Brammallite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Locally Abundant
Chemical Composition: Sodium-rich illite mica
Chemical Formula: (Na,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,H2O]
Method(s) of Verification: Llandebie - XRD (Natural History Museum, x2742) & wet chemistry (partial analysis) Pandora Mine – XRD (Natural History Museum, 19595).

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
Introduction: brammallite, from Llandebie, Dyfed was identified by A. Brammall (formerly of the Department of Geology, Imperial College of Science and Technology) and described as a new mineral by Bannister (1943) after his death. The mineral is a sodium-rich illite mica, and has subsequently been redefined as a mineral series (Rieder et al., 1999).
Occurrence in Wales: only one occurrence of brammallite in Wales, other than that from the type locality noted above, is cited in the literature. As the mineral is fine-grained, and can only be identified optically or by X-ray diffraction, it is quite possible that it has a far wider distribution than is currently documented.

Key Localities:

  • Llandebie, Dyfed: brammallite from the type locality forms a white infilling to fissures or as a coating on slickensided surfaces in shales overlying the Coal Measures. The soft fibrous mineral comprises small, compact tufts of elongated plates approximately 0.5 mm long x 0.1 mm wide.
  • Pandora Mine, Gwynedd: Natural History Museum X-ray powder diffraction data for colourless to white fibres from Pandora Mine, show a pattern close to that of brammallite. No other details of this occurrence are know.

References:

  1. Bannister, F.A., 1943. Brammallite (sodium-illite), a new mineral from Llandebie, South Wales. Mineralogical Magazine, 26, 304-307.
  2. Rieder, M., et al., 1999. Nomenclature of Mica. Mineralogical Magazine, 63, 267-281.