Halloysite-10Å

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Aluminium silicate hydroxide hydrate
Chemical Formula: Al2Si2O5(OH)4.2H2O
Method(s) of Verification: Benallt Mine - XRD (National Museum of Wales, NMW X-104).

Chemical Group:

  • Silicates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: halloysite-10Å belongs to the halloysite-allophane group of minerals, and is used in place of the term endellite, as suggested by the International Association for the study of clays (AIPEA, 1975). It typically forms as a product of hydrothermal alteration or surface weathering of aluminium silicate minerals, such as feldspars (Anthony et al., 1995).
Occurrence in Wales: two occurrence, both unpublished accounts, of halloysite are known from Wales.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: a specimen of prismatic natrolite collected by Arthur Russell in 1911 (NMW 27.11.GR.369, ex G.J. Williams Collection) is partially coated by white halloysite.
  • Prenteg, Tremadog, Gwynedd: at Fron Oleu, near Prenteg, halloysite (endellite) forms cream-white opaque masses filling voids between crystals of brookite, albite and quartz on specimens collected by G. Ryback in 1972 (G. Ryback, unpublished data).

References:

  1. Anthony J., Bideaux R., Bladh K. & Nichols M., 1995. Handbook of Mineralogy, vol. 2 (parts 2/1 and 2/2), Silica, Silicates
  2. Brindley, G. W. and Pedro, G., 1976. Meeting of the Nomenclature Committee of A.I.P.E.A., Mexico City, July 21, 1975. AIPEA Newsletter No. 12, 5-6.