Apatite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium phosphate carbonate fluoride hydroxide
Chemical Formula: Ca5(PO4,CO3)3(F,OH,Cl)
Method(s) of Verification: all descriptions are visual identifications based on optical properties (rock-forming occurrences) or distinctive crystal habit (alpine-type veins).

Chemical Group:

  • Phosphates

Geological Context:

  • Igneous
  • Sedimentary
  • Hydrothermal
SEM image of minute, but well-formed, euhedral apatite crystal (centre, bottom) associated with platy clinochlore and anatase (bottom left). Gloddfa Ganol Slate Quarry, Blaenau Ffestiniog. © National Museum of Wales.
Introduction: apatite is the group name for the hexagonal arsenates, vanadates and phosphates, and is used more specifically for undifferentiated calcium phosphates containing varying proportions of fluoride, hydroxide, chloride and carbonate. In this context, it is used to cover occurrences of apatite-group minerals that have not undergone sufficient analyses to determine precisely which member of the group is present. See the entries for carbonate-fluorapatite, carbonate-hydroxylapatite, fluorapatite and hydroxylapatite for confirmed occurrences of these species. Apatite group minerals occur in a wide variety of geological settings. In general, fluorapatite is most frequently found in igneous rocks and veins, while the carbonate and hydroxyl-bearing varieties are found in concretions in sedimentary rocks. As a vein mineral, apatite occurs most frequently in Wales in Alpine fissure-type vein associations with quartz, anatase, albite, chlorite and a range of rarer minerals.
Occurrence in Wales: apatite occurs widely in Wales with examples from igneous, sedimentary and hydrothermal geological settings. Undifferentiated apatite of igneous origin is documented from the St. David's head intrusion in Pembrokeshire, where it is most frequently encountered in relatively fractionated rocks such as granophyric gabbros and aplites (Roach, 1969). A similar occurrence is in fractionated gabbros in the nearby Fishguard area (Bevins, 1982). It also occurs in the Rhiw Intrusion on the Llyn Peninsula, in plagioclase-apatite-hornblende mesocumulates (Cattermole, 1976). The other localities given below pertain to Alpine fissure-type veins, although, given that fluorapatite has been analytically confirmed as the species present at similar localities, these may well all be examples of that species.

Key Localities:

  • Brynyrafr Mine, Ponterwyd, Ceredigion: apatite occurs with albite, rutile and quartz in small (<10 mm long) open cavities in sandstone and mudstone clasts in a quartz-cemented breccia (British Micromount Sociery Newsletter 35). These are clearly post-brecciation and a mechanism to explain their formation has yet to be developed. The assemblage has also been found, in a less well-developed form, at Castell, Darren, Erglodd, Ystrad Einion and Bwlchysgellyn Mines, and although widespread, is restricted paragenetically to the early or 'A1' assemblages (J.S. Mason, unpublished data).
  • Gloddfa Ganol Quarry, near Blaenau Ffestiniog, Gwynedd: complex euhedral crystals, associated with clinochlore and anatase, were discovered during Minescan follow-up work in 2001 (NMW/Manchester Museum, unpublished data).
  • Hendre Quarry, Glyn Ceiriog, Clwyd: small (0.5 mm), colourless crystals sometimes showing complex forms were described from this locality (Starkey et al., 1991).
  • Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: apatite occurs as complex hexagonal tabular crystals at this locality (Green & Middleton, 1996).

References:

  1. Bevins, R.E., 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal, 17, 1-21
  2. Cattermole, P.J., 1976. The crystallization and differentiation of a layered intrusion of hydrated alkali olivine-basalt parentage at Rhiw, North Wales. Geological Journal,11,45-70.
  3. Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.
  4. Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.
  5. 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.