Kilauea Lava Lake

Figure 8.00. The earth's moon from northern Vermont, March 2018. The light colored crust of the moon is made of anorthosite, consisting principally of plagioclase feldspar. The plagioclase is believed to have floated to the surface of a lunar magma ocean shortly after the moon formed. The bright moon is, therefore, a visible consquence of fractional crystallization.

Fractional Crystallization

"Part of it, plus the rest of it, equals all of it."
John Rosenfeld, citing "Baldy's Law"
8.1 Overview

Fractional crystallization occurs when the minerals that grow from a cooling magma are physically separated from the liquid, changing the chemical composition of that liquid. There is good evidence that fractional crystallization occurs in many common magmas. This chapter considers some of the features of fractional crystallization and their effects on the major element chemistry of igneous rocks. It is possible to use knowledge of these effects to detect fractional crystallization by examining the chemistry of the resulting rocks. Concepts that are developed in this chapter include:
  • crystal settling
  • fractionation trends
  • detecting fractionation
  • exchange coefficients
  • Bowen's Reaction Series
Concepts that are used in this chapter, include chemical equilibrium, composition axes, ternary diagrams, binary solid solution diagrams, and ternary eutectic diagrams.