Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270107041169/fa3103sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270107041169/fa3103Isup2.hkl |
The isokite specimen used in this study is from Kjorrestad, near Bamle, Norway, and is in the collection of the RRUFF project (deposition No. R070526; https://rruff.info), donated by the University of Arizona Mineral Museum (No. 4797). It formed a rim on a large sample of wagnerite, Mg2(PO4)F (RRUFF deposition No. R050519). The average chemical composition of the sample studied, Ca1.00Mg1.00(P1.00O4)[F0.80(OH)0.20]Σ=1, was determined with a CAMECA SX50 electron microprobe.
The atomic occupancy of the octahedral chain bridging site was constrained to that determined by microprobe analysis (0.8 F + 0.2 OH) throughout the structure refinements.
Data collection: APEX2 (Bruker, 2003); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: SHELXTL (Bruker, 1997).
Fig. 1. The crystal structure of isokite, CaMg(PO4)F. The octahedra and tetrahedra represent the MgO4F2 and PO4 groups. |
CaMg(PO4)O0.20F0.80 | F(000) = 352 |
Mr = 177.76 | Dx = 3.250 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 868 reflections |
a = 6.5109 (3) Å | θ = 9.3–69.6° |
b = 8.7301 (5) Å | µ = 2.25 mm−1 |
c = 6.9046 (5) Å | T = 293 K |
β = 112.246 (2)° | Block, colourless |
V = 363.25 (4) Å3 | 0.06 × 0.05 × 0.05 mm |
Z = 4 |
Bruker APEXII ? CCD area-detector diffractometer | 801 independent reflections |
Radiation source: fine-focus sealed tube | 660 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.031 |
ϕ and ω scans | θmax = 35.1°, θmin = 4.1° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2005) | h = −10→9 |
Tmin = 0.877, Tmax = 0.896 | k = −14→11 |
3037 measured reflections | l = −11→10 |
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F2 > 2σ(F2)] = 0.030 | Secondary atom site location: difference Fourier map |
wR(F2) = 0.081 | w = 1/[σ2(Fo2) + (0.0379P)2 + 0.3685P] where P = (Fo2 + 2Fc2)/3 |
S = 1.05 | (Δ/σ)max < 0.001 |
801 reflections | Δρmax = 0.62 e Å−3 |
40 parameters | Δρmin = −0.66 e Å−3 |
CaMg(PO4)O0.20F0.80 | V = 363.25 (4) Å3 |
Mr = 177.76 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 6.5109 (3) Å | µ = 2.25 mm−1 |
b = 8.7301 (5) Å | T = 293 K |
c = 6.9046 (5) Å | 0.06 × 0.05 × 0.05 mm |
β = 112.246 (2)° |
Bruker APEXII ? CCD area-detector diffractometer | 801 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2005) | 660 reflections with I > 2σ(I) |
Tmin = 0.877, Tmax = 0.896 | Rint = 0.031 |
3037 measured reflections |
R[F2 > 2σ(F2)] = 0.030 | 40 parameters |
wR(F2) = 0.081 | 0 restraints |
S = 1.05 | Δρmax = 0.62 e Å−3 |
801 reflections | Δρmin = −0.66 e Å−3 |
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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Mg1 | 0.5000 | 0.0000 | 0.5000 | 0.00979 (18) | |
Ca1 | 0.5000 | 0.33652 (6) | 0.2500 | 0.01762 (15) | |
P1 | 0.0000 | 0.17921 (7) | 0.2500 | 0.00759 (14) | |
O1 | 0.1091 (2) | 0.28009 (14) | 0.1325 (2) | 0.0107 (2) | |
O2 | 0.1690 (2) | 0.07291 (14) | 0.4092 (2) | 0.0116 (2) | |
F1 | 0.5000 | 0.08302 (17) | 0.2500 | 0.0110 (3) | 0.80 |
OH1 | 0.5000 | 0.08302 (17) | 0.2500 | 0.0110 (3) | 0.20 |
U11 | U22 | U33 | U12 | U13 | U23 | |
Mg1 | 0.0105 (4) | 0.0103 (4) | 0.0089 (4) | −0.0001 (3) | 0.0041 (3) | 0.0013 (3) |
Ca1 | 0.0107 (2) | 0.0083 (2) | 0.0276 (3) | 0.000 | 0.0003 (2) | 0.000 |
P1 | 0.0066 (3) | 0.0073 (3) | 0.0088 (3) | 0.000 | 0.0029 (2) | 0.000 |
O1 | 0.0105 (5) | 0.0102 (5) | 0.0128 (6) | 0.0004 (4) | 0.0058 (5) | 0.0023 (4) |
O2 | 0.0095 (5) | 0.0106 (6) | 0.0128 (6) | −0.0001 (4) | 0.0023 (4) | 0.0022 (4) |
F1 | 0.0147 (7) | 0.0098 (7) | 0.0094 (6) | 0.000 | 0.0053 (6) | 0.000 |
OH1 | 0.0147 (7) | 0.0098 (7) | 0.0094 (6) | 0.000 | 0.0053 (6) | 0.000 |
Mg1—F1 | 1.8721 (6) | Ca1—O1 | 2.4113 (12) |
Mg1—O2 | 2.1021 (12) | Ca1—O1iii | 2.6635 (13) |
Mg1—O1i | 2.1295 (12) | P1—O2 | 1.5378 (13) |
Ca1—F1 | 2.2131 (16) | P1—O1 | 1.5413 (12) |
Ca1—O2ii | 2.4005 (13) | ||
F1—Mg1—O2 | 88.03 (4) | Mg1vii—O1—Ca1 | 95.64 (5) |
F1—Mg1—O2iv | 91.97 (4) | P1—O1—Ca1iii | 107.49 (6) |
F1—Mg1—O1v | 86.02 (5) | Mg1vii—O1—Ca1iii | 89.91 (4) |
F1—Mg1—O1i | 93.98 (5) | Ca1—O1—Ca1iii | 105.26 (4) |
O2—Mg1—O1v | 89.58 (5) | P1—O2—Mg1 | 140.48 (8) |
O2—Mg1—O1i | 90.42 (5) | P1—O2—Ca1viii | 96.40 (6) |
O2vi—P1—O2 | 105.76 (10) | Mg1—O2—Ca1viii | 96.70 (5) |
O2—P1—O1vi | 108.44 (7) | P1—O2—Ca1ix | 91.45 (5) |
O2—P1—O1 | 111.93 (7) | Mg1—O2—Ca1ix | 118.44 (5) |
O1vi—P1—O1 | 110.30 (10) | Ca1viii—O2—Ca1ix | 109.80 (4) |
P1—O1—Mg1vii | 126.58 (7) | Mg1x—F1—Mg1 | 134.45 (8) |
P1—O1—Ca1 | 125.29 (7) | Mg1—F1—Ca1 | 112.78 (4) |
Symmetry codes: (i) −x+1/2, y−1/2, −z+1/2; (ii) x+1/2, y+1/2, z; (iii) −x+1/2, −y+1/2, −z; (iv) −x+1, −y, −z+1; (v) x+1/2, −y+1/2, z+1/2; (vi) −x, y, −z+1/2; (vii) −x+1/2, y+1/2, −z+1/2; (viii) x−1/2, y−1/2, z; (ix) −x+1/2, −y+1/2, −z+1; (x) −x+1, y, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | CaMg(PO4)O0.20F0.80 |
Mr | 177.76 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 293 |
a, b, c (Å) | 6.5109 (3), 8.7301 (5), 6.9046 (5) |
β (°) | 112.246 (2) |
V (Å3) | 363.25 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 2.25 |
Crystal size (mm) | 0.06 × 0.05 × 0.05 |
Data collection | |
Diffractometer | Bruker APEXII ? CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 2005) |
Tmin, Tmax | 0.877, 0.896 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3037, 801, 660 |
Rint | 0.031 |
(sin θ/λ)max (Å−1) | 0.809 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.030, 0.081, 1.05 |
No. of reflections | 801 |
No. of parameters | 40 |
Δρmax, Δρmin (e Å−3) | 0.62, −0.66 |
Computer programs: APEX2 (Bruker, 2003), SAINT (Bruker, 2005), SAINT, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), XtalDraw (Downs & Hall-Wallace, 2003), SHELXTL (Bruker, 1997).
The C2/c titanite structure-type is very flexible and capable of accommodating a wide range of chemical components (Hawthorne, 1990; Sebastian et al., 2002). Minerals belonging to this group include more than a dozen silicates, arsenates, phosphates and sulfates (Groat et al., 1990). A list of synthetic analogues of titanite was given by Sebastian et al. (2002). Isokite is a fluor-bearing calcium–magnesium phosphate mineral and was first described by Deans & McConnell (1955) with the ideal chemical formula CaMg(PO4)F. Recently, Hochleitner & Fehr (2005) presented a summary on the paragenesis, chemistry and physical properties of isokite on the basis of a new occurrence at Senhora de Assunção, Portugal, and experimental data. Although all previous studies (e.g. Deans & McConnell, 1955; Isaacs & Peacor, 1981; Strunz & Nickel, 2001; Hochleitner & Fehr, 2005) noted the similarities between isokite and minerals of the titanite group in terms of unit-cell parameters and crystal chemistry, the structure of isokite remained undetermined. This study presents the first structure refinement of isokite based on single-crystal X-ray diffraction data.
Isokite is homologous with minerals of the C2/c titanite group (e.g. Hawthorne et al., 1991; Oberti et al., 1991; Troitzsch et al., 1999) and topologically very similar to the minerals of the C1 amblygonite (LiAlPO4F)–montebrasite (LiAlPO4OH) group (Groat et al., 1990). Its structure, in which Mg1, Ca1, P1 and F1 (= 0.8 F + 0.2 OH) are located at special positions with site symmetries 2, 1, 1 and 1, respectively, is characterized by kinked chains of corner-sharing MgO4F2 octahedra (parallel to c) that are cross-linked by isolated PO4 tetrahedra, forming a three-dimensional polyhedral network. The Ca1 cations occupy the interstitial sites coordinated by six O atoms and one F anion (Fig. 1). Compared with the structure of tilasite (CaMgAsO4F) (Bermanec, 1994), a member of the C2/c titanite mineral group and the As analogue of isokite, both Mg—F and Ca—F bond distances in isokite, which are 1.872 (1) and 2.213 (1) Å, respectively, are noticeably shorter than the corresponding ones [1.910 (1) and 2.246 (5) Å, respectively] in tilasite. The calculation of bond-valence sums using the parameters given by Brese & O'Keeffe (1991) yields a value of 1.28 valence units (v.u.) for the bridging F− anion in the octahedral chain in isokite, indicating that F− is more over-bonded than in tilasite, which has a bond-valence sum of 1.16 v.u. In addition, the isokite structure appears to provide a better bonding environment for Ca2+, as indicated by its bond-valence sum of 1.90 v.u., compared with that in tilasite (1.78 v.u.).
The substitution of OH for F in minerals of the C2/c titanite group has been a matter of discussion (e.g. Cooper & Hawthorne, 1995; Troitzsch et al., 1999). Both tilasite CaMg(AsO4)F (Bermanec, 1994) and synthetic CaAl(SiO4)F (Troitzsch et al., 1999) have monoclinic C2/c symmetry, but their OH analogues, adelite CaMg(AsO4)OH (Effenberger et al., 2002) and vuagnatite CaAl(SiO4)OH (McNear et al., 1976), respectively, are orthorhombic with space group P212121. Interestingly, Isaacs & Peacor (1981) reported a new mineral, panasqueiraite, with stoichiometry CaMg(PO4)(OH0.7F0.3) and unit-cell parameters a = 6.535 (3), b = 8.753 (4) and c = 6.919 (4) Å, and β = 112.33 (4)°, suggesting that panasqueiraite and isokite are isomorphous. Apparently, further research is needed to clarify whether a complete solid solution exists between the two phosphate end-members of CaMg(PO4)F and CaMg(PO4)OH, and if not, to what extent OH can substitute for F without modifying the C2/c titanite-type structure.