Oligoclase is a common plagioclase feldspar that represents a solid solution between albite and anorthite. It is frequently found in igneous rocks and can exhibit a unique schiller effect known as aventurescence, especially in the variety called sunstone.

Hardness

6-6.5

Mohs

Luster

Vitreous to pearly

Streak

White

Transparency

Transparent to translucent

mineralcommonPlagioclase Feldspar Group family

Oligoclase

Hardness: 6-6.5
Luster: Vitreous to pearly
Streak: White
Transparency: Transparent to translucent

Is this oligoclase?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch oligoclase with a known reference. Oligoclase sits at Mohs 6-6.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous to pearly.
4
Match the color range
Compare against the expected color range: white, colorless, gray, pale yellow, pale green, reddish, brown, greenish, yellowish.
5
Look at form & habit
Crystal system: triclinic. Typical habit: euhedral crystals, granular.

Often confused with

Oligoclase vs. its common look-alikes, and how to tell them apart in the field.

How to tell apart: Luster reads vitreous to pearly on Oligoclase and subvitreous to pearly on Andesine.

Often found alongside oligoclase

Minerals reported to co-occur with oligoclase. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: (Na,Ca)(Si,Al)₄O₈
Mohs hardness: 6-6.5
Density: 2.63-2.66 g/cm³
Colors: White Colorless Gray Pale yellow Pale green Reddish Brown Greenish Yellowish
Streak: White
Luster: Vitreous to pearly
Transparency: Transparent to translucent
Crystal system: Triclinic
Crystal habit: Euhedral crystals, granular
Cleavage: Perfect on {001}, less perfect on {010}, imperfect on {110}, (001) ^ (010) ~94°
Fluorescence: Yellow under LW UV
Rarity: Common
Uses: Collector, Petrological studies, Lapidary
Host rock: Common in granite, syenite, nepheline syenite, and their pegmatites; in diorite, rhyolite, andesite

Where rockhounds find oligoclase

Classic worldwide localities

Fine and Macomb, St. Lawrence Co., New York, USA
Corundum Hill, Chester Co., and at Media, Delaware Co., Pennsylvania, USA
Hawk, east of Bakersville, Mitchell Co., North Carolina, USA
Danviken, near Stockholm, Sweden
Arendal, on Tromöy Island at Alve, Tvedestrand, Norway
Ytterby, Sweden

Field-hunting tip

Where it forms: Common in granite, syenite, nepheline syenite, and their pegmatites; in diorite, rhyolite, andesite. If you start seeing quartz, orthoclase, sanidine in float, you are in the right ground. Typical habit: euhedral crystals, granular. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify oligoclase?+

Mohs hardness is 6-6.5. Luster: vitreous to pearly. The streak is white. Common colors include white, colorless, gray, pale yellow.

Where is oligoclase found?+

Notable localities include Fine and Macomb, St. Lawrence Co., New York, USA; Corundum Hill, Chester Co., and at Media, Delaware Co., Pennsylvania, USA; Hawk, east of Bakersville, Mitchell Co., North Carolina, USA; Danviken, near Stockholm, Sweden; Arendal, on Tromöy Island at Alve, Tvedestrand, Norway.

What rocks look like oligoclase?+

Oligoclase is most often confused with Albite, Andesine, Orthoclase. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with oligoclase?+

Oligoclase commonly co-occurs with Quartz, Orthoclase, Sanidine, Tourmaline, Corundum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does oligoclase form in?+

Where it forms: Common in granite, syenite, nepheline syenite, and their pegmatites; in diorite, rhyolite, andesite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is oligoclase used for?+

Oligoclase is used in collector, petrological studies, lapidary.

Find oligoclase on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.