Pumpellyite-(Fe)

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: calcium iron aluminium silicate hydroxide hydrate
Chemical Formula: Ca2Fe2+Al2(Si2O7)(OH)2.H2O
Method(s) of Verification: various localities - EMPA (Bevins & Rowbotham, 1983; Roberts, 1981).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Introduction: pumpellyite-(Fe) is a member of the pumpellyite group (see pumpellyite entry for more details), and can be divided into pumpellyite-(Fe2+) and pumpellyite-(Fe3+). However it is difficult to differentiate between these two species on the basis of electron microprobe data alone, as all iron is calculated on the basis of FeO (Fe2+). More recent work using Mössbauer spectrometry suggests that all iron within low-grade metamorphic pumpellyite is present as Fe3+.
Occurrence in Wales: the Lower Palaeozoic igneous and sedimentary rocks of the Welsh Basin were metamorphosed predominantly under prehnite-pumpellyite facies conditions, which equates roughly to heating to around 300°C and burial to a depth of at least 4.5 km. The widespread occurrence of pumpellyite in the Welsh Basin was demonstrated by Bevins & Rowbotham (1983) and Roberts (1981). In light of the work of Artioli & Geiger, 1994 and Artioli et al., 1995, it is reasonable to classify all reported occurrences of pumpellyite in Wales as pumpellyite-(Fe|3|+).

Key Localities:

  • Fishguard, Pembrokeshire: Bevins (1978) published the first electron microprobe data for Welsh pumpellyite from two localities in the Fishguard area (Y Garn and Treseissyllt). These analyses can be re-interpreted as pumpellyite-(Fe3+). The pumpellyite occurs in a variety of forms within basalt, dolerite and gabbro (i) in veins as stout prismatic crystals (up to 0.04 mm long) and is frequently associated with prehnite (ii) as amygdale infillings (iii) scattered through the matrix of the rock as small prismatic crystals (iv) replacing plagioclase or more rarely pyroxene.
  • Llanelwedd Quarry, Builth Wells, Powys: Metcalfe (1990) in his detailed study of the low-grade metamorphic mineralogy of this site provides 147 electron microprobe analyses of pumpellyite, which, for the reasons given above, can be regarded as pumpellyite-(Fe3+). He shows that this data includes a range of composition, as wide as that recorded throughout the rest of the Welsh Basin. Metcalfe (1990) describes a range of textures (metadomains) in which pumpellyite may occur alone replacing matrix pyroxene or plagioclase; pumpellyite may be associated with prehnite, and calcite where these minerals pseudomorph plagioclase; or pumpellyite-bearing veins. The metadomains formed in an Ordovician hydrothermal system, where water (possibly sea-water) was heated by shallow intrusions.
  • Penmaenmawr, Conwy: Durham (2004) notes dark patches within quartz veins as consisting of aggregates of minute pumpellyite crystals. Rarer, coarser-grained patches of quartz are typicaly rimmed by pumpellyite (with crystals up to 0.05 mm across) and prehnite.
  • Tal y Fan, Conway: Bevins & Merriman (1988) from a detailed study of the Tal y Fan Intrusion, reported pumpellyite as part of a prehnite-pumpellyite-epidote assemblage formed during low-grade regional metamorphism. Pumpellyite forms aggregates of small, randomly orientated crystals in chlorite or small inclusions within plagioclase.

There are no key localities for this specimen.

References:

  1. Artioli, G. & Geiger, C.A., 1994. The crystal chemistry of pumpellyite: an X-ray Rietveld refinement and 57Fe Mössbauer study. Physics and Chemistry of Minerals, 20, pp 443–453.
  2. Artioli, G. & Quartieri, S. & Deriu, A., 1995. Spectroscopic data on coexisting prehnite-pumpellyite and epidote-pumpellyite. Canadian Mineralogist, 33, 67–75.
  3. Bevins, R.E., 1978. Pumpellyite-bearing basic igneous rocks from the Lower Ordovician of North Pembrokeshire, Wales. Mineralogical Magazine, 42, 81-83.
  4. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  5. Bevins,R.E. & Merriman, R. J., 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphism field gradients. Journal of Metamorphic Geology, 6, 17-39.
  6. Durham, J., 2004. The Petrogenesis of the Penmaenmawr Intrusion, North Wales. Unpublished M.Phil thesis, The Open University, 307pp.
  7. 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.
  8. Roberts, B., 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llyn and Snowdonia, Gwynedd, North Wales. Geological Magazine, 118, 189-200.

There are no references for this specimen.