(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.
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: tetragonal (+)
Colour in hand specimen: reddish brown, yellow, grey, green or colourless
Hardness: 7.5
Colour in plane polarised light: colourless to pale brown
Pleochroism: very weak
Relief: high
Birefringence: 0.042-0.065; 3rd order green to 4th order green
Extinction: straight
Twinning: rare
Orthosilicate
Crystal system: tetragonal (+)
Colour in hand specimen: reddish brown, yellow, grey, green or colourless
Hardness: 7.5
Colour in plane polarised light: colourless to pale brown
Pleochroism: very weak
Relief: high
Birefringence: 0.042-0.065; 3rd order green to 4th order green
Extinction: straight
Twinning: rare
Medium to coarse-grained rock with a granular texture.
White feldspar laths 2-4 mm long are clearly visible.
Other phases: pink feldspar, black biotite mica, black hornblende amphibole and pale grey quartz. These grains are more equant, typically 1-2 mm across.
Biotite may be distinguished from hornblende by its flaky habit and one good cleavage. It is shinier than hornblende.
Plagioclase feldspar
- The larger laths (2-4 mm long) are strongly zoned. The cores of these laths, which display clear lamellar twinning, have been overgrown by rims of more feldspar. Some alteration to white mica.
Biotite mica
- Grains typically 1 mm across. Strongly pleochroic in reddish brown and pale straw. Often associated with small grains of an opaque accessory mineral, probably magnetite.
Alkali feldspar
- Anhedral grains up to 5mm long. Some alteration to white mica. Some grains display a microperthite texture.
Hornblende amphibole
- Pale green, anhedral. Grains appear to have been “eaten away”, reducing their prismatic form (diamond) to a ragged anhedral form. Amphiboles have two cleavage planes which intersect at 56°.
This rock is classified as an adamellite because it contains roughly equal proportions of alkali feldspar and plagioclase feldspar, with additional quartz and biotite.
hornblende biotite granite
adamellite
Caledonian, Devonian
Glencoe
Medium to coarse-grained rock with a granular texture.
White feldspar laths 2-4 mm long are clearly visible.
Other phases: pink feldspar, black biotite mica, black hornblende amphibole and pale grey quartz. These grains are more equant, typically 1-2 mm across.
Biotite may be distinguished from hornblende by its flaky habit and one good cleavage. It is shinier than hornblende.
Plagioclase feldspar
- The larger laths (2-4 mm long) are strongly zoned. The cores of these laths, which display clear lamellar twinning, have been overgrown by rims of more feldspar. Some alteration to white mica.
Biotite mica
- Grains typically 1 mm across. Strongly pleochroic in reddish brown and pale straw. Often associated with small grains of an opaque accessory mineral, probably magnetite.
Alkali feldspar
- Anhedral grains up to 5mm long. Some alteration to white mica. Some grains display a microperthite texture.
Hornblende amphibole
- Pale green, anhedral. Grains appear to have been “eaten away”, reducing their prismatic form (diamond) to a ragged anhedral form. Amphiboles have two cleavage planes which intersect at 56°.
This rock is classified as an adamellite because it contains roughly equal proportions of alkali feldspar and plagioclase feldspar, with additional quartz and biotite.
hornblende biotite granite
adamellite