Cacoxenite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Iron aluminium phosphate hydroxide hydrate
Chemical Formula: (Fe3+,Al)25(PO4)17O6(OH)12.17H2O
Method(s) of Verification: Pwll-du Beach - XRD & EMPA at the National Museum of Wales (XRD film no. NMW X-1309).

Chemical Group:

  • Phosphates
Radiating sprays of golden cacoxenite from Pwll-du Beach, Gower. Each individual spray is approximately 0.35 mm across. Photo D.I. Green, © D.I. Green.
Introduction: cacoxenite occurs typically in phosphate-dominated assemblages with associated minerals including strengite, beraunite, variscite and wavellite plus quartz. It commonly forms inclusions in quartz and may be of detriment to semi-precious varieties of this mineral, such as amethyst, since it imparts its own colour to the quartz, giving it a brownish tint.
Occurrence in Wales: first identified from Wales on a specimen collected by Mr A. Dean at Pwll-du Beach, Bishopston, Gower. Crystals are very small, forming radiating sprays that rarely exceed 0.3 mm in diameter, but are clearly visible using magnification.

Key Localities:

  • Pwll-du storm beach, Bishopston, Gower, South Wales: cacoxenite occurs rarely as minute (<0.3 mm) radial sprays and spherules of velvety golden-orange acicular crystals on fracture surfaces in chert. Associated minerals are wavellite, variscite and crandallite with quartz. Cacoxenite is late in the paragenesis, and forms partial overgrowths on wavellite (Dean & Cotterell, 2003).

References:

  1. Dean, A. & Cotterell, T.F., 2003. Cacoxenite and crandallite from Pwlldu Beach, Bishopston, Gower, Swansea, South Wales: The first Welsh occurrence. Journal of the Russell Society, 8(1), 30-32.