Allophane

Crystal System: Amorphous
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Hydrous aluminium silicate
Chemical Formula: Al2O3.1.3-2.0(SiO2).2.5-3.0(H2O)
Method(s) of Verification: Welsh Foxdale - unspecified analysis (by Prior, 1906), presumably wet chemistry; Cwm Dwyfnant - wet chemical analysis (G.T. Prior), X-ray diffraction analysis (Natural History Museum, no. x1663).

Chemical Group:

  • Silicates

Geological Context:

  • Supergene
Introduction: allophane is a found as a low temperature secondary mineral, forming by the alteration of volcanic ash: by hydrothermal alteration of feldspar in igneous rocks or in hydrothermal veins where is can be associated with metallic ore, particularly copper.
Occurrence in Wales: four occurrences of allophane have been recorded from Wales. Although little detail is available regarding allophane from the iron mines at Betws Garmon, Snowdonia cited by Jenkins & Johnson (1993), the three remaining localities described below have formed by hydrothermal activity associated with vein mineralization.

Key Localities:

  • Cwm Dwyfnant, Llangammarch Wells, Powys: the occurrence of allophane from a level driven on a marcasite vein at Cwm Dwyfnant was confirmed by X-ray diffraction and chemical analysis by G.T. Prior (Natural History Museum specimen B.M. 1918,403).
  • Gorlan Mine, Trefriw, Gwynedd: Russell (1944) reported ‘gum-like’, brownish-yellow allophane coated by dundasite and enveloping crystals of cerussite, quartz and anglesite, all these minerals occurred as a secondary assemblage on the galena, blende and marcasite lode. Allophane was identified by wet chemistry but no analyses were published.
  • Welsh Foxdale (New Pandora) Mine, Trefriw, Gwynedd: Prior (1906) described the occurrence of allophane from Welsh Foxdale (=New Pandora) Mine, near Trefriw, Gwynedd, where it occurs as ‘beautiful, small glassy spherules associated with dundasite and cerussite’.

There are no key localities for this specimen.

References:

  1. Jenkins, D.A. & Johnson, D.B., 1993. Abandoned metal mines: a unique mineralogical and microbiological resource. Journal of the Russell Society, 5, 40-44.
  2. Prior, G.T., 1906. Dundasite from North Wales. Mineralogical Magazine, 14, 167-169
  3. Russell, A., 1944. Notes on some minerals either new or rare in Britain. Mineralogical Magazine, 27, 1-10.

There are no references for this specimen.