Читать книгу Earth Materials - John O'Brien - Страница 15

Earth is largely composed of various types of rock. A rock is an aggregate of mineral crystals and/or mineraloids. Scarce monominerallic rocks consist of multiple crystals of a single mineral. Examples include the sedimentary rock quartz sandstone which may consist entirely of quartz grains held together by quartz cement and the igneous rock dunite which can consist entirely of olivine crystals. The vast majority of rocks are polyminerallic; they are composed of many types of mineral crystals. For example, granite commonly contains quartz, potassium feldspar, plagioclase, hornblende and/or biotite, and various other minerals in small amounts.

Mineral composition is one of the major defining characteristics of rocks. Rock textures and structures are also important defining characteristics. It is not surprising that the number of rock types is very large indeed, given the large number of different minerals that occur in nature, the different conditions under which they form, and the different proportions in which they can combine to form aggregates with various textures and structures. Helping students to understand the properties, classification, origin, and significance of minerals and rocks is the major emphasis of this text.