Andesite

Andesite is an extrusive igneous rock formed when igneous material, magma, erupts on or very close to the earths surface. Chemically, andesite often has at or a little over 50% (but less than 63%) silica content, as such it has a slightly lighter density compared to mafic and ultra mafic rocks such as basalt as well as a lighter colouration. It will also often contain expected minerals expected from igneous rocks such as pyroxenes, usually augite crystals as well as plagioclase feldspar, both of these crystals can sometimes form large enough to be easily viewed by the naked eye and are known as phenocrysts. The name is derived from the type locality, the Andes mountains in South America, where it is found in abundance. These particular samples come from Barrowdale, Cumbria, UK and formed during the late Ordovician period and are part of the Barrowdale volcanic group. This group is believed to be a pre-caldera succession where abundance of basalt, andesite and dacite are found, the formation is thought top be roughly 2.6 km in thickness. An ideal rock for higher education use, though collectors may also consider these pieces to complete their collections, andesite is ideal to display the different main rocks found in the main groups of igneous rocks classified by chemical composition. They are available in three various sizes and come supplied in a card tray with an information label.