Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium calcium magnesium silicate hydroxide
Chemical Formula: (Ca0.5,Na)0.3Mg3Si4O10(OH)2
Method(s) of Verification: Gimlet Quarry - XRD (National Museum of Wales, NMW X-737)

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
Dull, fleshy pink coloured stevensite crystals forming by the alteration of pectolite. Gimlet Quarry. Specimen National Museum of Wales (NMW 27.111.GR.256), ex G.J. Williams collection, photo D.I. Green, © National Museum of Wales.
Introduction: stevensite is a smectite group mineral that forms typically as an alteration product of pectolite or in altered igneous rocks.
Occurrence in Wales: stevensite has been recorded from just one locality in Wales where it forms a late stage hydrothermal alteration product on pectolite within an Ordovician gabbroic intrusion (National Museum of Wales unpublished data).

Key Localities:

  • Gimlet Quarry, Pwllheli, Gwynedd: stevensite occurs as fleshy pink, silky to dull, masses extensively coating or totally replacing pectolite. The mineral is no longer found at this site as the quarry has been in-filled, but it has been identified from specimens in the National Museum of Wales Collection (e.g. NMW 27.111.GR.29 and NMW 27.111.GR.30).

There are no key localities for this specimen.


There are no references for this specimen.