Porphyritic.
Large, tabular, pale grey-yellow phenocrysts of plagioclase feldspar, randomly oriented, twin striations are visible on many of them.
Pink-brown cryptocrystalline groundmass contains black, shiny flakes of biotite.
Plagioclase feldspar
- Euhedral phenocrysts, 2-20 mm long, displaying the characteristic lamellar twinning. Some alteration to white mica. In places they have been replaced by chlorite and calcite.
Biotite
- Phenocrysts, most <1mm long, with characteristic brown to pale straw pleochroism and straight extinction.
Accessory minerals
- Quartz
- Apatite
Groundmass
- Mainly cryptocrystalline. A small number of feldspar grains can be identified. Small ragged vesicles.
Two-stage cooling: phenocrysts followed by groundmass.
Phenocrysts are euhedral indicating that they crystallised free-floating in a magma chamber.
The fine-grained groundmass must have crystallised rapidly due to cooling at the Earth’s surface.
Alteration occurred after crystallisation.
Early Paleozoic
Cansip Porphyry Sills
Sutherland
Porphyritic.
Large, tabular, pale grey-yellow phenocrysts of plagioclase feldspar, randomly oriented, twin striations are visible on many of them.
Pink-brown cryptocrystalline groundmass contains black, shiny flakes of biotite.
Plagioclase feldspar
- Euhedral phenocrysts, 2-20 mm long, displaying the characteristic lamellar twinning. Some alteration to white mica. In places they have been replaced by chlorite and calcite.
Biotite
- Phenocrysts, most <1mm long, with characteristic brown to pale straw pleochroism and straight extinction.
Accessory minerals
- Quartz
- Apatite
Groundmass
- Mainly cryptocrystalline. A small number of feldspar grains can be identified. Small ragged vesicles.
Two-stage cooling: phenocrysts followed by groundmass.
Phenocrysts are euhedral indicating that they crystallised free-floating in a magma chamber.
The fine-grained groundmass must have crystallised rapidly due to cooling at the Earth’s surface.
Alteration occurred after crystallisation.
Pink-brown, porphyritic rock with a very fine-grained groundmass.
Abundant pink macrocrysts (5-10 mm).
Phenocrysts of feldspar (2-4 mm) and quartz (2-4 mm).
Accessory black mineral.
Quartz
- Rounded phenocrysts (1-4 mm) with embayments/cavities and tiny inclusions.
Feldspar
- Phenocrysts ~2mm, subhedral. Mainly alkali feldspar, some plagioclase. Alteration to white mica.
Garnet
- Very rare, euhedral phenocrysts (0.5-1 mm). Isotropic, high relief, colourless to pale pink.
Biotite
- Very rare fragments with chlorite alteration.
Groundmass
- Microcrystalline quartz and feldspar, with some muscovite and biotite.
Calcite
- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite. These are the pink macrocrysts visible in hand specimen.
Ordovician
Armboth Dyke
Thirlmere, Cumbria
Pink-brown, porphyritic rock with a very fine-grained groundmass.
Abundant pink macrocrysts (5-10 mm).
Phenocrysts of feldspar (2-4 mm) and quartz (2-4 mm).
Accessory black mineral.
Quartz
- Rounded phenocrysts (1-4 mm) with embayments/cavities and tiny inclusions.
Feldspar
- Phenocrysts ~2mm, subhedral. Mainly alkali feldspar, some plagioclase. Alteration to white mica.
Garnet
- Very rare, euhedral phenocrysts (0.5-1 mm). Isotropic, high relief, colourless to pale pink.
Biotite
- Very rare fragments with chlorite alteration.
Groundmass
- Microcrystalline quartz and feldspar, with some muscovite and biotite.
Calcite
- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite. These are the pink macrocrysts visible in hand specimen.