Sanidine Identification Guide

What Sanidine Looks Like

Sanidine is the high-temperature form of potassium feldspar (KAlSi3O8), crystallizing rapidly in volcanic and shallow intrusive rocks. It is typically colorless, glassy gray, smoky, or pale yellow, and is often transparent to translucent — clearer than most other feldspars. Crystals are tabular to blocky and commonly form prominent phenocrysts (large crystals) set in fine-grained volcanic groundmass. Some sanidine shows a faint bluish or silvery schiller (moonstone-like adularescence). Carlsbad twinning is common, and good crystals display two cleavage directions meeting near 90 degrees.

Step-by-Step Field Checklist

1. Note the host rock. Sanidine occurs as glassy phenocrysts in young volcanic rocks like rhyolite, trachyte, and obsidian — context is a major clue.
2. Check transparency. Look for unusually clear, glassy feldspar crystals, often colorless or pale gray.
3. Find cleavage. Rotate in light to see two flat cleavage planes intersecting at about 90 degrees with a pearly to vitreous sheen.
4. Look for twinning. Carlsbad twins (a crystal appearing split lengthwise) are common; sanidine lacks the fine parallel striations (polysynthetic twinning) of plagioclase.
5. Test hardness. It is Mohs 6, scratches glass, and is not scratched by a steel knife edge easily.
6. Inspect for schiller. Some specimens flash a bluish moonstone-like glow.

Key Diagnostic Tests

• Hardness: 6 — scratches glass.
• Cleavage: two good directions at ~90 degrees (basal and prismatic).
• Luster: vitreous, sometimes pearly on cleavage faces.
• Streak: white.
• Specific gravity: ~2.56–2.62, light.
• Twinning: Carlsbad (simple), not the fine striations of plagioclase.

Common Look-Alikes and How to Tell Them Apart

• Orthoclase/microcline: same composition but lower-temperature; they are usually opaque pink, cream, or white and occur in granites and pegmatites, whereas sanidine is glassy and found in volcanics. Microcline (amazonite) can be green.
• Plagioclase (oligoclase, andesine): shows fine parallel striations (albite twinning) on cleavage faces under good light — sanidine does not.
• Quartz: harder (Mohs 7), has no cleavage and breaks with conchoidal fracture, and shows hexagonal prisms rather than flat cleavage planes.
• Clear glass/obsidian: truly glassy, with conchoidal fracture and no cleavage.
• Adularia/moonstone: a low-temperature feldspar with similar schiller, but tied to hydrothermal veins rather than volcanic phenocrysts.

Where Sanidine Is Found

Sanidine is essentially a volcanic mineral, forming where potassium-rich magma cooled quickly. Classic localities include the trachytes of the Eifel district (Germany), Mont Dore (France), and rhyolites and obsidians of the western United States such as in Nevada and California. It is also a useful index of high crystallization temperatures and is used in argon dating of eruptions.