Chemistry and physical properties of axinites

Abstract

Electron microprobe analyses of 37 axinites substantiate the conclusions of Sanero and Gottardi (1968) as to the substitutional solid solution between octahedrally coordinated divalent cations Ca, Fe, and Mn. The analyses also suggest the substitution of Fe³⁺ for Al in the AlO₅(OH) octahedra for six axinites and the substitution of Al for Si in tetrahedral coordination for one specimen. The chemistry of the axinite group is best described by a new structural formula: [(Mn, Fe²⁺, Mg, Zn)(Ca_2-xMn_x)(Al_2-yFe_y³⁺)]₂^VI(OH)₂[(B₂Si₈_zAl_z)^IVO₃₀], where x < 1, y << 1, and z << 1.

Pronounced layering approximately parallel to (lll), bands of B₂Si₈O₃₀ groups parallel to [011], and four types of octahedral chains influence indicatrix orientation, external morphology, and cleavage. Due to their proximity to octahedral chains, the b and c cell edges have high correlations with <r>, the mean radius of octahedral cations. Density and mean refractive index are shown to be highly correlated with the amount of transition metal oxides present for all axinites and 2Vα is highly correlated with Mg content for the non-zincian 2Ca-axinites. Reasonable estimates of composition can be obtained for the non-zincian 2Ca-axinites by using the following equations for the mole fraction of Mg, Fe, and Mn:

Mg = -14.333 <R.I.>+ 0.012(2Vα) + 23.487

Fe= 71.937 <R.I.> - 11.810(p_obs) - 81.890

Mn = -42.903 <R.I.> + 12.523(p_obs) + 31.473.