Pectolite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium calcium silicate hydroxide
Chemical Formula: NaCa2Si3O8(OH)
Method(s) of Verification: Gimlet Quarry - XRD (National Museum of Wales, specimens NMW 27.111.GR.30, 258, 259 & 262, XRD nos. NMW X-738, X-740, X-742, & X-743 respectively and Natural History Museum, specimen no. B.M. 1944.3, x5302).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Hydrothermal: amygdale infill & veins in volcanic rocks
Botryoidal pectolite crystal masses, up to 15 mm in diameter, from Hendre Quarry. National Museum of Wales Collection (NMW 27.111.GR.252), ex G.J. Williams Collection. © National Museum of Wales.
Radiating masses of pectolite crystals coated by apophyllite, Hendre Quarry. Specimen 8 cm x 5.5 cm. National Museum of Wales Collection (NMW 27.111.GR.265), ex G.J. Williams Collection. © National Museum of Wales.
Introduction: pectolite forms a series with serandite - NaMn2Si3O8(OH). It occurs typically with zeolite minerals or prehnite and forms during hydrothermal alteration of basic igneous rocks. It may also occur as a rock-forming mineral in alkaline igneous rocks.
Occurrence in Wales: pectolite has been confirmed from two localities in Wales. In both instances it occurs in veins cutting basic intrusions and is interpreted as having formed by hydrothermal alteration during the cooling of the magma. Pectolite was first described from Hendre Quarry, Clwyd by Wedd et al. (1929) although no specimens have been collected in recent times (e.g. Starkey, et al., 1991). Fine specimens from this locality and from Gimlet Quarry, Pwllheli are held in the National Museum of Wales Collection and were acquired in 1927 as part of the G.J. Williams (H.M. Inspector of Mines, North Wales) Collection. A third occurrence of pectolite in Wales was reported by Sargent (1925), from the Graig Llwyd area of the Penmaenmawr Intrusion, North Wales. However analysis of so-called pectolite, in the National Museum of Wales Collection, from the Penmaenmawr Intrusion, has proved to be prehnite (NMW X-59). Sargent's original description of veins with drusy clusters of brownish elongate pectolite crystals associated with axinite is now considered questionable.

Key Localities:

  • Gimlet Quarry, Pwllheli, Gwynedd: pectolite from Gimlet Quarry occurs in quartz veins cutting gabbro (e.g. NMW 27.111.GR.29). It forms botryoidal masses of white silky fibres, up to 25 mm long, associated with calcite, and less commonly apophyllite (e.g. NMW 27.111.GR.262) and may be partially altered to stevensite (e.g. NMW 27.111.GR.259).
  • Hendre Quarry, Glyn Ceiriog, Clwyd: the finest examples of pectolite from Wales come from Hendre Quarry (e.g. NMW 27.111.GR.252). From the description of Wedd et al. (1929), pectolite is interpreted as occurring along calcite-filled, E-W orientated, joint planes. Radiating aggregates of silky white, or off-white, crystals several cm across are coated by apophyllite (e.g. 27.111.GR.255) and associated with prehnite (Bevins, 1994). The pectolite may be partially altered to stevensite (e.g NMW 27.111.GR.254).

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Starkey, R.E., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.
  3. Wedd, C.B., Smith, B., King, W.B.R. & Wray, D.A., 1929. The Country around Oswestry. Memoirs of the Geological Survey of England and Wales. Explanation of Sheet 137.

There are no references for this specimen.