THE CRYSTAL STRUCTURE OF NEAR-END-MEMBER FERROAXINITE FROM AN IRON-CONTAMINATED PEGMATITE AT MALEšOV, CZECH REPUBLIC

Abstract

We have characterized ferroaxinite (up to 95 mol.% of end-member composition) from a contaminated granitic pegmatite cutting the magnetite-rich portion of an Fe-skarn body near Malešov, Czech Republic, by means of electron-microprobe analysis, single-crystal X-ray diffraction, infrared spectroscopy and temperature-dependent 57Fe Mössbauer spectroscopy. The following crystal-chemical formula was obtained: VI[Ca2.00 Ca2.00 (Fe2+1.83Mn0.10 Mg0.06)∑1.99 (Al1.97 Fe3+0.04)∑2.01 Al2.00] IV[B2.00Si8.00] O30 OH2.00. The crystal structure was refined to R(F) = 0.0184 for 4667 observed reflections with Fo > 4∑(Fo). We obtained the following unit-cell parameters: a 7.152(1), b 9.206(2), c 8.962(2) Å, α 91.84(3), β 98.17(3), γ 77.33(3)° V 569.86(19) Å3. Relative to the other end-members of axinite group, the only significant differences of structural parameters occur at the Y site, where Fe2+ atoms predominate; the mean Y-O bond length is 2.229 Å. The unusually long Fe-O(14) bond (2.704 Å) of the distorted Y site suggests a [5 + 1] coordination rather than the previously reported distorted [6] coordination for the group. The scarcity of near-end-member ferroaxinite in nature is due to preferred incorporation of Mn into the structure. The formation of near-end-member ferroaxinite in the late stage of pegmatite evolution at Malešov was favored by high activity of Fe, and lack of Mn and Mg in parent low-temperature fluids.