Oligoclase Identification Guide
Identify oligoclase, a sodium-rich plagioclase feldspar, by its twinning striations, cleavage, hardness, and aventurescent sunstone variety.
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What Oligoclase Looks Like
Oligoclase is a plagioclase feldspar near the sodic end of the series (more sodic than andesine, less so than albite). It is usually white, gray, cream, or pale greenish, with a vitreous to pearly luster and blocky to tabular crystals. The gem variety oligoclase-sunstone shows coppery aventurescence (schiller) from tiny platy hematite/copper inclusions; some material shows faint moonstone-like sheen. It commonly appears as grains in igneous rocks rather than large crystals.
- Color: white, gray, cream, pale green; sunstone variety orange with copper glitter
- Luster: vitreous to pearly on cleavage faces
- Transparency: translucent to transparent
- Habit: tabular/blocky crystals; usually anhedral grains in rock; twin striations on cleavage
Step-by-Step Field ID Checklist
- Confirm it is feldspar: hardness ~6 (scratches glass), two cleavages meeting near 90°, blocky habit.
- Look for plagioclase striations: fine parallel lines (albite polysynthetic twinning) on the best cleavage face — diagnostic for plagioclase and absent in K-feldspar.
- Note pale color: white to gray; helps separate from pink alkali feldspar.
- Check for sunstone schiller: tilt the stone to see coppery/golden glints if it is sunstone-type oligoclase.
- Examine the setting: common in granodiorite, diorite, and felsic igneous/metamorphic rocks.
- Caveat: exact plagioclase species (oligoclase vs andesine vs albite) needs optical/chemical work.
Key Diagnostic Tests
- Hardness: ~6–6.5; scratches glass, not scratched by a knife easily.
- Cleavage: two good cleavages intersecting ~86–90°, with striations on one set.
- Streak: white.
- Acid: no reaction.
- Density: ~2.64–2.66 g/cm³ (slightly higher than K-feldspar).
Common Look-Alikes and How to Tell Them Apart
- Alkali/potassium feldspar (orthoclase, microcline): these lack albite twin striations (though microcline may show grid twinning) and are often pink/salmon; oligoclase's fine parallel striations on cleavage are the tell. K-feldspar may show perthite streaks instead.
- Other plagioclase (albite, andesine, labradorite): all show striations; distinction is compositional. Labradorite typically shows blue/green labradorescence; oligoclase sunstone shows coppery aventurescence. Precise species needs lab methods.
- Quartz: no cleavage (conchoidal fracture), no striations, glassy; oligoclase has cleavage and twinning.
- Moonstone (orthoclase/microcline): adularescence is bluish floating sheen; oligoclase sunstone glitter is metallic and directional.
- Aventurine quartz/goldstone: quartz aventurine is harder with green mica flecks; goldstone is manmade glass with uniform sparkle.
Where Oligoclase Is Found
Oligoclase is a widespread rock-forming feldspar in intermediate to felsic igneous rocks (granodiorite, tonalite, diorite, syenite) and many metamorphic rocks. Gem sunstone oligoclase comes from Norway (historic), India, Russia, Canada, and Tanzania; note that the famous Oregon Sunstone is actually a calcic labradorite, not oligoclase. Aventurescent and moonstone-like oligoclase is found in pegmatites and feldspar-rich rocks worldwide.
Frequently asked questions
How can you tell if a feldspar is oligoclase?
Confirm feldspar (Mohs ~6, two cleavages near 90°), then look for fine parallel striations from albite twinning on the cleavage face, which mark it as plagioclase. A pale white-gray color and, in sunstone material, coppery glitter point to oligoclase, though exact species needs lab testing.
What is the difference between oligoclase and orthoclase?
Both are feldspars, but oligoclase is a sodic plagioclase showing fine albite twin striations on its cleavage, while orthoclase is a potassium feldspar without those striations and is often pinkish, sometimes showing perthite streaks instead.
Is Oregon Sunstone oligoclase?
No. Although sunstone is often associated with oligoclase, the famous Oregon Sunstone is a calcic plagioclase (labradorite). Sunstone-type oligoclase from Norway, India, and elsewhere is a separate, more sodic feldspar.
What does oligoclase look like?
Usually a white, gray, or cream blocky to tabular feldspar with a glassy-to-pearly luster and fine twin striations on cleavage faces; the gem sunstone variety adds coppery or golden aventurescent glints.
Oligoclase identified by the community
Recent Oligoclase specimens identified with Rock Identifier.
