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
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
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).
Strongly developed planar fabric made up of platy biotite mica and muscovite mica grains and lenses of quartz grains. Fabric wraps around large, euhedral garnets, typically ~2mm across. Quartz, the most soluble mineral, has recrystallised at the corners of the “eyes”, in the pressure shadows of the garnets.
Bands of interlocking quartz grains and some feldspar (plagioclase and alkali, lamellar twinning and alteration), alternate with bands composed of needles of biotite mica and muscovite mica, about 50:50.
Quartz displays undulose extinction, and being the most soluble mineral in this rock has recrystallised into the pressure shadows around the garnets.
The fabric is interrupted by large euhedral garnet crystals. Characteristically the garnets are isotropic and have high relief. They contain inclusions of quartz and mica.
Accessory minerals:
- chlorite
- staurolite
- tourmaline
This schist contains large quantities of muscovite indicating that it had a pelitic protolith. This pelite was subjected to high pressures and temperatures during regional metamorphism, resulting in the crystallisation and alignment of metamorphic minerals.
The foliation of this schist may be due to original compositional banding in the pelite; the quartz-rich layers having been sandy layers, while the mica-rich layers were muddy.
Some evidence for post-metamorphic deformation: quartz displaying undulose extinction; micas are wrapped around the garnet grains.
Tiny grains of chlorite found around the edges of the garnets indicate that some retrogradation has taken place. Chlorite is a common break-down product of garnets.
In thin-section staurolite is pale-golden in colour. It has high relief, and displays straight extinction and 1st order birefringence colours.
Pre-Devonian
Ny-Friesland, Spitsbergen
Strongly developed planar fabric made up of platy biotite mica and muscovite mica grains and lenses of quartz grains. Fabric wraps around large, euhedral garnets, typically ~2mm across. Quartz, the most soluble mineral, has recrystallised at the corners of the “eyes”, in the pressure shadows of the garnets.
Bands of interlocking quartz grains and some feldspar (plagioclase and alkali, lamellar twinning and alteration), alternate with bands composed of needles of biotite mica and muscovite mica, about 50:50.
Quartz displays undulose extinction, and being the most soluble mineral in this rock has recrystallised into the pressure shadows around the garnets.
The fabric is interrupted by large euhedral garnet crystals. Characteristically the garnets are isotropic and have high relief. They contain inclusions of quartz and mica.
Accessory minerals:
- chlorite
- staurolite
- tourmaline
This schist contains large quantities of muscovite indicating that it had a pelitic protolith. This pelite was subjected to high pressures and temperatures during regional metamorphism, resulting in the crystallisation and alignment of metamorphic minerals.
The foliation of this schist may be due to original compositional banding in the pelite; the quartz-rich layers having been sandy layers, while the mica-rich layers were muddy.
Some evidence for post-metamorphic deformation: quartz displaying undulose extinction; micas are wrapped around the garnet grains.
Tiny grains of chlorite found around the edges of the garnets indicate that some retrogradation has taken place. Chlorite is a common break-down product of garnets.
In thin-section staurolite is pale-golden in colour. It has high relief, and displays straight extinction and 1st order birefringence colours.
Very fine-grained rock with a prominent, well-developed cleavage. The dark red-purple colour is due to iron-staining. The light green smudge is a reduction spot.
Mud size grains.
Mainly indistinguishable clay minerals.
Some quartz, muscovite, opaques.
The protolith of this rock was a pelite (mudstone). Under low-grade, regional metamorphic conditions, crystals within the pelite were aligned, producing the slaty cleavage.
Iron staining
Hematite(Fe2O3) → red (blood-red, cherry-red, brown-red) iron-staining
Limonite (FeO(OH)·nH2O) → yellow or brown iron-staining
The red-purple colour of this rock is probably produced by hematite.
Reduction spot
The dark red-purple of the majority of this rock is due to the presence of iron in the 3+ oxidation state (Fe3+). The light green colour of the reduction spot is instead due to the presence of iron in the 2+ oxidation state (Fe2+). Hence, reduction of the iron ions from Fe3+ to Fe2+ has taken place in the reduction spot.
Lower Cambrian
Llanberis, North Wales
Very fine-grained rock with a prominent, well-developed cleavage. The dark red-purple colour is due to iron-staining. The light green smudge is a reduction spot.
Mud size grains.
Mainly indistinguishable clay minerals.
Some quartz, muscovite, opaques.
The protolith of this rock was a pelite (mudstone). Under low-grade, regional metamorphic conditions, crystals within the pelite were aligned, producing the slaty cleavage.
Iron staining
Hematite(Fe2O3) → red (blood-red, cherry-red, brown-red) iron-staining
Limonite (FeO(OH)·nH2O) → yellow or brown iron-staining
The red-purple colour of this rock is probably produced by hematite.
Reduction spot
The dark red-purple of the majority of this rock is due to the presence of iron in the 3+ oxidation state (Fe3+). The light green colour of the reduction spot is instead due to the presence of iron in the 2+ oxidation state (Fe2+). Hence, reduction of the iron ions from Fe3+ to Fe2+ has taken place in the reduction spot.