Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Potassium magnesium iron aluminium silicate hydroxide.
Chemical Formula: K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2
Method(s) of Verification: see under individual mineral entries (annite, phlogopite & siderophyllite).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic: medium-grade
  • Sedimentary: allogenic (detrital)
Biotite, shown in plane polarized light, partially altered to chlorite and containing a small zircon crystal with a radiation damage halo. Coedana Complex Gneisses, Anglesey. Specimen National Museum of Wales (93.12G). © National Museum of Wales
Introduction: biotite is now used as a series name for annite-phlogopite and siderophyllite-eastonite, and is no longer a valid species name (Rieder et al., 1999).
Occurrence in Wales: biotite is widely distributed across Wales in medium to high grade metamorphic rocks; as a minor component in intermediate to acid igneous rocks and less commonly as a detrital mineral in sediments. Details of the specific occurrences are given under the entries for annite, phlogopite and siderophyllite.

Key Localities:

There are no key localities for this specimen.


  1. Rieder, M., et al., 1999. Nomenclature of Mica. Mineralogical Magazine, 63, 267-281.