This chapter deals with the classification of igneous rocks. This reduces the thousands of rock names found in the literature down to a manageable number and links them to a logical classification based on their mineral content and chemical composition. The chapter presents the classification adopted by the International Union of Geological Sciences (IUGS), which uses the abundance of the major rock-forming minerals (the mode) to place rocks in compositional fields for which there are commonly accepted names. Some rocks are too fine-grained, or even glassy, for this modal classification to be applied.
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