Alkali feldspar

(K,Na)[AlSi₃O₈]

Framework silicate

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

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

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

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.

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