Structural study of magnesioaxinite and its crystal-chemical relations with axinite-group minerals

The magnesium end-member of the axinite mineral group was studied by single-crystal X-ray diffraction and refined in the P (1) over bar space group (R = 2.2 %). Cell parameters are a = 7.1381(3) Angstrom, b = 9.1626(4) Angstrom, c = 8.9421(4) Angstrom, alpha = 91.903(4)degrees beta = 98.105(3)degrees gamma = 77.468(4)degrees, V = 565.21(4) Angstrom (3). Magnesioaxinite is isostructural with (Mn,Fe)axinites, but shows structural features which may account for the compositional gap observed between magnesioaxinite and manganaxinite. Electron and ion microprobe analyses and refined site occupancies gave the following crystal chemical formula: ([6])[Ca-2.00(Ca1.95Mn0.03)(Mg1.92Mn0.04V0.03Cr0.01)Al2.00Al2.00]([4])[B-2.00Si8.02O30](OH1.9O0.1), very close to the ideal Mg end-member. The octahedron occupied by Mg ([Mg-O] = 2.17 Angstrom) is distorted and closely connected with both tetrahedral and octahedral structure components. Its modification (when Mg substitutes for Fe2+ and Mn) causes lattice deformations and also influences T-O-T angles and geometry of octahedral sheets.