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

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).

Sheet silicate

Crystal system: monoclinic (-)

Colour in hand specimen: brown or black 
Cleavage: one, good 
Hardness: 2-3

Colour in plane polarised light: pale yellow, pale green, green or brown 
Pleochroism: strong 
Relief: low 
Birefringence: 0.028-0.08; 2^nd order green-yellow to >4^th order 
Extinction: straight

M303 Mica, biotite 
Cornwall 
Large irregular biotite crystals in a feathery arrangement 
Dark brown / black / golden 
Metallic lustre 
Platy habit due to the single perfect cleavage

M304 Mica, biotite 
Orlando, Canada 
Irregular plates of biotite split along cleavage planes 
One good cleavage 
Brilliant metallic lustre

L100 Alkali granite 
L101 Granite 
L102 Hornblende biotite granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L504 Garnet schist

Sheet silicate

Crystal system: monoclinic (-)

Colour in hand specimen: colourless 
Cleavage: one, good 
Hardness: 2.5-3

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.036-0.049; 2^nd order pink to 3^rd order yellow 
Extinction: straight

M301 Mica, muscovite 
Lincoln County, North Carolina 
Well-formed tabular crystals showing the pseudohexangonal symmetry 
Colourless, mirror- like, metallic lustre 
Platy, flaky habit 
Soft

M302 Mica, muscovite    
Mysore, India 
Plates of mica artificially cut to shape 
Colourless 
One perfect cleavage

L110 Rhyolite, porphyritic (quartz porphyry) 
L201 Tuff 
L504 Garnet schist

(Na,K)_0-1Ca₂(Mg,Fe²⁺,Fe³⁺,Al)₅Si_6-7.5Al_2-0.5O₂₂(OH)₂

Chain silicate

Crystal system: monoclinic (+) (-)

Colour in hand specimen: black 
Cleavage: two at ~56° 
Hardness: 5-6

Colour in plane polarised light: pale green, green, light yellow-brown to brown 
Pleochroism: yes 
Relief: moderate 
Birefringence: ~0.02; 2^nd order purple 
Extinction: inclined 
Twinning: simple, lamellar, common

M210 Amphibole, hornblende 
Bilen, Bohemia 
Single, black prismatic crystal. 
At the ends of the crystal two sets of cleavage planes are visible, intersecting at ~56°

M211 Amphibolite 
Isle of Coll, Inner Hebrides 
A metamorphic rock consisting almost entirely of crystals of the amphibole hornblende 
Black 
Brilliant lustre 
Two sets of cleavage planes intersect at ~56°

L102 Hornblende biotite granite 
L120 Granodiorite

The amphibole group encompasses a wide range of compositions. Even the hornblendes, which are a sub-group of the amphiboles, encompass a range of compositions (see mineral formula, above). The properties above refer to hornblende.