Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Rare
Chemical Composition: Sodium aluminium silicate hydroxide hydrate
Chemical Formula: (Na,K,Ca)~x(Al,Fe,Mg)4(Si,Al)8O20(OH)4•3.54(H2O)
Method(s) of Verification: Bangor - XRD & TGA (Dong et al., 2002).

Geological Context:

  • Metamorphic: low-grade
Introduction: brinrobertsite is a clay mineral formed of an equal proportions of interleaved pyrophyllite [(Al2Si4O10(OH)2·] and smectite [NaKCa)(AlMgFe)(SiAl)O20(OH)4] layers. It is therefore similar to rectorite (intelayered illite and smectite). Brinrobertsite is interpreted as having formed from the alteration of pyrophyllite by hydrothermal activity (Dong et al., 2002).
Occurrence in Wales: the first (and to date only) occurrence of brinrobertsite was described from an Ordovician metabentonite (altered volcanic tuff) from near Bangor, North Wales (Dong et al., 2002). Brinrobertsite, like other clay minerals does not have crystals which can be identified with the naked eye, so its occurrence must be confirmed by analytical techniques such as XRD (see below). The mineral is named after Dr Brinley Roberts of the University of London who has worked on the clay mineralogy of Welsh mudrocks.

Key Localities:

  • Bangor, Gwynedd, North Wales: brinrobertsite occurs in thin, pale-cream metabentonite within Ordovician mudstone, 750 m north-east of Penrhyn Castle, near Bangor [SH 60907235]. This is the only recorded occurrence of the mineral in Wales, although it may be present in other rocks


  1. Dong, H., Peacor, D.R., Merriman, R.J. & Kemp, S.J.,, 2002. Brinrobersite: a new R1 interstratified pyrophyllite/smectite-like clay mineral: characterization and geological origin. Mineralogical Magazine, 66, 605-617.