Oligoclase is a member of the solid solution plagioclase feldspar group between albite (pure NaAlSi3O8) and anorthite (pure CaAl2Si2O8). Oligoclase has between 70% - 90% Na and 10% - 30% Ca. Better known are two varieties of oligoclase, Sunstone and Moonstone. Sunstone has a golden shimmer caused by included flecks of hematite that reflect the light. Moonstone has an irredescent glow caused by internal intergrowths of the oligoclase crystal.
| (Na,Ca)(Al,Si)AlSi2O8 |
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| Tectosilicate |
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| Triclinic |
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| -1 |
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| 6-6.5 |
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| 2.64-2.68 |
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| {001} perfect, {010} good |
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| Conchoidal |
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| White, gray, various tints |
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| White |
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| Vitreous, dull |
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| Translucent/transparent |
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| Tabular |
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| Fluorescent, Carlsbad, albite and pericline twinning common |
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| Notable Localities: |
Sri Lanka; New York; Russia; Sweden; Norway; Canada |
| Uses: |
Ornamental stone, sometimes cut as a gem |
Specimens:
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