Gibbsite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Aluminium hydroxide
Chemical Formula: Al(OH)3
Method(s) of Verification: cited occurrences - visual identification.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: gibbsite is a secondary mineral which develops typically by the break-down of aluminous or aluminosilicate minerals in tropical or sub-tropical conditions. It is characteristic of laterites, clay deposits and highly-weathered soils; being the most commonly occurring crystalline aluminium hydroxide mineral in the latter. In general gibbsite is very stable under surface conditions, however under exceptional conditions it alters to clay minerals such as kaolin.
Occurrence in Wales: the two occurrence of gibbsite from Wales both have a supergene origin, although one occurs on a natural substrate, the other on mine waste. Although gibbsite is indicative of weathering under tropical or sub-tropical conditions, it’s occurrence at Glasdir Mine, first worked in the mid 1800s, clearly shows that it can form under range of conditions that is typically reported in the literature.

Key Localities:

  • Glasdir Mine, Dolgellau Gold-belt, Gwynedd: a second confirmed occurrence of gibbsite comes from the mine dumps at Glasdir where white globules (0.2 mm in diameter) occur on clear, glassy, colourless to pale blue allophane (G. Ryback, unpublished data, as quoted in Bevins, 1994).
  • Y Llymllwyd, near Nant Ffrancon, Snowdonia: Ball (1964) identified gibbsite as the only crystalline component in the clay fraction of a < 25 cm thick soil horizon developed over the Bwlch y Cywion Granite.

References:

  1. Ball, D.F., 1964. Gibbsite in altered granitic rock in North Wales. Nature, 204, 673-674.
  2. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.