Mineralogy

Soils are weathering products of rocks and minerals. The rocks in Earth’s outer surface can be classified as igneous, sedimentary, or metamorphic rocks. Igneous rocks are formed from molten magma. They are composed of primary minerals, which are minerals that have not been altered chemically since they formed as molten lava solidified. Examples of primary minerals are the light-colored minerals quartz, muscovite, feldspars, and orthoclase, and the dark-colored minerals biotite, augite, and hornblende. In general, dark-colored minerals contain iron (Fe) and magnesium (Mg) and are more easily weathered than light-colored minerals. Coarse-grained igneous rocks, such as granite and diorite, contain mainly lightcolored minerals, while medium-grained igneous rocks such as gabbro, peridotite, and hornblendite are composed of dark-colored primary minerals. Rhyolite and andesite are medium-grained igneous rocks containing light-colored primary minerals. Basalt is dark-colored with an intermediate to fine rock texture, and basaltic volcanic glass has a fine texture. Examples of light-colored igneous rocks with a fine texture are felsite and obsidian. Sedimentary rocks are the most common type of rock, covering about 75% of Earth’s land surface. They are mainly composed of secondary minerals, which are minerals that are recrystallized products of the chemical breakdown and/or alteration of primary minerals. Sedimentary rocks form when weathering products from rocks are cemented or compacted. For example, quartz (SiO2) sand, a weathering product of granite, may become cemented into sandstone. Another common sedimentary rock is limestone. There are two types of limestone, namely, calcite (CaCO3), and dolomite (CaCO3.MgCO3). Clays may become cemented into a sedimentary rock, which is known as shale. A sedimentary rock with several dominant minerals is called a conglomerate, in which small stones with different mineralogy are cemented together. Metamorphic rocks are formed by the metamorphism of igneous or sedimentary rocks. Great pressure and high temperatures, caused by the shifting of continental plates, can compress, distort, and/or partially re-melt the original rocks. Igneous rocks are commonly modified to form schist and gneiss, in which light and dark minerals have been reoriented into bands. Sedimentary rocks, such as limestone and shale, may be metamorphosed to form marble and slate, respectively.