Calcioferrite with composition (Ca_3.94Sr_0.06)Mg _1.01 (Fe_2.93A_l1.07)(PO₄) _6- (OH)₄·12H₂O

Calcioferrite, ideally Ca₄MgFe³⁺ 4(PO ₄)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 (Ca_3.94Sr_0.06)Mg _1.01 (Fe_2.93A_l1.07)(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 Mg ²⁺ 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).