Orthosilicate
Crystal system: cubic
Colour in hand specimen: typically red
Cleavage: none
Hardness: 6-7.5
Colour in plane polarised light: colourless, pale pink
Pleochroism: none
Relief: high
Isotropic
M130 Garnet schist
Portsoy, Banffshire
Euhedral porphyroblasts of dark-red garnet, with quartz, mica and other minerals in a medium-grade metamorphic rock.
The garnet group encompasses a wide range of compositions, and consequently a range of optical and physical properties. The properties detailed above are typical of pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12) and grossular (Ca3Al2Si3O12).
Orthosilicate
Crystal system: cubic
Colour in hand specimen: typically red
Cleavage: none
Hardness: 6-7.5
Colour in plane polarised light: colourless, pale pink
Pleochroism: none
Relief: high
Isotropic
M130 Garnet schist
Portsoy, Banffshire
Euhedral porphyroblasts of dark-red garnet, with quartz, mica and other minerals in a medium-grade metamorphic rock.
The garnet group encompasses a wide range of compositions, and consequently a range of optical and physical properties. The properties detailed above are typical of pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12) and grossular (Ca3Al2Si3O12).
Orthosilicate
Crystal system: monoclinic (-)
Colour in hand specimen: green
Cleavage: one, perfect
Hardness: 6
Colour in plane polarised light: yellow-green
Pleochroism: yes
Relief: high
Birefringence: 0.015-0.051; 1st order orange to 3rd order pink
Extinction: inclined
Twinning: lamellar, not common
Orthosilicate
Crystal system: monoclinic (-)
Colour in hand specimen: green
Cleavage: one, perfect
Hardness: 6
Colour in plane polarised light: yellow-green
Pleochroism: yes
Relief: high
Birefringence: 0.015-0.051; 1st order orange to 3rd order pink
Extinction: inclined
Twinning: lamellar, not common
Ring silicate
Crystal system: trigonal (-)
Colour in hand specimen: black
Hardness: 7
Colour in plane polarised light: typically brown-green or green
Pleochroism: strongly pleochroic in many colours
Relief: moderate
Birefringence: typically 2nd order
Ring silicate
Crystal system: trigonal (-)
Colour in hand specimen: black
Hardness: 7
Colour in plane polarised light: typically brown-green or green
Pleochroism: strongly pleochroic in many colours
Relief: moderate
Birefringence: typically 2nd order
Chain silicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5-6
Colour in plane polarised light: colourless, reddish or greenish
Pleochroism: variable weak to strong
Relief: moderate
Birefringence: 0.007-0.020; 1st order grey to 2nd order purple
Extinction: straight
Orthopyroxene has two end-member compositions: enstatite, Mg2Si2O6, and ferrosilite, Fe2+2Si2O6.
Chain silicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5-6
Colour in plane polarised light: colourless, reddish or greenish
Pleochroism: variable weak to strong
Relief: moderate
Birefringence: 0.007-0.020; 1st order grey to 2nd order purple
Extinction: straight
Orthopyroxene has two end-member compositions: enstatite, Mg2Si2O6, and ferrosilite, Fe2+2Si2O6.
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.
(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.
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
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: 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
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