Oligoclase feldspar: The mineral Oligoclase information and pictures

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. Oligoclase is an intermediary member of this series. Oligoclase is considered by some authorities as a variety of Albite rather than a separate mineral. The acclaimed Dana's System of Mineralogy lists Oligoclase as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a calcium-rich variety of Albite.

Identification

Chemical Formula: (Na,Ca)Al_1-2Si_3-2O₈
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).
Color: Colorless, white, cream, yellow, orange, pink, light blue, light green, brown, gray. Often iridescent with schillers.
Streak: White
Hardness: 6–6.5
Crystal System: Triclinic
Luster: Vitreous. Pearly on cleavage surfaces.
Cleavage: 2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Fracture: Conchoidal to uneven
Tenacity: Brittle
Transparency: Transparent to translucent
Specific Gravity: 2.63–2.67
Group: Feldspar Group
Rock Type: Igneous, Metamorphic

Rankings

Popularity (1–4): 2
Prevalence (1–3): 1
Demand (1–3): 2

Crystal Forms & Aggregates

Crystals are usually short and stubby, prismatic, thin tabular, and occasionally elongated. Crystals are often twinned. Also occurs grainy, massive, columnar, and as rounded waterworn stones.

Complex Tests

Soluble in hydrofluoric acid.

Striking Features

Color, crystal habits, cleavage, and hardness.

Environment

Most often in granite pegmatites, also in metamorphic rocks.

Varieties

✓ common · ✗ uncommon

Moonstone— Form of feldspar that displays a color sheen known as adularescence. Depending on the locality, Moonstone may be Orthoclase feldspar (Adularia), or it may be the Plagioclase feldspar Oligoclase.
Sunstone— Form of orange feldspar that has inclusions of Hematite (or in one locality Copper) that give it a glittering effect. Depending on the locality, Sunstone may be Microcline feldspar, or it may be the Plagioclase feldspar Oligoclase.

Uses

Oligoclase is industrially important in the manufacture of ceramics. Crystals are popular among mineral collectors. Oligoclase is also known for its gem varieties: Sunstone and Moonstone.

Noteworthy Localities

Good Oligoclase crystals come from the Montijos Quarry, Monte Redondo, Leiria, Portugal. Masses of Moonstone Oligoclase come from Spruce Pine, Mitchell Co., North Carolina; and well-shaped white and colorless crystals were found in the Hawk Mine, Mitchell Co., North Carolina. Very good Moonstone Oligoclase was found in Middletown Township, Delaware County, Pennsylvania. An important Connecticut locality is Timms Hill, Haddam, Middlesex Co., Connecticut. Much of the Feldspar in rock on the island of Manhattan (New York City) is Oligoclase.

Excellent gem quality Oligoclase Sunstone comes from Norway at Bamble, Telemark; and at Tvedestrand, near Arendal.

Common Mineral Associations

Quartz, Muscovite, Biotite, Potassium Feldspar Group

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 lower hardness.