Alkali Feldspar

Alkali feldspar hand specimen, pink hue.

Alkali feldspar hand specimen, milky white.

Simply twinned alkali feldspar in granite.

Alkali feldspar unit cell, CrystalMaker.

Formula

(K,Na)[AlSi₃O₈]

Structure

Framework silicate

Crystal System

Crystal system: monoclinic (-) or triclinic (+) (-)

Physical Properties

Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5

Optical Properties

Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1^st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi₃O₈) displays characteristic cross-hatched twinning

Mineral Specimens

M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.

M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.

M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.

Rock Specimens

L100 Alkali granite
L101 Granite
L102 Hornblende biotite granite
L110 Rhyolite, porphyritic (quartz porphyry)

Mineral Name

alkali feldspar