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 references for this specimen.