Partly bluish white fluorescens.
Partly white fluorescens.
|Mineral class||Silicates : Tektosilicates : Feldspar group.|
|Chemical formula||(Na,Ca)(AlSi3O8) (90-70% Na, 10-30% Ca)|
|Habitus||Tabular to prismatic, commonly with polysyntetic twinning.|
|Cleavage|| Perfect,  good, 90 degrees angle. Uneven fracture.|
|Color||White to grayish, or pale nuances of green yellow or brown.|
|Description||Oligoclase is a member of the plagioclase feldspar series.|
|Occurance||As a major part of many rocks, especially igneous rocks as granites, syenites and pegmatites.|
|Notes|| Flourescent in some cases.|
Two varietys of oligoclase that's been used as precious stones are moonstone and sunstone.
Moonstone is a variety where the chemistry of high temperature allowed a mixture of two phases. When the temperature dropped the different phases separated in the feldspar creating thin layers which reflects the light.
Sunstone is a gray to yellowish variety with a characteristic red to yellow metallic lustre. It's caused by fine inclusions of hematite.
Twinning often produces many layers on top of each other, each only a fraction of a mm to several mm thick. Theese layers can be seen as striations not only on the surface of the crystal, but also on cleavages. This is a characteristic sign of the plagioclase feldspar series.
|Locations|| A very common mineral. |
| To silicate index.
|| Mineral group index.
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