Kaolinite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Aluminium silicate hydroxide
Chemical Formula: Al2Si2O5(OH)4
Method(s) of Verification: Very little information is known. Gill et al. (1977) carried out detailed X-ray diffraction investigations into the clay mineral fraction of Carboniferous strata in the South Wales Coalfield.

Chemical Group:

  • Silicates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: a clay mineral, polymorphous with dickite, halloysite and nacrite; a member of the kaolinite-serpentine group. Kaolinite replaces other aluminosilicate minerals during hydrothermal alteration and weathering and is a common constituent of the clay-size fraction of sediments, where it may be formed by direct precipitation.
Occurrence in Wales: kaolinite has been reported from a number of localities - all in South Wales - but the accurancy of these identifications is uncertain. Kaolinite group minerals are visually identical and what was at one time known as kaolinite is now split into four polymorphs. North (1916) described kaolinite as present in Carboniferous rocks in the South Wales Coalfield, especially in fault zones. Detailed X-ray diffraction investigations by Gill et al. (1977) revealed that kaolinite is actually widespread in the clay mineral fraction of these strata. The presence of kaolinite has also been detected in some of the Carboniferous rocks of Pembrokeshire (Robinson et al., 1980). Elsewhere in Pembrokeshire kaolinite is reported from heavily altered rhyolitic lavas and tuffs in a borehole near Treffgarne (Brown et al., 1987).

Key Localities:

  • South Wales Coalfield: North (1916) noted films of kaolinite lining fissures in the Pennant Grit, particularly where the strata have been affected by faults, such as at Cwm Fforch-wen in the Garw Valley, Mid Glamorgan. It is not certain whether this is kaolinite or in fact dickite, which has since been confirmed as pearlescent white powdery films in clay ironstone from the Coal Measures in South Wales. Kaolinite is however widespread in the clay mineral fraction of Carboniferous rocks as a rock-forming mineral. This was proved during detailed X-ray diffraction investigations by Gill et al. (1977).
  • Treffgarne, Pembrokeshire: Brown et al. (1987) reported kaolinite from heavily altered rhyolitic lavas and tuffs in a borehole near Treffgarne.

References:

  1. Brown, M.J. and others, 1987. Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales. Mineral Reconnaissance Programme Report, British Geological Survey, No. 86.
  2. Gill, W.D., Khalaf, F.I. and Massoud, M.S., 1977. Clay minerals as an index of the degree of metamorphism of the carbonate and terrigenous rocks in the South Wales Coalfield. Sedimentology, 24, 675-691.
  3. North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.
  4. Robinson, D., Nicholls, R.A. and Thomas, L.J., 1980. Clay mineral evidence for low-grade Caledonian and Variscan metamorphism in south-western Dyfed, South Wales. Mineralogical Magazine, 43, 857-863.