gems, garnet, garnet composition, garnet testing

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 Fe²⁺ and Fe³⁺ (and Mn³⁺, if necessary) are calculated by stoichiometric constraints if these quantities have not been measured. The input data can include: SiO₂, TiO₂, ZrO₂, SnO₂, Y₂O₃, Al₂O₃, Sc₂O₃, Cr₂O₃, V₂O₃, FeO, Fe₂O₃, MnO, MgO, CaO, Na₂O, H₂O⁺ 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.