Hornblende amphibole
- Pleochroic in greens and cream. Some of the grains display simple twinning. Two cleavages intersect at 56°.
Biotite
- Subhedral to anhedral grains, pleochroic in brown and straw. Straight, speckled extinction. Chlorite partially replaces biotite – late stage alteration.
Quartz
- Anhedral, interstitial grains. They display undulose extinction and contain abundant tiny inclusions, but are characteristically fresh and unaltered.
Feldspar, mainly plagioclase
- Mostly anhedral grains, though some are lath-shaped. Alteration. Zonation.
Accessory minerals:
- Sphene
- Magnetite
- Apatite
Rock history Hornblende is euhedral. It is, therefore a primary igneous phase, and must have crystallised from a wet melt.
Magma cooled relatively slowly, allowing large crystals to grow.
Hornblende amphibole
- Pleochroic in greens and cream. Some of the grains display simple twinning. Two cleavages intersect at 56°.
Biotite
- Subhedral to anhedral grains, pleochroic in brown and straw. Straight, speckled extinction. Chlorite partially replaces biotite – late stage alteration.
Quartz
- Anhedral, interstitial grains. They display undulose extinction and contain abundant tiny inclusions, but are characteristically fresh and unaltered.
Feldspar, mainly plagioclase
- Mostly anhedral grains, though some are lath-shaped. Alteration. Zonation.
Accessory minerals:
- Sphene
- Magnetite
- Apatite
Rock history Hornblende is euhedral. It is, therefore a primary igneous phase, and must have crystallised from a wet melt.
Magma cooled relatively slowly, allowing large crystals to grow.
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
This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar.
Alkali feldspar (orthoclase)
- Grains typically 2-4mm long, subhedral to anhedral, dirty looking due to alteration. Many display a microperthite texture.
Plagioclase feldspar
- Grains typically 1-4mm long, subhedral, characteristic lamellar twinning. Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. They have been partially altered to white mica. Some grains are zoned.
Quartz
- Grains typically 1-4mm across, anhedral. They are characteristically clean and unaltered when compared with feldspar grains, but do contain many tiny inclusions.
Biotite
- Grains similar size as the rest of the groundmass. Strongly pleochroic in pale straw and dark reddish brown. Speckled extinction. Some sections show the one good cleavage of biotite. Small black blobs and circles are dotted all over the crystals. These are pleochroic haloes, produced by zircons. Partially decomposed to chlorite.
Accessory minerals
- Apatite
- Small irregular grains of muscovite mica
Variscan , Permo-Carboniferous
This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar.
Alkali feldspar (orthoclase)
- Grains typically 2-4mm long, subhedral to anhedral, dirty looking due to alteration. Many display a microperthite texture.
Plagioclase feldspar
- Grains typically 1-4mm long, subhedral, characteristic lamellar twinning. Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. They have been partially altered to white mica. Some grains are zoned.
Quartz
- Grains typically 1-4mm across, anhedral. They are characteristically clean and unaltered when compared with feldspar grains, but do contain many tiny inclusions.
Biotite
- Grains similar size as the rest of the groundmass. Strongly pleochroic in pale straw and dark reddish brown. Speckled extinction. Some sections show the one good cleavage of biotite. Small black blobs and circles are dotted all over the crystals. These are pleochroic haloes, produced by zircons. Partially decomposed to chlorite.
Accessory minerals
- Apatite
- Small irregular grains of muscovite mica