An Excel spreadsheet to recast analyses of garnet into end-member components, and a synopsis of the crystal chemistry of natural silicate garnets

Abstract

A Microsoft Excel spreadsheet has been programmed that allows users to calculate with ease the molar proportions of garnet end-members from chemical analyses. Recent advances in the understanding of the crystal chemistry of natural garnets, especially of the Ti-bearing garnets, are used to evaluate 29 end-members (15 species and 14 hypothetical end-members) for each analysis. The amounts of Fe 2+ and Fe 3+ (and Mn 3+, if necessary) are calculated by stoichiometric constraints if these quantities have not been measured. The input data can include: SiO 2, TiO 2, ZrO 2, SnO 2, Y 2O 3, Al 2O 3, Sc 2O 3, Cr 2O 3, V 2O 3, FeO, Fe 2O 3, MnO, MgO, CaO, Na 2O, H 2O+ and F. The spreadsheet can be used with large data sets (up to 100 analyses at a time), and is accompanied by results calculated for 470 garnet analyses taken from the literature. The spreadsheet employs a simple scoring algorithm to measure the quality of a garnet analysis. The propagation of error from the input chemical data to the calculation of end-member proportions is also discussed briefly.