L172 Ultramafic cumulate

Greenish black, coarse-grained rock, comprising olivine, clinopyroxene and plagioclase.
Grains typically 1-2 mm across. 

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.

ultramafic cumulate