![]() This diagram shows how feldspar minerals are classified on the basis of their chemical composition. These minerals are volumetrically very restricted, nowhere near as common as other feldspars.įig.1: Feldspar classification. They are also structurally similar to K-feldspars. They are celsian (BaAl 2Si 2O 8) and hyalophane (compositionally between K-feldspar and celsian). Some uncommon feldspars contain barium in the crystal structure. There are no feldspars that are intermediate in composition between K (Or) and Ca (An) endmembers because these ions have different ionic radii and charges which would make the structure unstable. Plagioclase feldspars are between Ab and An. There are more feldspars (60%) than all the other minerals combined in the outer (13-17) km of the crust.įeldspars include three compositional endmembers: K-feldspar (KAlSi 3O 8), albite (NaAlSi 3O 8) and anorthite (CaAl 2Si 2O 8) Fig.1.Īlkali feldspars are compositionally between Or and Ab endmembers. Feldspar is the most abundant mineral group in the Earth’s crust. The change from Spat to -spar was influenced by the English word spar, meaning a non-opaque mineral with good cleavage. Feldspat was introduced in the 18th century as a more specific term, referring perhaps to its common occurrence in rocks found in fields (Urban Brückmann, 1783). ![]() Spat had long been used as the word for "a rock easily cleaved into flakes". The name feldspar derives from the German Feldspat, a compound of the words Feld (field) and Spat (flake). ![]()
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