Redetermination of junitoite, CaZn2Si2O7·H2O

The crystal structure of the mineral junitoite, ideally CaZn2Si2O7·H2O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91–95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO4 tetrahedra parallel to the b axis, cross-linked by Si2O7 tetrahedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca2+ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H2O molecule (site symmetry ..2) in a distorted octahedral coordination environment. However, some bond lengths, especially for the SiO4 tetrahedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water molecule and a framework O atom.

The crystal structure of the mineral junitoite, ideally CaZn 2 Si 2 O 7 ÁH 2 O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91-95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO 4 tetrahedra parallel to the b axis, cross-linked by Si 2 O 7 tetrahedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca 2+ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H 2 O molecule (site symmetry ..2) in a distorted octahedral coordination environment. However, some bond lengths, especially for the SiO 4 tetrahedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water molecule and a framework O atom.

Crystal data
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Xtal-Draw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010 Williams (1976) with orthorhombic symmetry in space group Bbm2 (non-standard setting of space group No. 40) and unit-cell parameters a = 6.309, b = 12.503, c = 8.549 Å. By adopting the standard unit-cell setting of this space group in Ama2 (a = 12.510, b = 6.318, c = 8.561 Å) for this mineral, Hamilton & Finney (1985) noted that while the Weissenberg photographic data pointed to Ama2, the X-ray diffractometer data were also compatible with the space group Aea2. Although the two space groups yielded similar reliability factors R1 ~ 0.10 with isotropic displacement parameters for all atoms (H atoms were not located), Hamilton & Finney (1985) chose Ama2 for their final structure report because it "produces less distortion of the coordination polyhedra and provides a structure in which the site symmetry of the cations is more similar to other zinc silicates". Their attempts at refinement with anisotropic displacement parameters resulted in non-positive definite displacement parameters for a number of atoms. In our efforts to understand the hydrogen bonding environments in minerals and their relationships to Raman spectra, we concluded that the structural model for junitoite needed improvement. This study reports a structure redetermination of junitoite from the type locality by means of single-crystal X-ray diffraction data, demonstrating that the space group of this mineral is actually Aea2, rather than Ama2.
The crystal structure of junitoite consists of chains of corner-sharing ZnO 4 tetrahedra parallel to the b axis, cross-linked by Si 2 O 7 tetrahedral dimers along a and c to form a three-dimensional framework. The Ca 2+ cations, situated in cavities of the framework, are bonded to five O atoms and one H 2 O molecule in a distorted octahedral [CaO 5 (H 2 O)] coordination environment (Figs. 1, 2). As described below, it may be useful to consider that there is a Ca-H 2 O bonded pair in the cavity. The structure of junitoite in space group Aea2 resembles that in space group Ama2 (Hamilton & Finney, 1985). In fact, as noted by Hamilton & Finney (1985), the structure model in Aea2 can be attained simply by shifting the origin of the structure model in Ama2 from (x, y, z) to (x -1/4, y -1/4, z). Upon this shift, the only major structural change is that the two unique Zn atoms at the 4a sites in the Ama2 structure model are transformed into a single atom at the 8b site in the Aea2 structure model. The numbers and coordination polyhedra of the distinct Ca, Si, and O sites remain unaffected.
The hydrogen bond in junitoite is found between Ow5 and O1, with Ow5 as the donor and O1 as the acceptor. This agrees with the calculated bond-valence sums of 0.42 valence units for Ow5 and 1.77 valence units for O1 by using the parameters given by Brese & O′Keeffe (1991). For numerical details of the hydrogen-bonding geometry, see:  (Lin et al., 1999), high-pressure rare earth (RE) disilicates RE 2 Si 2 O 7 (Fleet & Liu, 2001) or BaKY(Si 2 O 7 ) (Kolitsch et al., 2009). Accordingly, any sizable substitution of smaller Sr 2+ for Ba 2+ would worsen the bonding energetics for this site and thus destabilize the structure. For junitoite, Ca 2+ by itself, which is even smaller than Sr 2+ , is apparently too small to occupy the cavities in the framework. Therefore, the presence of the H 2 O-Ca 2+ bonded pair is essential to stabilize its structure. By the same token, one could argue that the pair (Sr 2+ + H 2 O) together may be too large for the cavities in the structures analogous to those for the BaM 2 Si 2 O 7 materials, since there is no report for any SrM 2+ 2 Si 2 O 7 .H 2 O compound up to date. Based on this reasoning, we postulate that more compounds with composition CaM 2+ 2 Si 2 O 7 .H 2 O may be found in nature or synthesized in laboratories. Furthermore, it would be interesting if the Sr-H 2 O pair might be found in digermanates, where this structural unit is even larger.

Experimental
The junitoite crystal used in this study is from the type locality, the Christmas Mine, Gila County, Arizona and is in the collection of the RRUFF project (http://rruff.info/R120100). Its chemical composition measured by Williams (1976) is Ca 0.98 Zn 1.96 Si 1.84 O 6.6 .1.13H 2 O.

Refinement
The H atom was located near Ow5 from difference Fourier syntheses and its position refined freely with a fixed isotropic displacement (U iso = 0.03). For simplicity, the ideal chemistry, CaZn 2 Si 2 O 7 .H 2 O, was assumed during the final refinement.

Figure 2
Atoms in junitoite with corresponding ellipsoids at the 99.9% probability level. The gray, yellow, purple, red, and green ellipsoids represent Ca, Zn, Si, O, and Ow5, respectively. The small blue spheres represent H atoms. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.52 e Å −3 Δρ min = −0.65 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0120 (4) Absolute structure: Flack (1983), 580 Friedel pairs Flack parameter: 0.023 (12) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.