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stage hydrothermal alteration product on pectolite within an Ordovician gabbroic intrusion (National Museum of Wales unpublished data).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 references for this specimen.