Microcline Identification Guide

What Microcline Looks Like

Microcline is a triclinic potassium feldspar (KAlSi3O8), chemically identical to orthoclase but with a more ordered, lower-temperature structure. It forms blocky, prismatic crystals and large cleavable masses in pegmatites. Colors include white, cream, pale grey, salmon-pink, and — most famously — the blue-green variety amazonite. It has two good cleavages meeting at nearly 90 degrees and a vitreous luster, and often shows fine cross-hatch (tartan) twinning visible under magnification or in thin section.

• Color: white, cream, salmon-pink, pale yellow; green-blue as amazonite
• Luster: vitreous to pearly on cleavage
• Transparency: translucent to opaque
• Habit: blocky/prismatic crystals; large cleavage masses; perthitic intergrowths with albite common

Step-by-Step Field-ID Checklist

1. Confirm it is feldspar: hardness ~6 and two cleavages at ~90 degrees.
2. Look for blocky cleavage: flat reflective surfaces meeting near right angles, with step-like breaks.
3. Check for perthite: wispy, thread-like albite intergrowths (light streaks) in the feldspar.
4. Note color: green-blue strongly suggests amazonite (a microcline).
5. Look for grid/tartan twinning: with a hand lens or in thin section, a cross-hatched pattern is diagnostic of microcline (absent in orthoclase).

Key Diagnostic Tests

• Hardness: 6-6.5; scratches glass, not scratched by a steel knife.
• Cleavage: two good cleavages meeting at ~90 degrees (technically slightly off 90 in triclinic microcline).
• Streak: white.
• Density: ~2.55-2.63.
• Twinning: tartan (cross-hatch) twinning under crossed polars is the definitive separator from orthoclase.
• No acid reaction; no magnetism.

Common Look-Alikes and How to Tell Them Apart

• Orthoclase: same composition and color range (pink/white), but monoclinic and lacks tartan twinning. In hand sample they are very similar; microcline often occurs as larger pegmatite crystals and shows grid twinning microscopically.
• Plagioclase feldspar: shows fine parallel striations (albite twinning lamellae) on cleavage faces; microcline lacks these straight striations (it has cross-hatch twinning instead). Striations vs. tartan is the key.
• Quartz: harder (7) but has no cleavage (conchoidal fracture) and no flat reflective cleavage faces; feldspar cleaves in flat steps. Quartz is also usually more glassy and translucent.
• Amazonite vs other green stones: amazonite's feldspar cleavage and ~90-degree cleavage angles distinguish it from green jade (tougher, no cleavage) and aventurine (quartz, no cleavage).

Where It Is Typically Found

Microcline is a major constituent of granites, granitic pegmatites, and gneisses. The largest crystals come from pegmatites (Colorado, Brazil, Madagascar, Russia). Amazonite is famous from Colorado (Pikes Peak), Virginia, Russia (Kola/Ilmen), and Madagascar. As a stable feldspar it also occurs as detrital grains in arkosic sandstones.