Isokite, CaMg(PO₄)F_0.8(OH)_0.2, isomorphous with titanite

This study presents the first structural report of natural isokite (calcium magnesium phosphate fluoride), with the formula CaMg(PO₄)F_0.8(OH)_0.2 (i.e. some substitution of OH for F), based on single-crystal X-ray diffraction data. Isokite belongs to the C2/c titanite mineral group, in which Mg is on an inversion centre and the Ca, P and F/OH atoms are on twofold axes. The structure is composed of kinked chains of corner-sharing MgO₄F₂ octa­hedra that are crosslinked by isolated PO₄ tetra­hedra, forming a three-dimensional polyhedral network. The Ca²⁺ cations occupy the inter­stitial sites, coordinated by six O atoms and one F anion.

S1. Comment

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)(OH_0.7F_0.3) 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.

S2. Experimental

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, Ca_1.00Mg_1.00(P_1.00O₄)[F_0.80(OH)_0.20]_Σ=1, was determined with a CAMECA SX50 electron microprobe.

S3. Refinement

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.

Computing details

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).

Figures

Fig. 1. The crystal structure of isokite, CaMg(PO4)F. The octahedra and tetrahedra represent the MgO4F2 and PO4 groups.

calcium magnesium phosphate fluoride

Crystal data

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) Å^3	0.06 × 0.05 × 0.05 mm
Z = 4

Data collection

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

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.030	Secondary atom site location: difference Fourier map
wR(F^2) = 0.081	w = 1/[σ^2(Fo^2) + (0.0379P)^2 + 0.3685P] where P = (Fo^2 + 2Fc^2)/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

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 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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

Geometric parameters (Å, º)

Mg1—F1	1.8721 (6)	Ca1—O1	2.4113 (12)
Mg1—O2	2.1021 (12)	Ca1—O1^iii	2.6635 (13)
Mg1—O1^i	2.1295 (12)	P1—O2	1.5378 (13)
Ca1—F1	2.2131 (16)	P1—O1	1.5413 (12)
Ca1—O2^ii	2.4005 (13)
F1—Mg1—O2	88.03 (4)	Mg1^vii—O1—Ca1	95.64 (5)
F1—Mg1—O2^iv	91.97 (4)	P1—O1—Ca1^iii	107.49 (6)
F1—Mg1—O1^v	86.02 (5)	Mg1^vii—O1—Ca1^iii	89.91 (4)
F1—Mg1—O1^i	93.98 (5)	Ca1—O1—Ca1^iii	105.26 (4)
O2—Mg1—O1^v	89.58 (5)	P1—O2—Mg1	140.48 (8)
O2—Mg1—O1^i	90.42 (5)	P1—O2—Ca1^viii	96.40 (6)
O2^vi—P1—O2	105.76 (10)	Mg1—O2—Ca1^viii	96.70 (5)
O2—P1—O1^vi	108.44 (7)	P1—O2—Ca1^ix	91.45 (5)
O2—P1—O1	111.93 (7)	Mg1—O2—Ca1^ix	118.44 (5)
O1^vi—P1—O1	110.30 (10)	Ca1^viii—O2—Ca1^ix	109.80 (4)
P1—O1—Mg1^vii	126.58 (7)	Mg1^x—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.