Mineral description : Oligoclase

Oligoclase from Granmuren.
Partly bluish white fluorescens.
Oligoclase from Ytterklippen.
Partly white fluorescens.

Mineral class Silicates : Tektosilicates : Feldspar group.
Chemical formula (Na,Ca)(AlSi3O8) (90-70% Na, 10-30% Ca)
Crystal system Triclinic.
Habitus Tabular to prismatic, commonly with polysyntetic twinning.
Cleavage [001] Perfect, [010] good, 90 degrees angle. Uneven fracture.
Hardness 6-6.5
Density 2.64-2.66
Color White to grayish, or pale nuances of green yellow or brown.
Streak White
Luster Vitreous.
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.
Associates  
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.
  • Granmuren pegmatite quarry, Sala, Västmanland. As big crystals and cleavage masses.
  • Malsjö limestone quarry, Grums, Värmland. As crystals.
  • Ytterby feldspar quarry, Stockholm. As crystals.
  • Hult, Burseryd, Småland.

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Last changed : 1998-09-02