A feldspar mineral common in siliceous igneous rocks, consisting of a sodium-rich plagioclase (with more calcium than albite).‘It is composed of quartz, oligoclase, and microcline, with minor garnet, biotite, muscovite and iron-oxides.’
- ‘Here it was found sparsely as rounded grains associated with pargasite, diopside, oligoclase and lazurite.’
- ‘They range from bytownite - labradorite in basalts and gabbros, andesine - oligoclase in diorites and andesites to albite in granites and rhyolites.’
- ‘They recognized kyanite-staurolite assemblages in pelites to the north of the Annagh Gneiss Complex exposures, but only garnet and oligoclase to the south.’
- ‘However, this assumption is inconsistent with the fairly simultaneous appearance of the second garnet generation and oligoclase.’
- ‘Granite sample RHGI can be modelled by continuing this felsic scheme, for example with 22% fractionation of oligoclase, amphibole, biotite and the usual minor apatite and titanite.’
Mid 19th century from oligo-‘relatively little’ + Greek klasis ‘breaking’ (because thought to have a less perfect cleavage than albite).