Anorthite Identification Guide
How to identify anorthite, the calcium end-member plagioclase feldspar, by its twinning striations, hardness, and the mafic rocks that host it.
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What Anorthite Looks Like
Anorthite is the calcium-rich end-member of the plagioclase feldspar series (An90–An100, CaAl2Si2O8). It is usually white, gray, or colorless, occasionally tinted reddish or greenish, with a vitreous luster and translucent-to-transparent appearance. It forms blocky, lath-shaped, or tabular crystals (triclinic system) and is most often seen as grains within dark igneous rocks rather than as showy crystals. Like all plagioclase, it shows polysynthetic twinning — fine parallel striations on cleavage faces.
Step-by-Step Field-ID Checklist
- Confirm feldspar — blocky habit, flat reflective cleavage surfaces, glassy luster.
- Find two cleavages meeting near 90° — the feldspar signature.
- Look for twinning striations — fine parallel grooves on a cleavage face confirm plagioclase (not alkali feldspar).
- Scratch test — Mohs 6–6.5; scratches glass, not scratched by a knife.
- Check the host rock — anorthite favors calcium-rich, mafic and metamorphic rocks (gabbro, anorthosite, basalt, contact-metamorphosed limestone).
Key Diagnostic Tests
- Mohs hardness: 6–6.5.
- Streak: white.
- Cleavage: two good cleavages intersecting at ~94° (near right angles).
- Specific gravity: ~2.74–2.76 — the highest of the plagioclase series (calcium-rich), slightly denser than albite.
- Plagioclase twinning striations present.
- Inert to dilute acid, non-magnetic.
Common Look-Alikes
- Albite and other plagioclase (oligoclase, andesine, labradorite): the same series; anorthite is the calcium end and is separated only by composition (optics, slightly higher SG). Labradorite often shows blue schiller that anorthite usually lacks.
- Alkali feldspar (orthoclase/microcline): lacks plagioclase twinning striations; microcline may show tartan cross-hatching. Striations = plagioclase.
- Quartz: harder (Mohs 7) and has no cleavage — only conchoidal fracture, versus feldspar's flat cleavage planes.
- Scapolite / nepheline: found in similar rocks but differ in cleavage and crystal form; nepheline is greasy-lustered and gelatinizes in acid (feldspar does not).
Where It Is Found
Anorthite is a rock-forming mineral of calcium-rich, silica-poor igneous rocks — gabbro, anorthosite, and basalt — and of contact-metamorphosed (skarn) limestones. It is also abundant in lunar highland rocks and meteorites. Terrestrial localities include volcanic ejecta at Monte Somma–Vesuvius (Italy), Japan (Miyake-jima), and many layered mafic intrusions worldwide.
Frequently asked questions
How can you tell if it's anorthite?
Confirm it is plagioclase feldspar: Mohs 6–6.5, two cleavages near 90 degrees, and fine parallel twinning striations on a cleavage face. Anorthite is the calcium end-member, identified precisely by optics or its slightly higher density (SG ~2.75) and its presence in calcium-rich mafic rocks.
What does anorthite look like?
It usually appears as white, gray, or colorless blocky-to-tabular feldspar grains with a glassy luster inside dark igneous rocks like gabbro and basalt, showing flat cleavage faces with fine parallel striations.
Anorthite vs albite — what's the difference?
They are the two end-members of the plagioclase series. Anorthite is calcium-rich (An90–100) and slightly denser; albite is sodium-rich (An0–10). They look similar, so composition (optics or density) separates them.
How is anorthite different from orthoclase?
Anorthite is a calcium plagioclase showing twinning striations, while orthoclase is a potassium alkali feldspar that lacks those striations. The presence of fine parallel striations on a cleavage face indicates plagioclase like anorthite.