Plagioclase feldspar

Na[AlSi₃O₈] – Ca[Al₂Si₂O₈]

Framework silicate

Crystal system: triclinic (+) (-)

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

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: ~0.010, 1^st order grey 
Extinction: inclined 
Twinning: lamellar, very common

M403 Plagioclase feldspar, oligoclase 
Ytterby, Sweden 
A cleavage fragment of white plagioclase feldspar 
Pearly lustre on cleavage planes 
Two sets of cleavage at 90° 
Fine parallel striations due to lamellar twinning

M404 Plagioclase feldspar, labradorite 
Labrador 
Dark grey, light where fragments are small due to cracking along cleavage 
Schiller (blue iridescence), characteristic of labradorite 
Very fine parallel striations due to lamellar twinning

L100 Alkali granite 
L101 Granite 
L102 Hornblende biotite granite 
L112 Rhyolite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L141 Gabbro 
L162 Basalt, porphyritic 
L172 Ultramafic cumulate 
L504 Garnet schist

Plagioclase feldspar forms a continuous solid solution between the sodic endmember (albite, or Ab) and the calcic endmember (anorthite, or An). The series is sub-divided into albite (An_0-10), oligoclase (An_10-30) andesine (An_30-50), labradorite (An_50-70), bytownite (An_70-90) and anorthite (An_90-100).