Olivine
- Subhedral to euhedral grains making up ~45% of the rock. Alteration to magnetite and serpentine, particularly along the curving cracks. Cracks across the thin-section, could be due to expansion of the rock as the olivine was altered.
Clinopyroxene
- Interstitial phase making up ~35% of the rock. Moderate relief, inclined extinction and first order pink to second order orange interference colours.
Plagioclase feldspar
- Interstitial phase making up ~20% of the rock. Low first order interference colours and lamellar twinning.
Accessory minerals - Biotite, small, interstitial grains, altered.
This is a cumulate rock. It has undergone late-stage shear banding.
Olivine phenocrysts crystallised first.
Clinopyroxene grew around the olivine phenocrysts.
Interstitial feldspar filled in the remaining gaps.
Shearing of the rock produced bands of deformation.
Hydrothermal alteration.
Olivine
- Subhedral to euhedral grains making up ~45% of the rock. Alteration to magnetite and serpentine, particularly along the curving cracks. Cracks across the thin-section, could be due to expansion of the rock as the olivine was altered.
Clinopyroxene
- Interstitial phase making up ~35% of the rock. Moderate relief, inclined extinction and first order pink to second order orange interference colours.
Plagioclase feldspar
- Interstitial phase making up ~20% of the rock. Low first order interference colours and lamellar twinning.
Accessory minerals - Biotite, small, interstitial grains, altered.
This is a cumulate rock. It has undergone late-stage shear banding.
Olivine phenocrysts crystallised first.
Clinopyroxene grew around the olivine phenocrysts.
Interstitial feldspar filled in the remaining gaps.
Shearing of the rock produced bands of deformation.
Hydrothermal alteration.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.
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.