O4—Si1—O6	91.1 (5)	B2^x—O8—Si1	120.8 (8)
O8—Si1—O6	84.2 (5)	B2^x—O8—Ca1^ix	104.4 (7)
O2—Si1—O6	89.4 (5)	Si1—O8—Ca1^ix	134.1 (5)
O4—Si1—O6^v	92.8 (5)	O7^v—B1—O1	106.6 (11)
O8—Si1—O6^v	87.9 (5)	O7^v—B1—O5ⁱ	100.9 (9)
O2—Si1—O6^v	94.9 (5)	O1—B1—O5ⁱ	114.2 (12)
O6—Si1—O6^v	172.0 (4)	O7^v—B1—O3	116.4 (12)
O4—Si1—Si2	28.4 (3)	O1—B1—O3	106.5 (10)
O8—Si1—Si2	94.8 (3)	O5ⁱ—B1—O3	112.4 (11)
O2—Si1—Si2	147.1 (4)	O7^v—B1—Ca1^x	98.8 (7)
O6—Si1—Si2	90.2 (3)	O1—B1—Ca1^x	61.4 (6)
O6^v—Si1—Si2	89.8 (3)	O5ⁱ—B1—Ca1^x	160.2 (8)
O4—Si1—Ca1^vii	133.8 (4)	O3—B1—Ca1^x	55.7 (6)
O8—Si1—Ca1^vii	85.9 (4)	O7^v—B1—Ca1^iv	48.5 (6)
O2—Si1—Ca1^vii	44.5 (4)	O1—B1—Ca1^iv	99.8 (8)
O6—Si1—Ca1^vii	53.8 (3)	O5ⁱ—B1—Ca1^iv	59.7 (6)
O6^v—Si1—Ca1^vii	126.3 (3)	O3—B1—Ca1^iv	153.1 (9)
Si2—Si1—Ca1^vii	143.79 (15)	Ca1^x—B1—Ca1^iv	138.5 (5)
O4—Si1—Ca1^viii	139.6 (4)	O7^v—B1—Ca1	82.5 (8)
O8—Si1—Ca1^viii	81.8 (3)	O1—B1—Ca1	136.2 (8)
O2—Si1—Ca1^viii	53.4 (3)	O5ⁱ—B1—Ca1	105.5 (9)
O6—Si1—Ca1^viii	125.3 (3)	O3—B1—Ca1	37.9 (6)
O6^v—Si1—Ca1^viii	53.8 (3)	Ca1^x—B1—Ca1	75.0 (3)
Si2—Si1—Ca1^viii	143.46 (15)	Ca1^iv—B1—Ca1	116.2 (4)
Ca1^vii—Si1—Ca1^viii	72.56 (10)	O8^v—B2—O2	106.9 (10)
O4—Si1—Ca1^iv	37.3 (3)	O8^v—B2—O6^xiii	101.1 (11)
O8—Si1—Ca1^iv	160.4 (3)	O2—B2—O6^xiii	113.2 (12)
O2—Si1—Ca1^iv	81.4 (3)	O8^v—B2—O3^xiv	118.1 (12)
O6—Si1—Ca1^iv	93.5 (3)	O2—B2—O3^xiv	105.6 (11)
O6^v—Si1—Ca1^iv	93.8 (3)	O6^xiii—B2—O3^xiv	112.1 (10)
Si2—Si1—Ca1^iv	65.74 (11)	O8^v—B2—Ca1^viii	99.3 (8)
Ca1^vii—Si1—Ca1^iv	108.40 (10)	O2—B2—Ca1^viii	61.2 (6)
Ca1^viii—Si1—Ca1^iv	114.71 (11)	O6^xiii—B2—Ca1^viii	159.6 (10)
O4—Si1—Ca1^ix	96.0 (3)	O3^xiv—B2—Ca1^viii	55.9 (6)
O8—Si1—Ca1^ix	27.4 (3)	O8^v—B2—Ca1^iv	48.4 (5)
O2—Si1—Ca1^ix	145.3 (4)	O2—B2—Ca1^iv	100.1 (7)
O6—Si1—Ca1^ix	87.7 (3)	O6^xiii—B2—Ca1^iv	59.5 (6)
O6^v—Si1—Ca1^ix	84.9 (3)	O3^xiv—B2—Ca1^iv	154.0 (10)
Si2—Si1—Ca1^ix	67.53 (10)	Ca1^viii—B2—Ca1^iv	138.9 (5)
Ca1^vii—Si1—Ca1^ix	109.65 (12)	O8^v—B2—Ca1^xiv	83.0 (8)
Ca1^viii—Si1—Ca1^ix	101.89 (9)	O2—B2—Ca1^xiv	136.4 (8)
Ca1^iv—Si1—Ca1^ix	133.26 (14)	O6^xiii—B2—Ca1^xiv	105.8 (8)
O4—Si2—O7	122.4 (5)	O3^xiv—B2—Ca1^xiv	39.1 (6)
O4—Si2—O1	119.0 (5)	Ca1^viii—B2—Ca1^xiv	75.4 (3)
O7—Si2—O1	118.4 (4)	Ca1^iv—B2—Ca1^xiv	116.3 (5)

Symmetry codes: (i) −x+1, y−1/2, −z+1/2; (ii) x, y, z+1; (iii) x+1, y, z+1; (iv) −x+1, −y, −z+1; (v) x, −y+1/2, z+1/2; (vi) x+1, −y+1/2, z+1/2; (vii) x−1, y, z−1; (viii) x−1, −y+1/2, z−1/2; (ix) −x+1, y+1/2, −z+1/2; (x) x, −y+1/2, z−1/2; (xi) x, y, z−1; (xii) x+1, y, z; (xiii) −x, −y, −z; (xiv) x−1, y, z.

Acknowledgements

The authors are grateful to A. Kurnosov for help with loading of the diamond anvil cells. Parts of this research were carried out at the PETRA III light source at DESY, a member of the Helmholtz Association (HGF), and at the European Synchrotron Radiation Facility.

Funding information

The following funding is acknowledged: Deutsche Forschungsgemeinschaft (project No. BO 2550/8-1 to Anna Pakhomova).

References

Agilent (2012). CrysAlisPRO. Agilent Technologies, Yarnton, Oxfordshire, England. Google Scholar
Albanese, G., Deriu, A., Lucchini, E. & Slokar, G. (1981). Appl. Phys. A, 26, 45–50. CrossRef Web of Science Google Scholar
Alberti, A., Sacerdoti, M., Quartieri, S. & Vezzalini, G. (1999). Phys. Chem. Miner. 26, 181–186. Web of Science CrossRef ICSD CAS Google Scholar
Angel, R. J. (1997). Am. Miner. 82, 836–839. CAS Google Scholar
Angel, R. J. (2000). Rev. Mineral. Geochem. 41, 35–59. Web of Science CrossRef Google Scholar
Angel, R. J., Gonzalez-Platas, J. & Alvaro, M. (2014). Z. Kristallogr. 229, 405–419. CAS Google Scholar
Angel, R. J., Ross, N. L., Seifert, F. & Fliervoet, T. F. (1996). Nature, 384, 441–444. CrossRef CAS Web of Science Google Scholar
Baus, J. A., Burschka, C., Bertermann, R., Guerra, C. F., Bickelhaupt, F. M. & Tacke, R. (2013). Inorg. Chem. 52, 10664–10676. Web of Science CSD CrossRef CAS PubMed Google Scholar
Belov, N. B. (1941). Structure of Ionic Crystals and Metallic Phases, p. 144. Moscow: Academy of Sciences USSR Publishers. Google Scholar
Best, S. P., Clark, R. J. H., Hayward, C. L. & Withnall, R. (1994). J. Raman Spectrosc. 25, 557–563. CrossRef CAS Web of Science Google Scholar
Finger, R. M. & Hazen, L. W. (2000). Rev. Mineral. Geochem. 41, 123–155. Web of Science CrossRef Google Scholar
Finkelstein, G. J., Dera, P. K. & Duffy, T. S. (2015). Phys. Earth Planet. Inter. 244, 78–86. Web of Science CrossRef ICSD CAS Google Scholar
Garbev, K., Stemmermann, P., Black, L., Breen, C., Yarwood, C. & Gasharova, B. (2007). J. Am. Ceram. Soc. 90, 900–907. Web of Science CrossRef CAS Google Scholar
Gautron, L., Angel, R. J. & Miletich, R. (1999). Phys. Chem. Miner. 27, 47–51. Web of Science CrossRef ICSD CAS Google Scholar
Giacovazzo, C., Monaco, H. L., Viterbo, D., Scordari, F., Gilli, G., Zanotti, G. & Catti, M. (2002). Fundamentals of Crystallography, pp. 825. New York: Oxford University Press. Google Scholar
Goryainov, S. V. (2016). J. Raman Spectrosc. 47, 984–992. Web of Science CrossRef CAS Google Scholar
Hackwell, T. H. & Angel, R. J. (1992). Eur. J. Mineral. 4, 1221–1228. CrossRef CAS Google Scholar
Isaeva, A. A., Makarevich, O. N., Kuznetsov, A. N., Doert, T., Abakumov, A. M. & Van Tendeloo, G. (2010). Eur. J. Inorg. Chem. 2010, 1395–1404. Web of Science CrossRef ICSD Google Scholar
Kantor, I., Prakapenka, V., Kantor, A., Dera, P., Kurnosov, A., Sinogeikin, S., Dubrovinskaia, N. & Dubrovinsky, L. (2012). Rev. Sci. Instrum. 83, 125102. Web of Science CrossRef PubMed Google Scholar
Karas, O. A., Pakhomova, V. A., Karabtsov, A. A. & Kononov, V. V. (2008). Proceedings of XIII International Conference on Thermobarogeochemistry and IV APIFIS Symposium, 22–25 September 2008, Moscow, Russia, Vol. 2, pp. 45–48. Moscow: INEG RAN. Google Scholar
Kreber, U. & Gonser, E. (1976). Appl. Phys. 10, 175–180. CrossRef CAS Web of Science Google Scholar
Krivovichev, S. V. (2009). Structural Crystallography of Inorganic Oxysalts, Vol. 22, p. 328. Oxford University Press. Google Scholar
Kurnosov, A., Kantor, I., Boffa-Ballaran, T., Lindhardt, S., Dubrovinsky, L., Kuznetsov, A. & Zehnder, B. H. (2008). Rev. Sci. Instrum. 79, 045110. Web of Science CrossRef PubMed Google Scholar
Larsson, A.-K., Stenberg, L., Lidin, S. & Hase, T. (1995). Acta Chem. Scand. 49, 800–802. CrossRef CAS Web of Science Google Scholar
Liebau, F. (1984). Inorg. Chim. Acta, 89, 1–7. CrossRef CAS Web of Science Google Scholar
Liebau, F. (1985). Structural Chemistry of Silicates, p. 354. Heidelberg: Springer-Verlag. Google Scholar
Lima-de-Faria, J. (1994). Structural Mineralogy: An Introduction, p. 346. Dordrecht: Springer Netherlands. Google Scholar
Mao, H. K., Xu, J. & Bell, P. M. (1986). J. Geophys. Res. 91, 4673–4676. CrossRef CAS Web of Science Google Scholar
Mück, F. M., Förster, B., Baus, J. A., Nutz, M., Burschka, C., Bertermann, R. & Tacke, R. (2016). Eur. J. Inorg. Chem. 3246–3252. Google Scholar
Pauling, L. (1929). J. Am. Chem. Soc. 51, 1010–1026. CrossRef CAS Google Scholar
Phillips, M. W., Gibbs, G. V. & Ribbe, P. H. (1974). Am. Mineral. 59, 79–85. CAS Google Scholar
Prewitt, C. T. & Downs, R. T. (1998). Rev. Mineral. Geochem. 37, 283–317. CAS Google Scholar
Rappoport, Z. & Apeloig, Y. (1998). The Chemistry of Organic Silicon Compounds, Vol. 2, p. 2830. Chichester: John Wiley & Sons Ltd. Google Scholar
Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567–570. CrossRef PubMed CAS Web of Science Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Thompson, R. M. & Downs, R. T. (2003). Am. Mineral. 88, 653–666. Web of Science CrossRef ICSD CAS Google Scholar
Townes, W. D., Fang, J. H. & Perrotta, A. J. (1967). Z. Kristallogr. 125, 437–449. CrossRef ICSD CAS Web of Science Google Scholar
Wagler, J., Böhme, U. & Kroke, E. (2014). Struct. Bond. 155, 29–106. Web of Science CrossRef CAS Google Scholar

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IUCrJ

Volume 4| Part 5| September 2017| Pages 671-677

ISSN: 2052-2525

https://doi.org/10.1107/S2052252517010612

CHEMISTRY | CRYSTENG

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research papers\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

A closer look into close packing: pentacoordinated silicon in a high-pressure polymorph of danburite

1. Introduction

2. Materials and methods

3. Results

3.1. Crystal structure of danburite under ambient conditions

3.2. High-pressure phase transitions of danburite

4. Discussion

Supporting information

Acknowledgements

Funding information

References

research papers