OLIGOCLASE
OLIGOCLASE See “PLAGIOCLASE”
OBSIDIAN
OBSIDIAN See “Volcanic Glass”
OLIVINE
OLIVINE Olivine – (Fe,Mg)SiO4 – is a very common nesosilicate, occurring mainly in basic igneous rocks. It is an important industrial mineral with a number of uses. “Olivine” is actually not a mineral, but just a generic term used for nesosilicates of the isomorphic series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4), a complete solid solution. […]
ORTHOCLASE
ORTHOCLASE Orthoclase – KAlSi3O8 – is a very common tectosilicate, an important rock forming mineral in various types of igneous rocks, as well as an industrial mineral with a number of uses. It is a polymorph of sanidine, microcline and kokchetavite. Forms a series with celsian, which is a barium containing feldspar. It has six […]
OPAL
OPAL Opal – SiO2.nH2O – is a tectosilicate (Dana, 8th ed.), found in many types of rock, it is very common. The precious variety fetches high prices. For historical reasons, “opal” is still considered a mineral, but in reality it is composed of a mixture of cristobalite and/or tridymite or amorphous silica. There are 4 […]