Apophyllite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Potassium sodium calcium fluorosilicate hydrate
Chemical Formula: (K,Na)Ca4Si8O20(OH,F).8H2O
Method(s) of Verification: Hendre Quarry - XRD at the National Museum of Wales (NMW X-834).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
Apophyllite on pectolite from Hendre Quarry, near Glyn Ceriog, Clwyd. 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: apophyllite is a group name for the three end-member species fluorapophyllite, hydroxyapophyllite and natroapophyllite. These are commonly found with calcite and various zeolite minerals in amygdaloidal cavities or secondary veins developed in altered basic igneous rocks.
Occurrence in Wales: although apophyllite is well-known as fine crystals from basaltic lava-fields from various parts of the world, in Wales there are few such terrains and consequently few apophyllite occurrences. Two of the three Welsh occurrences are from localities better-known for Alpine fissure-type vein assemblages.

Key Localities:

  • Gimlet Quarry, Pwllheli, Gwynedd: apophyllite occurs with pectolite in cavities within microgabbro (e.g. specimen NMW 27.111.GR.262, ex G.J. Williams Collection). Specimens can no longer be collected from this locality.
  • Hendre Quarry, Glyn Ceiriog, Clwyd: National Museum of Wales specimens NMW 27.111.GR.263-265 (ex G.J. Williams Collection) consist of blocky white pseudocubic crystals on pectolite. These specimens fail to indicate what, if any, relationship these occurrences have to the well known Alpine fissure-type mineralization also present at the quarry.
  • Holy Island, Anglesey: Maltman (1977) recorded blades of apophyllite in altered gabbros associated with the Monian Supergroup metasediments and volcanics.

References:

  1. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.
  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.