5.1 Overview

Much of the petrology of igneous and metamorphic rocks concerns change. Rocks melt, magmas crystallize, shale metamorphoses into schist, peridotite is changed into serpentinite. The rocks we find in the field are the end products of those changes. Petrologists study igneous and metamorphic rocks to try to discover and constrain the processes, the changes, that produced the rocks. Many of the changes involve chemical reactions. If we can identify the chemical reactions that a rock has experienced and the conditions required to cause those reactions, we can better interpret the processes that produced the rock. In the following paragraphs some principles of chemical reactions are presented that will help you understand the fundamentals of petrology.

5.2 What is a chemical reaction?

It is likely that you first thought seriously about chemical reactions in a chemistry course. Introductory chemistry courses typically emphasize liquids and gases, introducing concepts and a vocabulary helpful in describing reactions in liquids or gases such as solvent, solute, molecule, concentration, saturation, precipitation, etc. Although this terminology is also used by geologists, the importance of solids (minerals), the nature of magmatic liquids, and the occurrence of chemical reactions in rocks that may not involve liquids or gases leads petrologists to adopt a broader, largely macroscopic view of chemical reactions.

According to britanica.com, a chemical reaction is “…a process in which one or more substances, the reactants, are converted to one or more different substances, the products. …. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.” For most of the reactions discussed in this petrology textbook, the reactants and products are “phases,” where phase is a macroscopic term meaning a physically homogeneous substance such as a mineral or a magmatic liquid. Reactions that involve more than one phase are heterogeneous reactions. Reactions that involve only one phase are homogeneous reactions.

Examples of chemical reactions of interest to petrologists include partial melting of mantle peridotite during decompression to produce basaltic liquid, basaltic liquid crystallizing olivine and plagioclase when a lava is erupted and cooled, and muscovite interacting with quartz to produce sillimanite, orthoclase, and water as a schist is heated during a continental collision.

How can you tell if a chemical reaction has occurred in the history of a rock?

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