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.
Mid 19th century: from oligo- 'relatively little' + Greek klasis 'breaking' (because thought to have a less perfect cleavage than albite).
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