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Mineral group · Silicate · Tectosilicate (framework silicate)

Feldspar

(K,Na,Ca)(Al,Si)₄O₈

The most abundant mineral group in Earth’s crust — the pink in granite, the white in basalt, and the flash in moonstone and labradorite.

Hardness6–6.5
Crystal systemTriclinic
LusterVitreous
Sp. gravity2.55–2.76
ClassSilicate
←PrevFelbertaliteNextFelsőbányaite→
Feldspar specimen

Salmon-pink feldspar blocks in granite, beside glassy gray quartz — Photo: Robert M. Lavinsky · CC BY-SA 3.0

What is Feldspar?

Feldspar is not one mineral but a family — and the most successful one on Earth. Together, feldspars make up roughly 60% of Earth’s crust: the pink blocks in granite, the white laths in basalt, the sand between your toes, even the bright highlands of the Moon. If a rock exists, feldspar is probably in it.

All feldspars share one recipe: a 3-D framework of aluminium and silicon tetrahedra with room for a large cation — potassium, sodium or calcium. That single letter makes the family: KAlSi₃O₈ is the potassium branch (orthoclase, microcline, sanidine), NaAlSi₃O₈ is albite, and CaAl₂Si₂O₈ is anorthite — with sodium and calcium mixing freely in between to give the plagioclase series.

The name is practical rather than poetic: German miners called it Feldspat, the ‘field spar’ — the cleavable mineral weathering out of rocks into tilled fields.

In one sentence: Two flat cleavages meeting at about a right angle, hardness around 6, white streak — that trio says feldspar. Then check for striations: fine parallel lines mean plagioclase, smooth faces mean potassium feldspar.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Cleavage at right angles. Every feldspar breaks along two flat planes — {001} and {010} — that meet at or very near 90°. It is the family signature and the fastest field test: quartz chips, feldspar splits.

Two branches, one name. Alkali feldspar (K + Na) stays monoclinic or relaxes to triclinic on cooling; plagioclase (Na + Ca) is always triclinic and twins repeatedly, leaving fine parallel striations on its cleavage faces — the easiest way to tell the two branches apart by eye.

Light effects. When a cooling feldspar unmixes into microscopic layers, the layers scatter light: a soft floating glow gives moonstone (adularescence), a broad peacock flash gives labradorite (labradorescence), and included metal platelets make sunstone glitter (aventurescence).

Hardness and heft. All feldspars sit near 6 on the Mohs scale and 2.55–2.76 g/cm³ — hard enough to survive as sand, soft enough to weather into clay.

Members

Same chemistry, different faces — trace impurities and how finely it crystallizes give a whole family of familiar stones.

Orthoclase
Orthoclase
Pink to white · KAlSi₃O₈ — monoclinic K-feldspar of plutonic rocks
Microcline
Microcline
Green to pink · Triclinic K-feldspar; tartan twinning
Sanidine
Sanidine
Colorless, glassy · High-temperature K-feldspar of volcanic rocks
Albite
Albite
White · NaAlSi₃O₈ — sodium endmember
Oligoclase
Oligoclase
White to gray · Plagioclase An₁₀–An₃₀
Andesine
Andesine
White to gray · Plagioclase An₃₀–An₅₀
Labradorite
Labradorite
Gray with schiller · Plagioclase An₅₀–An₇₀
Bytownite
Bytownite
Gray · Plagioclase An₇₀–An₉₀
Anorthite
Anorthite
White to gray · CaAl₂Si₂O₈ — calcium endmember
Amazonite
Amazonite
Blue-green · Microcline colored by lead and water
Moonstone
Moonstone
White with blue sheen · Adularescent orthoclase/albite intergrowth
Sunstone
Sunstone
Orange to red · Aventurescent feldspar with hematite or copper platelets

How & Where It Forms

Feldspars crystallize first or second from almost every cooling magma. Calcium-rich plagioclase leads in basalt and gabbro; potassium feldspar arrives later in granite and syenite; pegmatites grow crystals metres long. The exact member records the magma’s chemistry — a dark basalt carries anorthite-rich plagioclase, while a granite brims with orthoclase and albite.

They persist through metamorphism in gneisses, then break down at the surface: feldspar weathering is the planet’s main factory for clay minerals such as kaolinite, which is why soils and sediments are full of it even after the crystals are gone.

Often grows alongside:

QuartzMuscoviteBiotiteHornblendeAugiteOlivine

What It’s Used For

Glass & ceramicsThe main industrial use — feldspar is the flux that lowers melting points in glass, porcelain, glazes and tiles.
FillersGround feldspar bulks and strengthens paint, plastics, rubber and adhesives.
GemstonesMoonstone, labradorite, sunstone and amazonite are all feldspars in disguise.
Mild abrasivesPowdered feldspar scours in household cleaners and soaps.
Dating rocksPotassium-bearing feldspars power K-Ar and ⁴⁰Ar/³⁹Ar geochronology.

How to Identify It

Scratch and split. Feldspar barely scratches glass, leaves a white streak, and breaks along two flat faces at ~90° — never the curved chips of quartz. Smooth cleavage faces with Carlsbad twins point to K-feldspar; fine parallel striations point to plagioclase; a flash of blue-green, floating sheen or golden glitter points to the gem varieties.

Two cleavages at or near a right angle — the ‘feld’ testHardness 6–6.5; scratches glass with effort, scratched by quartzWhite streakStriations on cleavage = plagioclase; smooth faces = K-feldsparSchiller flashes mark labradorite, moonstone, sunstone
Looks like…But differs because…Quick test
QuartzHarder (7), no cleavage, breaks conchoidally into curved chipsCleavage & scratch
CalciteMuch softer (3), rhombohedral cleavage, and effervesces in dilute acidVinegar drop
TopazHarder (8) with a single perfect basal cleavage, not two at 90°Cleavage count
ApatiteSofter (5), hexagonal prisms, no twin striationsHardness & shape

Where It’s Found

Notable localities for fine specimens:

Nain–Paul's Island, Canada Ilmen Mountains, Russia Meetiyagoda, Sri Lanka Pikes Peak–Crystal Peak, USA Arendal, Norway Larvik, NorwayOpen the locality map →

References & Further Reading

  1. Dana’s New Mineralogy, 8th ed. Feldspar group, tectosilicates.
  2. Mindat.org — Feldspar group mineral data and locality records.
  3. Wikipedia — Feldspar.

Last reviewed 2026-09-14 · How to cite this page.