Laumontite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium aluminium hydrous silicate
Chemical Formula: CaAl2Si4O12
Method(s) of Verification: Llanelwedd Quarry - XRD (National Museum of Wales, NMW X-149).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
Pink laumontite in calcite veins, from Llanelwedd Quarry, Builth Wells, Powys. Specimen 5 cm across. National Museum of Wales Collection (NMW 83.47G.M.1). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: laumontite belongs to the zeolite family, and is most commonly found with other zeolite minerals in cavities and veins in altered igneous rocks. It can form at very low temperatures (possibly as low as 50°C) but is know to have an upper stability of 210°C at shallow depths with this rising to a maximum of 300°C at depths of 10-11 kilometers (Boles, 1981). Laumontite readily dehydrates to a chalky material called leonhardite.
Occurrence in Wales: only one occurrence of laumontite is known from Wales, although it is possible that there are other occurrences yet to be documented.

Key Localities:

  • Llanelwedd Quarry, Builth Wells, Powys: National Museum of Wales specimens (e.g. NMW 83.47G.M.1 & 84.46G.M.1) show massive, salmon-pink laumontite within thin calcite veins (approximately 20-30 mm thick) which cut altered basic lava from Llanelwedd Quarry (Bevins & Horák, 1985). The original description was confirmed by more detailed work at this site by Metcalfe (1990).

There are no key localities for this specimen.

References:

  1. Bevins R.E. & Horák, J.M., 1985. A first occurrence of laumontite in Wales. Journal of the Russell Society, 1, 78-79.
  2. Boles, J.R., 1977. Zeolites in low-grade metamorphic rocks. In The mineralogy and geology of natural zeolites. Reviews in Mineralogy, No 4 (ed.) F.A. Mumpton, Mineralogical Society of America/Geochemical Society.
  3. Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.

There are no references for this specimen.