Graphical Representation of Metamorphic Mineral Parageneses

Abstract

Minerals composed of no more than three components may be represented within the plane of a triangle. As an example, wollastonite CaSiO₃, grossularite Ca₃Al₂(SiO₄), anorthite CaAl₂Si₂O₈, sillimanite Al₂SiO₅, quartz SiO₂, and corundum A1₂O₃ consist of the three components CaO, Al₂O₃, and SiO₂ in different proportions. Because of geometric advantages, compositions are most conveniently represented in an equilateral triangle [see Ferguson and Jones (1966)]. Each corner of the triangle represents a pure component, i.e., 100% CaO, 100% Al₂O₃, 100% SiO₂. Each side allows the representation of a mixture and a mineral consisting of two components, while the interior of the triangle allows the representation of mixtures and minerals consisting of all three components.