Calcioferrite with composition (Ca3.94Sr0.06)Mg1.01(Fe2.93Al1.07)(PO4)6(OH)4·12H2O

Calcioferrite, ideally Ca4MgFe3+ 4(PO4)6(OH)4·12H2O (tetracalcium magnesium tetrairon(III) hexakis-phosphate tetrahydroxide dodecahydrate), is a member of the calcioferrite group of hydrated calcium phosphate minerals with the general formula Ca4 AB 4(PO4)6(OH)4·12H2O, where A = Mg, Fe2+, Mn2+ and B = Al, Fe3+. Calcioferrite and the other three known members of the group, montgomeryite (A = Mg, B = Al), kingsmountite (A = Fe2+, B = Al), and zodacite (A = Mn2+, B = Fe3+), usually occur as very small crystals, making their structure refinements by conventional single-crystal X-ray diffraction challenging. This study presents the first structure determination of calcioferrite with composition (Ca3.94Sr0.06)Mg1.01(Fe2.93Al1.07)(PO4)6(OH)4·12H2O based on single-crystal X-ray diffraction data collected from a natural sample from the Moculta quarry in Angaston, Australia. Calcioferrite is isostructural with montgomeryite, the only member of the group with a reported structure. The calcioferrite structure is characterized by (Fe/Al)O6 octahedra (site symmetries 2 and -1) sharing corners (OH) to form chains running parallel to [101]. These chains are linked together by PO4 tetrahedra (site symmetries 2 and 1), forming [(Fe/Al)3(PO4)3(OH)2] layers stacking along [010], which are connected by (Ca/Sr)2+ cations (site symmetry 2) and Mg2+ cations (site symmetry 2; half-occupation). Hydrogen-bonding interactions involving the water molecules (one of which is equally disordered over two positions) and OH function are also present between these layers. The relatively weaker bonds between the layers account for the cleavage of the mineral parallel to (010).


Experimental
Crystal data (Ca 3.94 Table 1 Hydrogen-bond geometry (Å , ). 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 Larsen (1940) reported montgomeryite with an ideal chemical formula Ca 4 Al 5 (PO 4 ) 6 (OH) 5 .11H 2 O without recognizing its relationship to calcioferrite. Palache et al. (1951), on the basis of the chemistry given by Blum (1858), proposed the chemical formula Ca 3 Fe 3 (PO 4 ) 4 (OH) 3 .8H 2 O for calcioferrite. By comparing chemical compositions and Xray powder diffraction profiles between calcioferrite and montgomeryite, Mead & Mrose (1968) suggested that these two minerals are isostructural. Moore & Araki (1974) first solved the structure of montgomeryite in space group C2/c and revised its chemical formula to Ca 4 MgAl 4 (PO 4 ) 6 (OH) 4 .12H 2 O. Nevertheless, Fanfani et al. (1976) observed the presence of some weak reflections that violate the C2/c space group symmetry for montgomeryite, leading them to propose C2 as the actual space group for this mineral. Dunn et al. (1983) studied red montgomeryite from the Tip Top Pegmatite and also concluded that calcioferrite is the Fe 3+ analog of montgomeryite based on the similarity between their X-ray powder diffraction patterns. Consequently, they modified the ideal chemical formula of calcioferrite to its present form, Ca 4 MgFe 3+ 4 (PO 4 ) 6 (OH) 4 .12H 2 O. A second locality for calcioferrite was reported by Henderson & Peisley (1985) at the Moculta quarry in Angaston, South Australia, associated with apatite, jarosite, cacoxenite and altered pyrite, the latter probably being the source of Fe 3+ . The chemistry and X-ray power data of calcioferrite from this locality are consistent with the previous observations that calcioferrite is isotypic with montgomeryite. However, the structure of calcioferrite has remained undetermined because of its small crystal size and generally poor crystallinity. In the course of identifying minerals for the RRUFF Project (http://rruff.info), we were able to isolate a single crystal of calcioferrite and determine its structure by means of single-crystal X-ray diffraction, demonstrating that its space group is C2/c.
The [(Fe/Al) 3 (PO 4 ) 3 (OH) 2 ] layers are connected by Ca 2+ cations (coordination numbers of eight) and Mg 2+ cations (coordination number of six). The relatively weaker bonds between the layers account for the cleavage of the mineral parallel to (010).
The (Fe/Al)O 6 octahedral chains in calcioferrite have a repeat of ~7.1 Å, similar to those examined by Huminicki & Hawthorne (2002). Between the two distinct B sites, the B1 site is strongly preferred by Al. The average (Fe/Al)1-O distance is 1.962 Å, which is evidently shorter than the average (Fe/Al)2-O distance (1.997 Å). The analysis of the anisotropic displacement parameters of atoms indicates that PO 4 tetrahedra behave as rigid bodies, as should be expected for such strongly bonded tetrahedral groups (Downs, 2000). Both (Ca/Sr)1 and Ca2 are eight-coordinated, with the former by (4 O + 4 H 2 O) and the latter by (6 O + 2 H 2 O). The (Ca/Sr)1O 8 polyhedra are situated between the [(Fe/Al) 3 (PO 4 ) 3 (OH) 2 ] layers, whereas the Ca2O 8 polyhedra are located within the layers (Fig. 2). Hydrogen-bonding interactions involving the water molecules and OH-function are also present between these layers ( Table 1).
As observed for the (Ca/Sr)1O 8 polyhedra, the MgO 6 octahedra are also located between the [(Fe/Al) 3 (PO 4 ) 3 (OH) 2 ] layers (Fig. 2). The Mg-site is randomly half-occupied with an average Mg-O bond length of 1.988 Å. The water O atom OW3 appears to be split between two positions (OW3A and OW3B), representing the two sets of water molecules correlated to the occupancy of the Mg-site (Fig. 3). The displacement parameters for OW3A are significantly larger and elongated than those of OW3B, suggesting that OW3A correlates with the vacancy and therefore is in a "softer" potential well. Interatomic distances between Mg-OW3B are more similar to each other (2.145 (8) Å and 2.200 (9) Å) while those between Mg-OW3A are more dissimilar to each other (2.535 (11) Å and 2.117 (10) Å). This is consistent with our suggestion, based on displacement parameters, that OW3A is not bonded to Mg.

Experimental
The calcioferrite specimen used in this study is from Moculta quarry, Angaston, Australia, and is in the collection of the RRUFF project (deposition R120092: http://rruff.info/R120092). Its chemical composition was determined with a CAMECA SX100 electron microprobe at the conditions of 15kV, 1nA and a beam size of 5µm. These conditions were optimized to minimize sample damage by the electron beam due to the small size of the sample (Fig. 4) (86), with H 2 O 21.02 calculated by difference. Due to the significant dehydration of the sample during the electron microprobe analysis, this composition may not be very accurate and was used only for the estimation of cation ratios. By assuming six P cations per formula, the relative ratio of (Ca, Sr):Mg:(Fe, Al):P is 3.85:0.98:3.85:6.00. The composition of the crystal is then (Ca 3.94 Sr 0.06 ) Σ=4 Mg(Fe 2.93 Al 1.07 ) Σ=4 (PO 4 ) 6 (OH) 4 .12H 2 O as determined by the combination of the electron microprobe and the X-ray structural data.

Refinement
All non-hydrogen atoms were refined with anisotropic displacement parameters. Only H atoms bonded to OW1, OW2, and OH could be located from difference Fourier syntheses and their positions refined with a fixed isotropic displacement parameter (U iso = 0.03). The H atoms bonded to the disordered OW3 atom could not be located and were excluded from refinement.
The occupancies of Al and Fe of the two B sites were refined with their ratio determined from the electron microprobe analysis. The small amount of Sr detected from the electron microprobe analysis was assigned into the Ca1 site, because this site is significantly larger than the Ca2 site. The maximum residual electron density in the difference Fourier map, 0.57 e/Å 3 , was located at (0, 0.0340, 0.25), 0.69 Å from Sr1 and the minimum, -0.60 e/Å 3 , at (0.8637, 0.3318, 0.0082), 1.31 Å from OW1.     The crystal structure of calcioferrite showing atoms with anisotropic displacement ellipsoids at the 99% probability level.
Yellow, purple, green and grey ellipsoids represent (Fe/Al), P, Mg and Ca sites, respectively. Red and aquamarine ellipsoids represent O atoms and H 2 O groups, respectively. Small blue spheres represent H atoms. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.  (3)