L130 Andesite, porphyritic (porphyrite)

Hand Specimen

Porphyritic. 
Large, tabular, pale grey-yellow phenocrysts of plagioclase feldspar, randomly oriented, twin striations are visible on many of them. 
Pink-brown cryptocrystalline groundmass contains black, shiny flakes of biotite.

Plagioclase feldspar 
- Euhedral phenocrysts, 2-20 mm long, displaying the characteristic lamellar twinning. Some alteration to white mica. In places they have been replaced by chlorite and calcite.

Biotite 
- Phenocrysts, most <1mm long, with characteristic brown to pale straw pleochroism and straight extinction.

Accessory minerals 
- Quartz 
- Apatite

Groundmass 
- Mainly cryptocrystalline. A small number of feldspar grains can be identified. Small ragged vesicles.

Two-stage cooling: phenocrysts followed by groundmass. 
Phenocrysts are euhedral indicating that they crystallised free-floating in a magma chamber. 
The fine-grained groundmass must have crystallised rapidly due to cooling at the Earth’s surface. 
Alteration occurred after crystallisation.