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Chemical Formula | (Na,Ca)Al1-2Si3-2O8 |
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Composition | Sodium calcium aluminum silicate. The ratio
of sodium to calcium is about 9:1. The amount of aluminum
atoms are between 1 and 2, and the amount of silicon
atoms are between 3 and 2. In the Plagioclase Feldspar series, Oligoclase contains between 70 and 90 percent Albite (Ab), and between 10 and 30 percent Anorthite (An). |
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Color | Colorless, white, cream, yellow, orange, pink, light blue, light green, brown, gray |
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Streak | White |
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Hardness | 6 - 6½ |
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Crystal Forms and Aggregates |
(Triclinic) Crystals are short and stubby prismatic, thin tabular, and occasionally elongated. Crystals are often twinned. Also occurs grainy, massive, columnar, and as rounded stones. |
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Transparency | Transparent to translucent |
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Specific Gravity | 2.63 - 2.67 |
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Luster | Vitreous. Pearly on cleavage surfaces. |
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Cleavage | 2,1 - basal ; 2,1 - prismatic ; 3,1 -
pinacoidal The cleavage angle is about 90º |
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Fracture | Conchoidal to uneven |
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Tenacity | Brittle |
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Varieties |  Sunstone - Orange to reddish variety of
Oligoclase with inclusions that cause internal color flashes Aventurine
Feldspar - Synonym of Sunstone (above) Moonstone - Transparent, adularescent form of Oligoclase |
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In Group | Silicates ; Tectosilicates ; Feldspar group ; Plagioclase Feldspars |
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All About | Oligoclase belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. The intermediary members are Oligoclase, Andesine, Labradorite, and Bytownite. |
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Uses | Oligoclase is industrially important in the manufacture of ceramics. Crystals are popular among mineral collectors. Oligoclase is famous for its two gem varieties: Sunstone and Moonstone. |
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Striking Features | Color, crystal habits, cleavage, and hardness |
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Popularity (1-4) | 2 |
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Prevalence (1-3 | 1 |
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Demand (1-3) | 2 |
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Distinguishing Similar Minerals |
Potassium feldspars - don't exhibit striations on twinned crystal surfaces, whereas the
Plagioclase feldspars do Other Plagioclase Feldspars - usually cannot be determined by practical means Spodumene - has a splintery cleavage Calcite - much softer (3) |
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Commonly Occurs With |
Quartz, Muscovite, Biotite, Potassium Feldspar |
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Noteworthy Localities |
Oligoclase is a very common mineral, but it
is rare in nice crystals. Masses of Moonstone Oligoclase
come from Spruce Pine, Mitchell Co., North Carolina.
Well-shaped white and colorless crystals were found in
the Hawk Mine, Mitchell Co., North Carolina. Much of the
Feldspar in rock on the island of Manhattan is
Oligoclase. Quality Oligoclase crystals are reported from
a quarry in St. Lawrence Co., New York. Almost all gem
quality Sunstone is from Norway and Canada. Moonstone Oligoclase is found along many North American beaches in the form of naturally rounded pebbles. |
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