Feldspar Identification Guide

What Feldspar Looks Like

Feldspar is not a single mineral but a family of aluminosilicates — the most abundant mineral group in Earth's crust. It splits into alkali feldspars (orthoclase, microcline, sanidine) and plagioclase feldspars (albite through anorthite). In the field you will see blocky, opaque to translucent crystals or grains with a vitreous to slightly pearly luster.

• Color: white, cream, pink, salmon, grey, or green (amazonite); plagioclase often white to grey
• Luster: vitreous on fresh faces, pearly on cleavage surfaces
• Transparency: translucent to opaque (rarely transparent)
• Habit: blocky prismatic crystals; common as grains in granite and gneiss

Step-by-Step Field ID Checklist

1. Find a flat reflective face and rotate it in light — feldspar shows flat cleavage planes that flash as you tilt.
2. Look for two cleavage directions meeting at about 90° (the name plagioclase means "oblique fracture," but the angle is still close to right).
3. Check for twinning striations: fine parallel lines on a cleavage face indicate plagioclase (albite twinning).
4. Test hardness against glass and a steel file.
5. Note the color and host rock to narrow the species.

Key Diagnostic Tests

• Mohs hardness 6–6.5: Feldspar scratches glass but a quartz crystal or hardened steel will scratch it. This is the hardness benchmark at 6 on the Mohs scale.
• Cleavage: Two good cleavages intersecting at or near 90° — the single most reliable feldspar test.
• Streak: White.
• Striations: Parallel grooves on cleavage = plagioclase; their absence with simple Carlsbad twins = alkali feldspar.
• Density: 2.55–2.76 g/cm³, increasing from albite to anorthite.

Common Look-Alikes and How to Tell Them Apart

• Quartz: Quartz is harder (Mohs 7) and has no cleavage — it breaks with curved conchoidal surfaces, never flat reflective planes. Feldspar's flat cleavage flashes are diagnostic.
• Calcite: Much softer (Mohs 3), fizzes vigorously in dilute acid, and has rhombohedral cleavage at oblique angles. Feldspar is inert to acid and far harder.
• Nepheline: Greasier luster, slightly softer, and found in silica-poor rocks; lacks feldspar's clean right-angle cleavage.
• Barite: Heavier (density ~4.5) and softer; feels noticeably denser in the hand.

Where It Is Typically Found

Feldspar is ubiquitous: it dominates granite, syenite, gneiss, and many sandstones, and forms large crystals in pegmatites. Look for pink/salmon alkali feldspar in granite and grey/white plagioclase laths in basalt and diorite. Gem and collector varieties — amazonite, moonstone, labradorite, sunstone — come from pegmatites and specific igneous bodies worldwide, including Brazil, Madagascar, India, and the western United States.