Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium iron magnesium aluminium silicate hydroxide hydrate
Chemical Formula: Ca2Fe2+MgAl2(Si2O7)(OH)2.H2O
Method(s) of Verification: see pumpellyite-(Fe) entry.
- Metamorphic: low-grade
- Sedimentary: allogenic (detrital)
Sprays of pumpellyite (coloured crystals) in quartz, with minor prehnite (top right) from altered Ordovician dolerite, Fishguard area. Field of view 1.7 mm. Copyright: National Museum of Wales.
Introduction: pumpellyite, originally used as a species name, has now been redefined as a group name to cover the full range of compositions. These include; pumpellyite-(Al), pumpellyite-(Mg), pumpellyite-(Fe2+), pumpellyite-(Fe3+) and pumpellyite-(Mn) (Passaglia & Gottardi, 1973; Kato, et al., 1981). The precise mineral species can only be attributed on the basis of a full chemical analysis. Pumpellyite minerals occur in rocks of different composition and varying metamorphic grade. They are found most commonly in low grade rocks associated with zeolites, with prehnite in the prehnite-pumpellyite facies, with epidote and actinolite in higher grade rocks and in rocks of the blueschist facies (Deer et al., 1986).
Occurrence in Wales: pumpellyite is widely developed across Wales occurring as a product of low-grade metamorphic alteration of basic lavas and intrusions, principally of Palaeozoic age. The first description of pumpellyite in the British Isles was made by Nicholls (1957), who reported it from altered basic lavas of Ordovician age at Llanelwedd Quarry, near Builth Wells, Powys. This account was followed by a more complete description (Nicholls, 1958). More recently Bevins (1985) and Metcalfe, (1990) attributed the presence of pumpellyite-rich domains at Llanelwedd Quarry to the effects of an Ordovician hydrothermal system. Ridgeway (1971) noted the presence of pumpellyite in altered dolerites from the Aran Mountains of Mid Wales and suggested that it was of metamorphic origin. Bevins (1978) described the widespread occurrence of pumpellyite in the Fishguard area and presented the first chemical analysis from Welsh specimens. He also highlighted that the pleochroic sea-green to colourless mineral with a high refractive index described from this area by Reed (1895) as ‘anomalous chlorite’ was almost certainly pumpellyite; although pumpellyite was not formally described as a mineral until 1925 from the Keweenaw Peninsula, Michigan, U.S.A. (Palache & Vassar, 1925). Jenkins and Ball (1964) identified pumpellyite in soils of the Conway Valley area of North Wales, which they traced back to a source of altered gabbro and dolerite in the vicinity of Bwlch y Ddeufaen. In all instances microprobe or equivalent chemical analyses are required to make an accurate classification of this mineral. Most of the occurrences reported from Wales, including those listed above, are pumpellyite-(Fe). Further details and analytical data can be found under this entry.
There are no key localities for this specimen.
- Bevins, R.E., 1978. Pumpellyite-bearing basic igneous rocks from the Lower Ordovician of North Pembrokeshire, Wales. Mineralogical Magazine, 42, 81-83.
- Bevins, R.E., 1985. Pumpellyite-dominated metadomain alteration at Builth Wells, Wales – evidence for a fossil submarine hydrothermal system? Mineralogical Magazine, 49, 451-456.
- Deer, W.A., Howie, R.A., & Zussman, J., 1986. Rock-Forming Minerals Volume 1B 2nd Edition, Dilsilicates and Ring Silicates Longman Scientific & Technical.
- Kato, A., Matsubara, S. & Yamamoto, R., 1981. Pumpellyite-(Mn2+) from the Ochiai mine, Yamanashi Prefecture. Bulletin de Minéralogie, 104, 396-399.
- 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.
- Nicholls, G.D., 1957. Autometasomatism in the Lower Spilites of the Builth Volcanic Series. Proceedings of the Geological Society of London, 1543, 8-10.
- Palache, C. & Vassar, H.E., 1925. Some minerals of the Keweenawan copper deposits: pumpellyite, a new mineral; sericite; saponite. American Mineralogist, 10, 412.
- Passaglia, E. & Gottardi, G., 1973. Crystal chemistry and nomenclature of pumpellyites and julgodites. Canadian Mineralogist, 12, 219-223.
- Reed, F.R.C., 1995. The geology of the country around Fishguard, Pembrokeshire. Quarterly Journal of the Geological Society of London, 51, 149-195.
- Ridgeway, J., . The stratigraphy and petrology of Ordovician volcanic rocks adjacent to the Bala Fault in Merionethshire. Unpublished Ph.D., University of Liverpool.