Anorthoclase Identification Guide

What Anorthoclase Looks Like

Anorthoclase is a sodium-rich alkali feldspar [(Na,K)AlSi3O8] that sits compositionally between albite and sanidine. In the field it is usually colorless, white, gray, or pale cream, sometimes with a faint smoky or yellowish cast. Gem and collector material from Norway and Mongolia can show a soft blue, silvery, or gold floating sheen (schiller/aventurescence) when tilted to the light.

• Luster: vitreous, sometimes slightly pearly on cleavage faces
• Transparency: transparent to translucent
• Crystal habit: stout prismatic or tabular crystals; large rhomb-shaped phenocrysts ("rhomb porphyry") are diagnostic in some volcanic rocks
• Diagnostic clue: fine cross-hatched (tartan) twinning visible under magnification, similar to but finer than microcline

Step-by-Step Field ID Checklist

1. Confirm it is a feldspar: look for two cleavage directions meeting at roughly 90 degrees, glassy luster, and hardness near 6.
2. Check the host rock: anorthoclase is typical of sodium-rich volcanics (trachyte, phonolite) and the classic rhomb porphyry lavas of the Oslo Rift.
3. Look for rhomb-shaped phenocrysts: large diamond/rhomb outlines in a fine groundmass strongly suggest anorthoclase.
4. Tilt for schiller: rotate the specimen under a point light; a low, milky blue or golden sheen supports anorthoclase (a moonstone-type effect).
5. Magnify the surface: very fine cross-hatch (tartan) twinning indicates a triclinic alkali feldspar (anorthoclase or microcline).

Key Diagnostic Tests

• Mohs hardness: 6 to 6.5; scratches glass, not scratched by a steel knife easily
• Streak: white
• Cleavage: perfect in one direction, good in a second, intersecting near 90 degrees (a slightly oblique angle reflects the triclinic symmetry)
• Fracture: uneven to conchoidal on broken surfaces
• Specific gravity: about 2.56 to 2.62, typical feldspar heft
• No reaction to dilute acid and no magnetism
• Optical/lab note: positive ID often requires the recognition of anorthoclase's fine twinning and low optic angle; XRD or thin section is definitive

Common Look-Alikes and How to Tell Them Apart

• Sanidine: the high-temperature monoclinic alkali feldspar of similar volcanics. Sanidine is clearer and glassier and lacks the cross-hatch twinning; anorthoclase shows fine tartan twinning under the microscope.
• Microcline/orthoclase: these are potassium-rich; anorthoclase is sodium-dominant and tends to occur in volcanic, not granitic/pegmatitic, hosts. Microcline's tartan twinning is coarser.
• Moonstone (orthoclase/albite): a moonstone sheen can mimic anorthoclase schiller, but moonstone's adularescence is a deeper blue billow; check the host and twinning.
• Labradorite: a calcic plagioclase that shows broad spectral labradorescence rather than the soft monochrome sheen of anorthoclase.

Where Anorthoclase Is Typically Found

Classic localities include the rhomb porphyries of the Oslo Rift, Norway; Mount Erebus, Antarctica (famous large phenocrysts); Pantelleria, Italy; and gem-quality sheen material from Mongolia and Kenya. Look for it in alkaline volcanic and shallow intrusive rocks rather than ordinary granite.