Albite Identification Guide
A field guide to albite, the sodium end-member plagioclase feldspar, using its twin striations, two cleavages, hardness, and white color.
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What Albite Looks Like
Albite is the sodium-rich end member of the plagioclase feldspar series (NaAlSi3O8). It is an extremely common rock-forming mineral.
- Color: white to colorless, occasionally pale gray, bluish, or greenish; the platy variety cleavelandite is bright white.
- Luster: vitreous, pearly on cleavage faces.
- Transparency: transparent to translucent.
- Habit: tabular or bladed crystals; in pegmatites the platy 'cleavelandite' fans are diagnostic; commonly massive or as grains in rock. Peristerite albite can show a blue moonstone-like sheen.
Step-by-Step Field ID Checklist
- Find the cleavage. Two good cleavages meeting at close to 90 degrees (about 86 degrees) produce blocky stepped surfaces — the feldspar signature.
- Look for albite twinning. Fine, parallel striations (grooves) on the best cleavage face are diagnostic of plagioclase and absent in K-feldspar. Use a hand lens and rotate in light.
- Test hardness. Mohs 6–6.5; scratches glass, is scratched by quartz.
- Check color and form. White, often tabular or bladed; cleavelandite forms curved white plates.
- Streak. White.
- Heft. Light, SG ~2.6.
Key Diagnostic Tests
- Hardness: 6–6.5.
- Cleavage: two directions at ~86–90 degrees (the name plagioclase means 'oblique fracture').
- Twinning: polysynthetic albite twinning shows as fine striations — the key to separating it from orthoclase/microcline.
- Specific gravity: ~2.62 (lowest of the plagioclase series).
- Streak: white.
- Acid: none; not magnetic.
Common Look-Alikes and How to Tell Them Apart
- Potassium feldspar (orthoclase/microcline): also white-to-pink with two cleavages, but K-feldspar lacks the fine albite twin striations. Striations on the cleavage = plagioclase (albite); none = K-feldspar. Microcline may show grid (tartan) twinning only in thin section.
- Quartz: harder (7) and has NO cleavage (conchoidal fracture only); quartz won't show the blocky 90-degree cleavage steps.
- Calcite: softer (3), fizzes in acid, and has rhombohedral cleavage at non-90 angles.
- Other plagioclase (oligoclase–anorthite): chemically distinct but visually similar; albite is the most sodic and lightest, often confirmed only by optics or SG.
- Barite: higher SG (heavy) and different cleavage; albite feels light.
Where Albite Is Found
Albite is ubiquitous: in granites, syenites, and especially granite pegmatites (where cleavelandite and gem albite occur), in low-grade metamorphic rocks (it defines the greenschist facies), in Alpine fissures, and in sodium-metasomatized rocks (albitite, adinole). Fine crystals come from pegmatites in Brazil, Pakistan, and the USA, and from Alpine clefts in Switzerland and Italy. Look for white tabular crystals or platy cleavelandite among quartz, micas, and tourmaline.
Frequently asked questions
How can you tell if a mineral is albite?
Look for two cleavages near 90 degrees, fine parallel twin striations on the best cleavage face (the plagioclase signature), hardness 6–6.5, white color, and a white streak.
What is the difference between albite and orthoclase?
Both are feldspars with two near-90-degree cleavages, but albite (a plagioclase) shows fine albite-twin striations on its cleavage faces, while orthoclase (a potassium feldspar) does not.
Albite vs quartz — how are they different?
Albite has two good cleavages and a hardness of 6–6.5, while quartz has no cleavage (only conchoidal fracture) and is harder at 7. The blocky cleavage steps indicate feldspar.
What is cleavelandite?
Cleavelandite is a platy, lamellar variety of albite that forms fan-like bright-white plates, common in granite pegmatites.
Is albite a plagioclase feldspar?
Yes. Albite is the pure sodium end member of the plagioclase series, which ranges from albite (sodium) to anorthite (calcium).