THE SOCIAL LIVES OF MINERALS

Mineralogy, the study of minerals and their properties, is an ancient, useful, and intellectually stimulating branch of science. Petrology, the study of rocks and their mechanisms of formation, was whimsically described by J.B. Thompson, Jr., as a look at the social lives of minerals, as minerals may interact, or choose not to interact. His insight has led to a variety of important ways to view chemical reactions in rocks, particularly metamorphic rocks. In his landmark 1955 paper, Thompson (1921-2011) applied the thermodynamic rigor of J. Willard Gibbs (1839-1903) to the question of mineral facies as envisaged by Pentti Eskola (1883-1964).

The minerals in a metamorphic rock represent the products in an equation of the form [?] = m₁ + m₂ + m₃ + … + m_n

The essential riddle that metamorphic petrology attempts to solve is to identify the reactants, and the physical conditions (intensive variables) that obtained during the reaction. The number of product minerals is limited by the Gibbs Phase Rule or the Mineralogical Phase Rule as suggested by Goldschmidt, and therefore studying the mineral compositions leads to identifying the relevant thermodynamic components.

Index minerals (e.g. the aluminum silicates) and by extension index assemblages, help define metamorphic facies, and, when coupled with experimental data, lead to the development of petrogenetic grids. With the advent of plate tectonics, petrogenetic grids may in turn be related to tectonic environments. The appearance of uncommon minerals (e.g. yoderite) or uncommon mineral assemblages (e.g. chlorite + diopside + Ti-garnet) also carries a message that may ultimately be related to unusual metamorphic conditions, and possibly to unusual tectonic environments.

No discussion of the social lives of minerals would be complete without mention of fluids. In typical prograde or retrograde metamorphic reactions, fluids appear as reactants or products. However, even in reactions involving nominally anhydrous minerals, fluids are commonly present although they are neither produced nor consumed. In this sense fluids may play the role of transport agents, catalysts, or even as - Cupid!