Vivianite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Iron phosphate hydrate
Chemical Formula: Fe3(PO4)2.8H2O
Method(s) of Verification: Valley – XRD (National Museum of Wales, XRD no. NMW X-283); Threapwood/Tallarn Green – XRD (National Museum of Wales, XRD no. NMW X-818); Kidwelly - unrecorded.

Chemical Group:

  • Phosphates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: vivianite is a secondary phosphate mineral that forms in range of geological settings, which include gossans and more generally the oxidation zone of ore deposits; phosphate bearing pegmatites: clays and superficial deposits particularly where organic matter is present. Although vivianite can form radiating crystals, sedimentary and authigenic vivianite commonly occurs as an earthy coating with no visible crystals. Vivianite is a light-sensitive mineral and darkens from colourless to dark green-blue or black on exposure to light, this colour change and continued exposure to light may cause disintegration in larger crystals.
Occurrence in Wales: Two occurrences of vivianite in Wales were reported by Bevins (1994), both confirmed by X-ray diffraction. In both instances the mineral has formed in superficial deposits containing organic matter. A third occurrence was described by Archer (1968) within alluvium near the mouth of the River Gwendraeth, Kidwelly.

Key Localities:

  • Kidwelly, Carmarthenshire: vivianite was encountered with roots within a bluish grey clay up to 1 ft. 9 in. thick on the north side of an excavation for a sewage pump near the mouth of the River Gwendraeth (Archer, 1968).
  • Tallarn Green, Wrexham: vivianite was found in boulder clay during excavation of a culvert, near Threapwood, Clwyd (S. Conway, personal communication in Bevins, 1994). No further data is available at present on this occurrence.
  • Valley (Dyffryn), Anglesey: a fragment of iron found in peat bog at Valley has a coating of powdering blue vivianite (National Museum of Wales specimen NMW 43.282.GR.1 recorded in Bevins, 1994).

There are no key localities for this specimen.

References:

  1. Archer, A.A., 1968. Geology of the South Wales Coalfield. Special Memoir. The Upper Carboniferous and later formations of the Gwendraeth Valley and adjoining areas in parts of the Carmarthen (229), Ammanford (230) and Worms Head (246) sheets.
  2. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.

There are no references for this specimen.