Plagioclase Identification Guide

What Plagioclase Looks Like

Plagioclase is a rock-forming feldspar series running from albite (sodium-rich) to anorthite (calcium-rich). In the field it most often appears as white, gray, or bluish-gray blocky to tabular grains, though it can be greenish or, in labradorite, show a flashy blue-green play of color. Luster is vitreous to subvitreous, and crystals are translucent to nearly opaque. Habit is typically stubby prismatic or tabular crystals, but in granites and gabbros it shows up as interlocking grains rather than free crystals.

Step-by-Step Field ID Checklist

1. Confirm it's a feldspar. Look for two cleavage directions meeting at close to 90 degrees and a hardness around 6.
2. Hunt for striations. Tilt a flat cleavage face in the light. Fine, parallel albite twinning lines (like tiny scratches or stripes) are the diagnostic signature of plagioclase and are absent in alkali feldspar.
3. Check the color and host rock. Gray plagioclase in a dark igneous rock (basalt, gabbro, diorite, andesite) strongly favors plagioclase over pink K-feldspar.
4. Look for a play of color. A blue, green, or gold schiller (labradorescence) indicates calcic plagioclase (labradorite/bytownite).

Key Diagnostic Tests

• Mohs hardness: 6 to 6.5 — will scratch glass but not quartz.
• Cleavage: Two good directions at about 86–94 degrees, producing stepped, blocky surfaces.
• Streak: White.
• Specific gravity: 2.62 (albite) to 2.76 (anorthite).
• Twinning: Polysynthetic albite twinning visible as striations is the single best diagnostic.
• Acid: No reaction to dilute HCl (distinguishes from carbonate look-alikes).

Common Look-Alikes and How to Tell Them Apart

• Orthoclase / microcline (alkali feldspar): Often pink or cream and lack albite striations; instead they may show simple Carlsbad twinning or perthitic streaks. If you see fine parallel striations on cleavage, it's plagioclase.
• Quartz: No cleavage (conchoidal fracture), harder (7), glassy, and forms hexagonal points rather than blocky cleaved grains.
• Calcite: Softer (3), fizzes in acid, rhombohedral cleavage — easy to separate.
• Nepheline: Greasy luster, slightly softer, found in silica-poor rocks without quartz.

Where Plagioclase Is Found

Plagioclase is one of the most abundant minerals in Earth's crust. It dominates basalt, gabbro, diorite, andesite, and anorthosite, and is a major component of granite and gneiss. Labradorite-rich varieties come from Labrador (Canada), Finland, and Madagascar; gem albite and oligoclase occur in pegmatites worldwide.