Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Potassium magnesium iron aluminium silicate hydroxide hydrate
Chemical Formula: K(Mg,Fe)6(Si,Al)8O20(OH)4.nH2O
Method(s) of Verification: Coed-y-Brenin - XRD (Rice & Sharp, 1976).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Hydrothermal: porphyry-type mineralization
Introduction: hydrobiotite is a regular interstratification of biotite and vermiculite layers and belongs to the 'interlayer-cation-deficient' group of micas. These develop as an alteration product of primary biotite by hydrothermal activity.
Occurrence in Wales: Rice & Sharp (1976) described hydrobiotite from the Coed-y-Brenin copper deposit in Gwynedd.

Key Localities:

  • Coed-y-Brenin, Dolgellau, Gwynedd: hydrobiotite was recorded with illite and chlorite in highly fractured phyllitic rock affected by the copper mineralization at Coed-y-Brenin, NNE of Dolgellau, Gwynedd. It should be noted that hydrobiotite has been been redefined as a mineral since this work was undertaken (Banfield & Eggleton, 1988), although the occurrence of this mineral is within a geological setting in which hydrobiotite would be expected to occur.


  1. Banfield, J.F. & Eggleton, R.A., 1988. Transmission electron microscope study of biotite weathering. Clays and Clay Minerals, 36, 47-60.
  2. Rice, R. & Sharp, G.J., 1976. Copper mineralization in the forest of Coed-y-Brenin, North Wales. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied earth science), 85, B1-B13