L100 Alkali granite

This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar.

Alkali feldspar (orthoclase) 
- Grains typically 2-4mm long, subhedral to anhedral, dirty looking due to alteration. Many display a microperthite texture.

Plagioclase feldspar 
- Grains typically 1-4mm long, subhedral, characteristic lamellar twinning. Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. They have been partially altered to white mica. Some grains are zoned.

Quartz 
- Grains typically 1-4mm across, anhedral. They are characteristically clean and unaltered when compared with feldspar grains, but do contain many tiny inclusions.

Biotite 
- Grains similar size as the rest of the groundmass. Strongly pleochroic in pale straw and dark reddish brown. Speckled extinction. Some sections show the one good cleavage of biotite. Small black blobs and circles are dotted all over the crystals. These are pleochroic haloes, produced by zircons. Partially decomposed to chlorite.

Accessory minerals 
- Apatite 
- Small irregular grains of muscovite mica

This is a coarse-grained rock, which must have cooled slowly as part of an intrusion. Its granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting.

alkali granite