Mineral Database

Mineral Database
Mineralogy of Wales

Pennantite

Crystal System: Monoclinic
Formula: (Mn5Al)(Si3Al)O10(OH)8
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Uncommon
Chemical Composition: Manganese aluminium silicate hydroxide
Method(s) of Verification: The new mineral pennantite was described on the basis of specimens obtained from Benallt Mine. Characterization of this mineral was made using X-ray analysis and chemical analysis (in Campbell Smith et al., 1946).

Dark brown pennantite with celsian (grey to white) and hematite (red) from Benallt Mine, Pen Ll?n. Photo 1.5cm across. National Museum of Wales specimen, no. NMW 2006.15G.M.5. © National Museum of Wales.

Dark brown pennantite crystals embedded in massive celsian from Benallt Mine, Pen Ll?n. Field of view approximately 2 cm across. National Museum of Wales specimen, no. NMW 2006.15G.M.5. © National Museum of Wales.

Chemical Group:
  • Silicates
Geological Context:
  • Metamorphic : low-grade
Introduction:

a manganese-rich chlorite group mineral typically formed during hydrothermal alteration of manganese deposits. Pennantite was first discovered in manganese ores at Benallt Mine, Llŷn, Gwynedd, North Wales and is named in honour of the Welsh mineralogist Thomas Pennant (1726-1798). Two polytypes are known, pennantite-IIb and pennantite-Ia, of which IIb represents the original material described by Campbell Smith at al. (1946) and Ia the mineral grovesite, described as a new species by Bannister et al. in 1955, but shown by Bayliss (1983) to be a polytype of pennantite.

Occurrence in Wales:

the new mineral pennantite was first described by Campbell Smith et al. (1946) from specimens collected from the Ty Canol incline at Benallt Mine, Rhiw, Llŷn. Type material is held at the Natural History Museum, London, registered as B.M. 1947, 295. The mineral described by Bannister et al. (1955) as grovesite, but now regarded as pennantite-Ia was discovered in the No. 5 ore-body at Benallt Mine. More recently Bennett (1987b) has reported the occurrence of pennantite in the manganese ore bed of the Hafotty Formation of Cambrian age, exposed in the Harlech Dome area of Gwynedd.

Key Localities:
  • Benallt Mine, Llŷn, Gwyneddŷ: a detailed description of the new mineral pennantite was produced by Campbell Smith et al. (1946) from specimens collected in the Ty Canol incline. The pennantite forms small (0.5 mm) orange-brown micaceous flakes in thin veins traversing massive ore and within the ore itself. The mineral described by Bannister et al. (1955) as grovesite, but now regarded as pennantite-Ia was discovered in the No. 5 ore-body. It forms a thin dark brown (blackish-brown) crust on manganese ore on Natural History Museum specimen no. B.M. 1944,35, and consists of closely-packed, small rosettes, at most 0.5 mm across coated by a thin layer of baryte.
References:
  • Bannister, F.A., Hey, M.H. and Campbell Smith, W., 1955 Grovesite, the manganese-rich analogue of berthierine.  Mineralogical Magazine, 30, 645-647.
  • Bayliss, P., 1983 The polytypes of pennantite.  Canadian Mineralogist, 21(3), 545-547.
  • Bennett, M.A., 1987b Genesis and diagenesis of the Cambrian manganese deposits, Harlech, North Wales.  Geological Journal, 22, 7-18.
  • 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.