(Original Newsletter Date: 1/16/20)
A mineral is a naturally occurring, non-organic, pure element or chemical compound with a distinct and constant chemical formation and an ordered crystalline structure. Minerals occur in solid form, with rare exception, and are the products of geologic processes.
A rock is defined as a hard composite that may contain one or many minerals, mineraloids, or organic materials. Rocks are classified by the substances they contain and the processes by which they were formed. The mineral content of many rocks is valuable and they are mined for mineral extraction. There are three general categories of rocks:
Sedimentary rock is formed by the compression of accumulated layers of sediment. These layers can contain minerals, older rock fragments, detritus, and remains of plants and animals. Examples of sedimentary rocks are sandstone, shale, limestone and coal. Common rock-forming minerals found in sedimentary rock include quartz, alkali and plagioclase feldspars, gypsum, and calcite.
Igneous rock is created by the solidification of molten materials (magma) from within the earth. Intrusive igneous rock occurs when magma cools and hardens within layers of the earth's crust; examples are basalt and granite. When magma as lava reaches the earth's surface before it cools extrusive igneous rock is formed, such as obsidian and pumice. About 65% of the earth's crust is comprised of igneous rock. Common rock-forming minerals found in igneous rock include plagioclase feldspar, pyroxenes, amphiboles and olivine.
Metamorphic rocks are formed from a "parent" rock that undergoes transformation through changes in temperature and/or pressure, usually deep within the earth's crust. The original rock can be sedimentary, igneous or previously metamorphosed. The result is a significant change in the composition and appearance of the rock. Familiar metamorphic rocks are marble, lapis lazuli, gneiss and schist. Minerals involved in metamorphic processes form at extremely high temperatures and are known as index minerals; this category of minerals includes garnet, kyanite, andalusite and staurolite.