Pyrophanite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Manganese titanium oxide
Chemical Formula: MnTiO3
Method(s) of Verification: Benallt Mine - XRD and wet chemistry at the Natural History Museum, London (XRD ref. no. x3576); Harlech area - EMPA (Bennett, 1987a).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Hydrothermal
  • Metamorphic
Scanning electron micrograph of a pyrophanite crystal displaying stepped triangular growth patterns. Benallt Mine. National Museum of Wales Collection (NMW 2002.62G.M.1), collected by R. Bell. © National Museum of Wales.
Scanning electron micrograph of a euhedral pyrophanite crystal from Benallt Mine. Scale bar 250 microns (0.25 mm). National Museum of Wales Collection (NMW 2002.62G.M.1), collected by R. Bell. © National Museum of Wales.
Introduction: pyrophanite is a manganese titanium oxide mineral and the Mn-analogue of ilmenite (Fe2+TiO3). It is found most commonly in metamorphosed manganese deposits and also as a an accessory mineral in granite, amphibolite, and serpentinites.
Occurrence in Wales: pyrophanite is known from two manganese deposits in Wales; Benallt Mine and in the manganese-rich mudstones in the area to the east of Harlech, known as the Harlech Dome.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: pyrophanite is found at this locality as small, tabular, orange-yellow crystals up to 0.25 mm in diameter and 0.15 mm in thickness. In thin section the crystals are characterized by very high birefringence and a high refractive index (n > 1.84). Identification has been confirmed by X-ray diffraction of Natural History Museum sample B.N. 1944, 50 and wet chemistry has demonstrated the presence of manganese and titanium (Campbell Smith et al., 1946; Campbell Smith & Claringbull, 1947).
  • Harlech Dome area, Gwynedd: Bennett (1987a&b) reported the presence of small, sparse, scattered, opaque grains of titanium-manganese-iron oxides ('ferropyrophanite') throughout the manganese ore and pyritic mudstones of the Haffoty Formation of Cambrian age, exposed in the Harlech Dome area of Gwynedd. Electron microprobe analyses indicate that these crystals belong to the solid solution series between pyrophanite and ilmenite.

References:

  1. Bennett, M.A., 1987a. The Cambrian manganese deposits of North Wales. Unpublished Ph.D. thesis, University of Leeds.
  2. Bennett, M.A., 1987b. Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales. Geological Journal, 22, 7-18.
  3. Campbell Smith, W. & Claringbull, G.F., 1947. Pyrophanite from the Benallt mine, Rhiw, Carnarvonshire. Mineralogical Magazine, 28, 108-110.
  4. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1946. Pennantite, a new manganese-rich chlorite from Benallt Mine, Rhiw, Carnarvonshire. Mineralogical Magazine, 27, 217-220.