Crystal structure, EMPA and FTIR spectroscopy of green yoderite: (Mg2.02Al5.68Fe3+0.35)Σ8.05 (Si3.93 P5+0.03)Σ3.96O18(OH)2; from Mautia Hill, Tanzania

Abstract

Green yoderite, with the ideal crystal-chemical formula ^[6](MgAl₃)^[5](MgAl) ^[5](Al₂)O₂(SiO₄)₄(OH)₂, from the type locality, Mautia Hill, Tanzania, has been characterized by accurate X-ray diffraction data and crystal-structure refinement at ambient conditions, electron microprobe analysis and FTIR spectroscopy. Yoderite is monoclinic with a = 7.9979 (3), b = 5.8003 (2), c = 7.2299 (2), ß= 104.7398 (9) Å, V = 324.36 (2) ų, space group P2₁/n (no. 14), Z = 1. Crystal-structure refinement with all non-H atoms refined anisotropically converged to an R1 value of 2.50 % based on all 2156 unique reflections and 94 refined parameters. Chains of edge-sharing A(1)O₅(OH) octahedra extend in the b direction and the A(1) site is occupied by 0.75 Al + 0.25 Mg. These chains are linked by isolated SiO₄ tetrahedra and by two independent trigonal bipyramids A(2)O₄(OH) and A(3)O₅, where the A(2) site is occupied by 0.50 Al + 0.50 Mg and the A(3) site by 0.94 Al + 0.06 Fe³⁺. The hydrogen-bond scheme is established on the basis of the H-atom position observed in the final ΔF map and refined without restraint. Fitting of the FTIR spectrum in the principal (OH)-stretching region shows that ^A(2)Al and ^A(2)Mg are preferentially associated with certain ^A(1)(AlAl), ^A(1)(AlMg) and ^A(1)(MgMg) dimers. Comparison of the band positions, observed intensities and associated local short-range arrangements suggests that the bond valence requirements of such arrangements control the chemical composition of the crystal.