Non-silicate
Crystal system: hexagonal (-)
Colour in hand specimen: green, white, yellow, blue brown etc.
Hardness: 5
Colour in plane polarised light: generally colourless
Pleochroism: weak to moderate in coloured varieties
Relief: moderate
Birefrinence: 0.001-0.007; first order greys
Extinction: straight
Form: commonly hexagonal in cross-section
Non-silicate
Crystal system: hexagonal (-)
Colour in hand specimen: green, white, yellow, blue brown etc.
Hardness: 5
Colour in plane polarised light: generally colourless
Pleochroism: weak to moderate in coloured varieties
Relief: moderate
Birefrinence: 0.001-0.007; first order greys
Extinction: straight
Form: commonly hexagonal in cross-section
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
Magnetite is the most abundant oxide in igneous rock.
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
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; 1st 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”
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; 1st 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”
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, 1st 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
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 (An0-10), oligoclase (An10-30) andesine (An30-50), labradorite (An50-70), bytownite (An70-90) and anorthite (An90-100).
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, 1st 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
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 (An0-10), oligoclase (An10-30) andesine (An30-50), labradorite (An50-70), bytownite (An70-90) and anorthite (An90-100).
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; 2nd order green-yellow to >4th 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
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; 2nd order green-yellow to >4th 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
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; 2nd order pink to 3rd 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
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; 2nd order pink to 3rd 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
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(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; 2nd 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°
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.
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(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; 2nd 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°
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.