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
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).
Fine-grained, blue-green, sedimentary texture.
Slaty cleavage.
Some patches of lighter and finer-grained material.
Slaty cleavage due to the alignment of chlorite and muscovite.
Cross-cut by the blobs of the fine-grained material. These are orange-brown in thin-section. Could be water-lain ash, ripped-up to form clasts.
Very fine-grained, brown matrix, probably volcanic ash.
Carbonate, quartz and opaque clasts slightly larger than the matrix.
Fine-grained, blue-green, sedimentary texture.
Slaty cleavage.
Some patches of lighter and finer-grained material.
Slaty cleavage due to the alignment of chlorite and muscovite.
Cross-cut by the blobs of the fine-grained material. These are orange-brown in thin-section. Could be water-lain ash, ripped-up to form clasts.
Very fine-grained, brown matrix, probably volcanic ash.
Carbonate, quartz and opaque clasts slightly larger than the matrix.