Wavellite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Aluminium phosphate hydroxide fluoride hydrate
Chemical Formula: Al3(PO4)2(OH,F)3.5H2O
Method(s) of Verification: Gower - XRD (National Museum of Wales, NMW X-1188 & 1189)

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Radiating sprays of wavellite (to 38 mm across) from Pwlldu beach. I.E. Jones Collection. Photo D.I. Green, © D.I. Green.
Glassy prismatic wavellite with spherical cacoxenite from Pwlldu beach. Photo D.I. Green © D.I. Green.
Radiating aggregates of wavellite, up to 8 mm in diameter, from Cililfor Top, Llanrhidian, Gower. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Introduction: wavellite is found typically as globular radiating aggregates on fractures or joints in aluminium-rich sedimentary or metamorphic rocks.
Occurrence in Wales: Wavellite is restricted to Carboniferous sedimentary sequences in the south-west of Wales. W.E.Logan produced the first written account of wavellite from Wales in 1837, in a report in the Swansea Philosophical and Literary Institution Proceedings of the Second Annual Meeting. Logan described the presence of wavellite in sandstone at Cil Ifor Hill and in all the quarries from up to two miles east of Cil Ifor. However, Logan stated that these identifications are of interest because; they confirm Mr Conybeare's discovery of this mineral in the South Wales coal basin; suggesting that an earlier discovery had been made. Further occurrences of wavellite on the Gower are reported by Strahan (1907) and Trueman (1930), and a cluster of localities surrounding Tenby in Pembrokeshire are recorded by Dixon (1921).

Key Localities:

  • Bishopston, Gower, South Wales: wavellite occurs in roadside sections between Clyne Common and Bishopston, occurring as 'discs of radiating crystals' in joints of the harder bands of a sequence of bedded clays with thin seams of limestone and chert (Strahan, 1907).
  • Cil Ifor Hill, Llanrhidian, Gower, South Wales: found in the joints of a 'whitish-yellow close-grained sandstone … used for building purposes' (North, 1916). Plant and Jones (2001) noted wavellite in fossiliferous rottenstones, in a temporary roadside exposure near Cae-Morgan farm on the southern slopes of Cil Ifor Top hill.
  • Gumfreston, near Tenby, Pembrokeshire: in striped cherts of Carboniferous age exposed in a lane west of Gumfreston (Dixon, 1921).
  • Lydstep, near Tenby, Pembrokeshire: in chert debris at Lydstep Lodge and in shales exposed in the valley bottom a short distance to the north-west (Dixon, 1921).
  • Pwll-du Head, Bishopston, Gower, South Wales: wavellite of a greenish tint occurs in cracks in the clayey rocks associated with rottenstone in a quarry on the headland (North, 1917). Sir Arthur Russell collected wavellite from an exposure above the quarry in 1916, including sea-green hemispheres to 4-5 mm across and discs of golden radiating crystals to 30 mm in diameter. These specimens now reside in the Natural History Museum (specimen numbers BM 1921,246 and Russell Collection).
  • Pwll-du storm beach, Bishopston, Gower, South Wales: occasional pebbles of chert and siliceous breccia washed up on the storm beach contain white to green wavellite. The wavellite forms as veins cementing brecciated rock fragments. When broken the breccia's reveal radial growths of wavellite (to 25 mm across) and rarely hemispheres displaying terminated crystals (Plant & Jones, 2001). It is believed that the wavellite-bearing material is derived from a quarry at Pwll-du Head, but this has not been proven.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. De La Beche, H.T., 1846. On the formation of the Rocks of South Wales and South Western England. Memoirs of the Geological Survey, 1, 296pp.
  3. Dixon, E.E.L., 1921. The Geology of the South Wales Coalfield. Part XIII. The Country around Pembroke and Tenby. Memoirs of the Geological Survey of England and Wales. Explanation of Sheets 244 and 245.
  4. North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.
  5. Plant, S.P., & Jones, I.E., 2001. Wavellite and variscite on Gower, Swansea, South Wales. Journal of the Russell Society, 7 (2), 79-81.
  6. Strahan, A., 1907. The Geology of the South Wales Coalfield, Part VIII. The Country around Swansea. Memoirs of the Geological Survey of England and Wales, Sheet 247.
  7. Trueman, A.E., 1930. Wavellite in the cherts at Bishopston. Swansea Scientific and Field Naturalists' Society Proceedings, 1, 98-99.