Plagioclase Showing Chmical Zoning

Figure 5.00. Plagioclase zoning. Thin section of a plagiocalse crystal in a granodiorite. Chemical zoning in the plagioclase crystal is indicated by the concentric variations in interference color in crossed polarized light. This chapter will explain why chemical zoning is common in plagioclase phenocrysts in igneous rocks.

Binary Solid Solution Diagrams

5.1 Overview

Binary equilibrium digrams for the melting and crystallization of rocks involving solid solution minerals are qualitatively different from binary diagrams in which there is no compositional variation of the minerals. When a magma is saturated with a solid solution mineral, the chemical compositions of both the mineral and the magma change with temperature. Concepts that are developed include:
  • distillation
  • solidus curves
  • binary loop
  • chemical zoning
  • azeotropic point
Concepts that are used in this chapter, include chemical equilibrium, composition axes, saturation, saturation curves, and the the lever rule.