L110 Rhyolite, porphyritic (quartz porphyry)

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- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite.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.

Porphyritic texture indicates two-stage cooling. The phenocrysts grew slowly in a magma chamber; the fine-grained groundmass formed during rapid cooling at the Earth’s surface.

Late stage hydrothermal alteration to white mica and carbonate.

porphyritic rhyolite
quartz porphyry