gems, garnet, analytical, species

Tröger examines the relationship between the chemical composition of garnet and the types of rocks in which garnets occur. Using approximately 480 published garnet analyses, recalculated in terms of the principal garnet end-members, he divides the analyses into 28 compositional groups and compares their distributions among different host-rock associations. The study emphasizes that natural garnets are generally complex solid solutions rather than pure end-members and that variations in proportions of such components as almandine, pyrope, spessartine, grossular, and andradite are related to geological environment. By correlating mineral chemistry with host-rock type, Tröger develops a systematic framework for using garnet composition as a petrogenetic indicator and demonstrates both characteristic compositional fields and overlaps among garnets from different rock associations. The paper is an early quantitative treatment of garnet chemistry and its relationship to petrology.