Bytownite Identification Guide

What Bytownite Looks Like

Bytownite is a calcium-rich plagioclase feldspar in the albite–anorthite series (composition An70–90). Gem-quality bytownite is famous as a transparent straw-yellow to golden, pale champagne, or colorless faceting material; rock-forming bytownite is more often white, gray, or pale yellow. Luster is vitreous, sometimes pearly on cleavage faces; transparency ranges from transparent (gem) to translucent/opaque (rock).

Crystal habit / form

Triclinic plagioclase. It shows two cleavage directions meeting at close to 90° and, on cleavage surfaces, fine parallel albite-twinning striations (alternating light/dark lines)—a hallmark of plagioclase. It occurs as grains and blocky crystals in mafic igneous rocks, and as gem crystals in some volcanic deposits.

Step-by-Step Field ID Checklist

1. Look for plagioclase twinning striations on a cleavage face (fine parallel lines)—the key plagioclase clue.
2. Check cleavage—two directions near 90°.
3. Note color—straw/golden-yellow gem material or pale gray-white rock feldspar.
4. Test hardness. Mohs ~6–6.5; scratches glass.
5. Assess transparency—gem bytownite is clean and yellow; tilt for any faint sheen.
6. Streak—white.

Key Diagnostic Tests

• Mohs hardness: ~6–6.5. Scratches glass; not scratched by a knife.
• Streak: White.
• Cleavage: Two good directions at ~86–94°; twinning striations on one set.
• Density: ~2.7 g/cm³ (slightly higher than K-feldspar's ~2.56).
• Acid: Inert.
• Twinning: Fine albite-law striations distinguish plagioclase from K-feldspar.

Common Look-Alikes and How to Tell Them Apart

• Other plagioclase (labradorite, andesine, anorthite): Bytownite sits between labradorite and anorthite in composition; precise separation requires optical or chemical analysis, but gem bytownite's clean straw-yellow color is distinctive (labradorite usually shows blue/green schiller, which bytownite typically lacks).
• Orthoclase/K-feldspar: Lacks albite twinning striations and is slightly less dense; cleavage faces are plain. Yellow citrine-like color can overlap—look for the striations.
• Citrine/yellow quartz: Harder (Mohs 7), no cleavage (conchoidal fracture), no twinning striations.
• Yellow beryl (heliodor): Harder (7.5–8), hexagonal, no feldspar cleavage.
• Scapolite: Yellow and similar hardness, but tetragonal with different cleavage and often fluorescent.

Where It Is Found

Bytownite is named for Bytown (now Ottawa), Canada, and forms in calcium-rich mafic igneous rocks—gabbro, norite, anorthosite, and basalt. Gem-quality straw-yellow bytownite comes notably from Mexico (Chihuahua) and the United States (Oregon), plus other localities. Rock-forming bytownite is widespread in mafic intrusions worldwide.