The crystal chemistry of kornerupine

Abstract

Kornerupine is an accessory sorosilicate mineral found in high-grade metamorphic rocks. It has the general formula ($\square,$ Mg, Fe) (Al, Mg, Fe)$\sb9$ (Si, Al, B)$\sb5$ O$\sb{21}$ (OH,F). Previous studies have been unable to arrive at accurate structural formulae and corresponding detailed crystal-chemical relations. Forty seven ferrous-iron kornerupines $\rm(Fe\sp{3+}/Fe\sp{2+}\le0.5)$ from $\sim$35 localities were examined using SREF, EMPA, SIMS and H-extraction line techniques to acquire complete crystallographic and chemical data. EMP data was processed using an anion-normalization scheme whereby cations are scaled to O$\sb{21}$ (OH + F)$\sb1\ pfu,$ which is supported by bond-valence arguments and H values from H-extraction line analysis. B and Fe$\sp{3+}$ quantities were developed via site-occupancy refinement at the T(3) and M(4) sites, respectively. Excellent agreement was attained between B values arrived at by SIMS and SREF methods. Site populations for the three tetrahedra, five octahedra and single eight-coordinate site were developed using standard crystal-chemical criteria. (Abstract shortened by UMI.)