Igneous Rock Classification
"There are
a few different types of igneous
rock classification.."
They have been named on the basis of different characteristics such as chemical composition, mineral content, texture, even geographical locality. The historical nomenclature for igneous rocks is therefore confusing.
Nowadays, they are classified on the basis of mineralogy and sometimes texture.
Mineralogical Classification
Mineralogy, and particularly composition, tell us about the original magma and how it evolved. Rocks that contain mainly dark, ferromagnesian minerals such as pyroxenes and olivine are mafic. Rocks that contain mainly light, silica-rich minerals such as quartz and feldspars are felsic. The IUGS Classification System is based on what minerals are found in a rock.
Textural Classification
Texture of a rock tells us about the environment in which the rock was formed and the cooling rate.
Glassy texture indicates a very quick cooling rate. Silica-rich, felsic rocks can form glassy texture easier than mafic rocks, however there are mafic rocks that do it too. Rocks with glassy texture include obsidian, pumice, tachylite and scoria.
Crystalline textures show different types of grain size. Those are classified as coarse-grained, medium size, and fiine-grained. The larger the mineral grains, the slower was cooling of magma at the time when the minerals formed. This is a large group of rocks that contains granite, rhyolite, basalt, gabbro, and many, many others - the vast majority of all igneous rocks.
Fragmental textures are formed in the air from volcanic debris and ash. These include hyaloclasite, volcanic breccia and pyroclastic rocks such as tuff.
Crystalline texture of a granitoid. By Kyknoord
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