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).

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

Fe₂O₃

Non-silicate

Crystal system: trigonal (-)

Colour in hand specimen: black, steel-grey and bright to dull red 
Streak: cherry-red 
Cleavage: none 
Hardness: 5-6

Colour in plane polarised light: opaque; blood-red in very thin splinter 
Pleochroism: none 
Relief: high

M500 Hematite 
Cumberland 
A concretionary mass showing concentric and radial structure 
Silvery black with a red streak 
Oxidised to dark brown-red in places 
Metallic lustre

L310 Breccia 
L311 Conglomerate 
L324 Sandstone, feldspathic arenite 
L326 Sandstone, ferruginous arenite 
L340 Mudstone 
L354 Limestone, shelly
L500 Slate

Fe₃O₄

Non-silicate

Crystal system: cubic

Colour in hand specimen: black
Streak: black
Cleavage: none
Hardness: 7.5-8

Colour in plane polarised light: almost opaque
Relief: high
Isotropic

M501 Magnetite 
Brazil 
Crystals showing the characteristic octahedral form 
Silver-black colour 
Metallic lustre

M502 Magnetite 
Hay Tor, Devon 
A mass of small octahedral crystals embedded in a metamorphic rock 
Silver-black 
Metallic lustre 
No cleavage

L102 Hornblende biotite granite
L120 Granodiorite, accessory 
L141 Gabbro, olivine decomposition, accessory 
L162 Basalt, porphyritic, feldspar inclusion 
L172 Ultramafic cumulate, olivine alteration

Non-silicate

Crystal system: cubic

Colour in hand specimen: pale brassy yellow; black when fine-grained
Streak: greenish or brownish
Lustre: metallic
Cleavage: not obvious
Hardness: 6-6.5

Colour in plane polarised light: opaque

M520 Slate containing pyrite
Coniston, Lancashire 
Cubic crystals, embedded in a well-cleaved slate 
Golden colour 
Metallic lustre 
Indistinct cleavage in places

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”

L112 Rhyolite 
L371 Chert, bedded

PbS

Non-silicate

Crystal system: cubic

Colour in hand specimen: lead grey
Streak: lead grey
Lustre: metallic
Cleavage: one, perfect
Hardness: 2.5

Colour in plane polarised light: opaque
Relief: high

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”

CaSO₄∙2H₂0

Non-silicate

Crystal system: monoclinic (+)

Colour in hand specimen: usually white or colourless
Cleavage: rhombohedral, one perfect, two distinct
Hardness: 2

M530 Gypsum 
Shotover, Oxford 
Single euhedral crystal 
Colourless, transparent 
Hardness 2; Soft – scratched with your fingernail

M531 Massive gypsum 
Nottingham 
Fibrous habit – many fine crystals 
White-cream colour 
Translucent 
Pearly lustre

CaSO₄

Non-silicate

Crystal system: orthorhombic (+)

Colour in hand specimen: white or colourless when pure 
Cleavage: two at 90° 
Hardness: 3-3.5

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: ~0.04; 3^rd order blue 
Extinction: straight 
Twinning: simple or repeated

CaCO₃

Non-silicate

Crystal system: trigonal (-)

Colour in hand specimen: white
Cleavage: rhombohedral
Hardness: 3

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence:0.172-(0.190); >4^th order 
Extinction: inclined 
Twinning: lamellar, common

M540 Calcite with banded hematite 
Egremont, Cumberland 
Mass of acicular (needle-like) calcite crystals 
Colourless with some grey and red impurities

M541 Calcite 
Iceland spar 
Iskifjördhr, Iceland 
Single crystal of calcite 
Colourless, transparent 
Hardness 3 
Observe the double refraction 
Rhombohedral cleavage (three cleavages)

M542 Calcite vein material 
Minsterley, Shropshire 
Rhombohedral cleavage (three cleavages) 
Pearly lustre 
Hardness 3

L110 Rhyolite, porphyritic (quartz porphyry) 
L130 Andesite, porphyritic (porphyrite) 
L161 Basalt, amygdaloidal
L325 Sandstone, calcareous cement 
L341 Mudstone, calcareous 
L350 Limestone, argillaceous 
L353 Limestone, oolitic, packstone 
L371 Chert, bedded