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; 1^st order grey to 2^nd order purple 
Extinction: straight

Orthopyroxene has two end-member compositions: enstatite, Mg₂Si₂O₆, and ferrosilite, Fe²⁺₂Si₂O₆.

orthopyroxene

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

L504 Garnet schist

tourmaline

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

L354 Limestone, shelly

epidote

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.

L504 Garnet schist

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).

garnet

Orthosilicate

Crystal system: monoclinic (+)

Colour in hand specimen: colourless, yellow, green, brown or black
Cleavage: one, good
Hardness: 5

Colour in plane polarised light: colourless, yellow or typically brown
Pleochroism: moderate if coloured
Relief: high
Birefringence: 0.100-0.192; >4th order
Extinction: inclined
Twinning: simple twins; occasionally lamellar twins

L101 Granite 
L120 Granodiorite

sphene
titanite

Advanced notes

Formula

Orthosilicate

Crystal system: orthorhombic (+) (-)

Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7

Colour in plane polarised light: colourless
Relief: moderate 
Birefringence: 0.035; 2nd order pink

M100 Olivine 
North Carolina 
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage

L141 Gabbro 
L162 Basalt, porphyritic 
L172 Ultramafic cumulate

Olivine has two end-member compositions: forsterite, Mg₂[SiO₄], and fayalite, Fe₂[SiO₄]. The physical and optical properties detailed above refer to forsterite. Forsteritic olivine is a common constituent of mafic and ultra-mafic rocks.
Cracks are often filled with serpentinite alteration.

olivine
forsterite
fayallite

Buff coloured rock.
Almost pure quartz, which has been very well recrystallised.
Faint banding could be original bedding.

Fine to medium grained recrystallised quartz with sutured contacts.

The sutured (ragged) grain boundaries and undulose extinction of the quartz grains indicate that this rock formed under relatively low temperature metamorphism. Had the rock been raised to higher temperatures perfect 120° grain boundaries would have formed, and the quartz would not be strained.

Inserting the sensitive tint plate results in the vast majority of grains changing colour in the same way – most of the grains simultaneously display higher interference colours in one orientation, and lower interference colours in the other orientation. This strong fabric was produced by recrystallisation under directed stress, which caused all the grains to elongate in one direction. 

quartzite

Subangular quartz grains, typically 0.25 mm across with sutured contacts.

The sutured contacts between the quartz grains indicate that this rock has been metamorphosed to a quartzite. This is a very well recrystallised rock. The protolith was a fine-grained quartz arenite (<15% matrix), but we cannot tell how angular or spherical the original grains were as they have been partially dissolved by pressure solution to form sutured contacts.

quartzite

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.

garnet schist