Framework silicate

Crystal system: trigonal (+)

Colour in hand specimen: colourless 
Lustre: vitreous 
Pleochroism: none 
Cleavage: none 
Hardness: 7

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.009; 1^st order grey

M410 Quartz 
Characteristic prismatic crystal form: hexagonal cross-section, needle-like; hexagonal prism surmounted by a hexagonal pyramid 
Colourless, transparent, milky in places 

M411 Quartz 
Madagascar 
Colourless, transparent quartz fragment, milky due to cracks and crystal boundaries 
Vitreous lustre, conchoidal fracture, no cleavage

M412 Quartz (with galena and pyrite) 
North Wales 
A mass of small hexagonal quartz prisms, with galena, pyrite and zinc blende. 
Transparent, white to light grey 
Galena: purple-black, shiny, platy habit, metallic 
Pyrite: metallic lustre, gold colour, cubic habit, “Fools’ gold”

L101 Granite 
L110 Rhyolite, porphyritic (quartz porphyry) 
L120 Granodiorite 
L311 Conglomerate, matrix 
L323 Sandstone, arenite 
L370 Chert, flint nodule, cryptocrystalline 
L504 Garnet schist 
L521 Quartzite

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).