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
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
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).
(K,Na)[AlSi3O8]
Framework silicate
Crystal system: monoclinic (-) or triclinic (+) (-)
Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi3O8) displays characteristic cross-hatched twinning
M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.
M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.
M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.
(K,Na)[AlSi3O8]
Framework silicate
Crystal system: monoclinic (-) or triclinic (+) (-)
Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi3O8) displays characteristic cross-hatched twinning
M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.
M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.
M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.
Mg3[Si2O5](OH)4
Sheet silicate
Crystal system: monoclinic or orthorhombic (-)
Serpentine is commonly very fine-grained. It can be recognised by its textural association with olivine and by its low birefringence and habit.
Colour in plane polarised light: green
Relief: low
Birefringence: 1st order
Mg3[Si2O5](OH)4
Sheet silicate
Crystal system: monoclinic or orthorhombic (-)
Serpentine is commonly very fine-grained. It can be recognised by its textural association with olivine and by its low birefringence and habit.
Colour in plane polarised light: green
Relief: low
Birefringence: 1st order
Sheet silicate
Crystal system: monoclinic (+) (-)
Colour in hand specimen: green
Cleavage: one, good
Hardness: 2-3
Colour in plane polarised light: colourless or green
Pleochroism: weak to moderate
Relief: low
Birefringence: anomalous (blue or brown colours that do not appear in the Michel-Levy chart)
Extinction: straight
Sheet silicate
Crystal system: monoclinic (+) (-)
Colour in hand specimen: green
Cleavage: one, good
Hardness: 2-3
Colour in plane polarised light: colourless or green
Pleochroism: weak to moderate
Relief: low
Birefringence: anomalous (blue or brown colours that do not appear in the Michel-Levy chart)
Extinction: straight
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.
Chain silicate
Crystal system: monoclinic (+)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5.5-6
Colour in plane polarised light: colourless, pale brown or pale grey
Pleochroism: variable weak to moderate
Relief: moderate
Birefringence: 0.018-0.033; 1st order pink to 2nd order orange
Extinction: inclined
Twinning: simple, common
Clinopyroxene contains calcium. It encompasses a range of solid solutions from (Mg,Fe)2[Si2O6] to Ca(Mg,Fe)[Si2O6]. The physical and optical properties detailed above refer to augite, a clinopyroxene common in mafic igneous rocks.
Chain silicate
Crystal system: monoclinic (+)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5.5-6
Colour in plane polarised light: colourless, pale brown or pale grey
Pleochroism: variable weak to moderate
Relief: moderate
Birefringence: 0.018-0.033; 1st order pink to 2nd order orange
Extinction: inclined
Twinning: simple, common
Clinopyroxene contains calcium. It encompasses a range of solid solutions from (Mg,Fe)2[Si2O6] to Ca(Mg,Fe)[Si2O6]. The physical and optical properties detailed above refer to augite, a clinopyroxene common in mafic igneous rocks.