L120 Granodiorite

Mostly moderately coarse-grained (typically 1-2 mm), some regions finer-grained (typically ~0.5 mm). 
“Salt and pepper” appearance being speckled black and white. Two black phases, biotite mica and hornblende amphibole. White regions comprise quartz and white feldspar.

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.

granodiorite