Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Aluminium silicate hydroxide
Chemical Formula: Al2Si2O5(OH)4
Method(s) of Verification: Llechweddhelyg Mine - XRD (National Museum of Wales, NMW X-1131).

Chemical Group:

  • Silicates

Geological Context:

  • Supergene
Chalky-white metahalloysite from Llechweddhelyg Mine. National Museum of Wales Collection (NMW 90.14G.M.428). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: a product of low-temperature dehydration of halloysite (Al2Si2O5(OH)4·2H2O), often occurring within museum environments.
Occurrence in Wales: metahallyosite has been identified on a specimen in the collection of the National Museum of Wales during analysis of an unusual white powdery crust on quartz vein material.

Key Localities:

  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: a white powdery phase encrusting minute quartz crystals in highly altered veinstone on National Museum of Wales specimen no. NMW 90.14G.M.428.

There are no key localities for this specimen.


There are no references for this specimen.