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Occurrence in Wales: two occurrence, both unpublished accounts, of halloysite are known from Wales.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filling voids between crystals of brookite, albite and quartz on specimens collected by G. Ryback in 1972 (G. Ryback, unpublished data).


  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.