Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass.
Plagioclase feldspar
- Phenocrysts 1-4mm long, with a euhedral, tabular form. Randomly orientated. Characteristic lamellar twinning. Some are zoned. Inclusions of magnetite, clinopyroxene and other minerals. Extensive alteration along abundant cracks.
Clinopyroxene (augite)
- Euhedral phenocrysts, 1-2 mm across. Moderate relief, inclined extinction and first order pink to second order orange interference colours. Pale pink in thin-section. Some are zoned.
Olivine
- Very few phenocrysts are fresh enough to be easily identifiable. These are colourless in plane polarised light, have moderate relief, and up to second order pink interference colours. No cleavage.
- 1st phase of alteration along cracks: The majority of the phenocrysts have been completely altered to greenish serpentine.
- 2nd phase of alteration: orange brown mineral – probably the amphibole cummingtonite. Characteristic curving cracks and the prismatic form of olivine are still visible.
Groundmass
- Fine-grained. Plagioclase feldspar needles, clinopyroxene, some olivine (mostly altered), and opaques, probably magnetite. Occasional calcite grain.
Carboniferous
Volcanic neck, Lion's Haunch, Arthur's Seat, Edinburgh
Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass.
Plagioclase feldspar
- Phenocrysts 1-4mm long, with a euhedral, tabular form. Randomly orientated. Characteristic lamellar twinning. Some are zoned. Inclusions of magnetite, clinopyroxene and other minerals. Extensive alteration along abundant cracks.
Clinopyroxene (augite)
- Euhedral phenocrysts, 1-2 mm across. Moderate relief, inclined extinction and first order pink to second order orange interference colours. Pale pink in thin-section. Some are zoned.
Olivine
- Very few phenocrysts are fresh enough to be easily identifiable. These are colourless in plane polarised light, have moderate relief, and up to second order pink interference colours. No cleavage.
- 1st phase of alteration along cracks: The majority of the phenocrysts have been completely altered to greenish serpentine.
- 2nd phase of alteration: orange brown mineral – probably the amphibole cummingtonite. Characteristic curving cracks and the prismatic form of olivine are still visible.
Groundmass
- Fine-grained. Plagioclase feldspar needles, clinopyroxene, some olivine (mostly altered), and opaques, probably magnetite. Occasional calcite grain.