Alkali Feldspar Identification Guide

What Alkali Feldspar Looks Like

Alkali feldspar is the group of potassium-sodium feldspars: orthoclase, microcline, sanidine, and the sodic end albite, plus the intergrowth perthite. They are among the most abundant minerals in continental crust.

• Color: white, cream, salmon-pink, brick-red, or pale gray; the green variety amazonite is microcline.
• Luster: vitreous, pearly on cleavage faces.
• Transparency: translucent to transparent.
• Habit: blocky prismatic crystals; commonly as grains in granite; perthite shows wavy white/pink streaks (exsolution lamellae). Carlsbad twins (two intergrown crystals) are common.

Step-by-Step Field ID Checklist

1. Find two cleavages near 90 degrees. Blocky stepped cleavage surfaces meeting at right angles mark a feldspar.
2. Check the color. Pink/salmon/red strongly suggests alkali (K) feldspar; pure white is ambiguous (could be plagioclase).
3. Test hardness. Mohs 6–6.5; scratches glass.
4. Look for twin striations — and their absence. Alkali feldspar (except albite) does NOT show the fine albite-twin striations of plagioclase. No striations on cleavage = alkali feldspar; striations = plagioclase.
5. Look for perthite. Irregular wavy intergrowth streaks indicate alkali feldspar.
6. Streak. White.

Key Diagnostic Tests

• Hardness: 6–6.5.
• Cleavage: two directions at or near 90 degrees.
• Specific gravity: ~2.55–2.63 (light).
• Twinning: Carlsbad and (in microcline) microscopic tartan twinning; lacks plagioclase's visible polysynthetic striations.
• Streak: white.
• Acid/magnetism: none.

Common Look-Alikes and How to Tell Them Apart

• Plagioclase feldspar: the key partner to separate. Plagioclase shows fine parallel albite-twin striations on cleavage faces; alkali feldspar (orthoclase/microcline/sanidine) does not. Pink color also favors alkali feldspar.
• Quartz: no cleavage (conchoidal fracture), harder (7), and often glassy gray; alkali feldspar shows two right-angle cleavages and is slightly softer.
• Calcite: soft (3), fizzes in acid, rhombohedral cleavage; feldspar does none of these.
• Nepheline: greasy luster, slightly softer, in silica-poor rocks; lacks feldspar's good cleavage.
• Scapolite/other white silicates: distinguished by cleavage angle, hardness, and association.

Where Alkali Feldspar Is Found

Alkali feldspars are essential constituents of granite, syenite, rhyolite, and granite pegmatites, and are widespread in gneisses and arkosic sandstones. Large crystals (including amazonite microcline and gem orthoclase/sanidine) come from pegmatites in Colorado (USA), Brazil, Madagascar, and Russia; sanidine occurs in young volcanic rocks. Look for blocky pink or white grains making up much of any granitic or felsic rock.