NaCl
Non-silicate
Crystal system: cubic
Colour in hand specimen: colourless or white when pure, more typically orange or red due to inclusions of iron compounds, and may also be grey, yellow or blue
Cleavage: perfect cubic, three planes intersect at 90°
Hardness: 2.5
NaCl
Non-silicate
Crystal system: cubic
Colour in hand specimen: colourless or white when pure, more typically orange or red due to inclusions of iron compounds, and may also be grey, yellow or blue
Cleavage: perfect cubic, three planes intersect at 90°
Hardness: 2.5
Non-silicate
Crystal system: cubic
Colour in hand specimen: extremely variable: colourless, white, yellow, green, blue and purple most common
Cleavage: perfect octahedral, up to four visible at one time, breaks into dipyramids
Hardness: 4
Colour in plane polarised light: colourless, pale green or pale violet
Pleochroism: none
Relief: low
M560 Fluorite
Derbyshire
Cubic fluorite crystals
Generally very variable; in this case white to purple
Glassy lustre
Hardness 4
Octahedral cleavage
Non-silicate
Crystal system: cubic
Colour in hand specimen: extremely variable: colourless, white, yellow, green, blue and purple most common
Cleavage: perfect octahedral, up to four visible at one time, breaks into dipyramids
Hardness: 4
Colour in plane polarised light: colourless, pale green or pale violet
Pleochroism: none
Relief: low
M560 Fluorite
Derbyshire
Cubic fluorite crystals
Generally very variable; in this case white to purple
Glassy lustre
Hardness 4
Octahedral cleavage
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
Non-silicate
Crystal system: trigonal (-)
Colour in hand specimen: white, often tinged with yellow or brown
Cleavage: rhombohedral
Hardness: 3.5-4
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: 0.179-(0.185); >4th order
Extinction: inclined
Twinning: lamellar, common
M543 Dolomite, associated with siderite and copper pyrites
Cerro Blanco, Chile
Rhombohedral crystals of dolomite with curved faces, associated with siderite (iron carbonate) and copper pyrite
White, slightly grey
Hardness 3.5 – 4
Non-silicate
Crystal system: trigonal (-)
Colour in hand specimen: white, often tinged with yellow or brown
Cleavage: rhombohedral
Hardness: 3.5-4
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: 0.179-(0.185); >4th order
Extinction: inclined
Twinning: lamellar, common
M543 Dolomite, associated with siderite and copper pyrites
Cerro Blanco, Chile
Rhombohedral crystals of dolomite with curved faces, associated with siderite (iron carbonate) and copper pyrite
White, slightly grey
Hardness 3.5 – 4
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); >4th 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
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); >4th 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
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; 3rd order blue
Extinction: straight
Twinning: simple or repeated
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; 3rd order blue
Extinction: straight
Twinning: simple or repeated
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∙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
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”
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”
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”
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”
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