RocksdayOrthoclase vs Scapolite
How to tell them apart
A potassium feldspar and one of the most common minerals in granite, where it forms the pink or cream-colored blocks.
Photo: Didier Descouens, CC BY-SA 4.0
A group of silicates forming square prisms in metamorphosed limestones. Some glow bright yellow under UV.
Photo: Géry PARENT, CC BY-SA 4.0
Key differences
- Feldspar has two good cleavages at right angles and a blocky shape.
Side by side
| Property | Orthoclase | Scapolite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 6 | 5.5–6 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | Pink, White, Orange | White, Gray, Yellow, Purple |
| Crystal system | Monoclinic | Tetragonal |
| Specific gravity | 2.55–2.63 | 2.5–2.78 |
| Formula | KAlSi₃O₈ | (Na,Ca)₄[Al(Al,Si)Si₂O₈]₃(Cl,CO₃,SO₄) |
| Found in | Granites worldwide; Karlovy Vary, Czech Republic | Ontario, Canada; Madagascar |
Properties that are the same for both are grayed out.
How to identify orthoclase
- Blocky pink to cream crystals in granite.
- Two cleavages meeting at a right angle — flat, shiny faces.
- Hardness 6 — scratches glass with effort.
- No fine parallel striations (unlike plagioclase).
Crystal system: Monoclinic
Full Orthoclase guide →How to identify scapolite
- Square prisms with a slightly greasy luster.
- Often fluoresces yellow under longwave UV.
- Hardness about 6.
Crystal system: Tetragonal
Full Scapolite guide →More comparisons
Could you tell them apart from a blurry close-up?
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