Feldspar (Orthoclase & Plagioclase)
KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈ · Monoclinic / Triclinic · also called potassium feldspar, K-spar, plagioclase, moonstone, labradorite, amazonite
Feldspar is the most abundant mineral group in the Earth's crust — the pink and white grains that make up most of a granite. Separating it from quartz is the single most useful skill in identifying igneous rock: feldspar has flat cleavage faces and is slightly softer, quartz has curved fracture and no flat faces.
Properties
- Mohs hardness
- 6–6.5
- Streak
- white
- Lustre
- vitreous, pearly
- Colour
- Salmon pink and creamy white are the classic colours. Green feldspar is amazonite; the blue-flashing variety is labradorite.
- Cleavage
- Two good cleavages meeting at very close to 90°, producing flat, stepped, reflective faces. Tilt the specimen in sunlight and whole facets flash at once.
- Fracture
- Uneven where cleavage does not control the break
- Specific gravity
- 2.55–2.76 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm feldspar
- Flat, mirror-flat cleavage faces that flash as a single plane — quartz never shows this.
- Hardness 6: it scratches glass, but quartz scratches it.
- Plagioclase often shows fine parallel striations, like the grooves on a record, on one cleavage face.
What feldspar gets confused with
Each of these is settled by one test. Run that test first.
Varieties
- Labradorite
- Grey plagioclase that flashes blue and gold when tilted — an optical effect from internal layering.
- Amazonite
- Blue-green microcline, coloured by trace lead.
- Moonstone
- Shows a floating blue-white sheen (adularescence).
Where it is found
Granite, gneiss, and pegmatite; the pink or white blocky grains in almost any coarse igneous rock.
Is it worth anything?
No value as ordinary rock-forming material. Labradorite with strong blue flash, amazonite, and moonstone are cut as gems and sell modestly.