Mineral Database

Mineral Database
Mineralogy of Wales

Pyrosmalite

Crystal System: Hexagonal
Formula: (Fe2+,Mn)8Si6O15(OH,Cl)10
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Iron manganese silicate hydroxide chloride
Method(s) of Verification: Blaen-y-Nant - optically and X-ray powder diffraction (Brown, 1959).
Chemical Group:
  • Silicates
Geological Context:
  • Metamorphic
Introduction: pyrosmalite is a series with iron- and manganese-rich end members - ferropyrosmalite and manganpyrosmalite respectively. Pyrosmalite is often the name given to undifferentiated members of this series, although has in the past been used for iron-rich varieties. Pyrosmalite typically occurs within metamorphosed rocks.
Occurrence in Wales: this rare mineral was described by Brown (1959), from the Nant Ffrancon area in Gwynedd, where it occurs as small clusters of radiating crystals in metasomatized Ordovician slates. Brown suggested that given the preponderance of iron over manganese in the host rock that,'it seems likely that the mineral is pyrosmalite rather than manganpyrosmalite'. However, Brown also notes that the X-ray powder pattern agrees well with that of manganpyrosmalite.
Key Localities:
  • Blaen-y-Nant, Nant Ffrancon, Gwynedd: Brown (1959) describes pyrosmalite as a very minor constituent in metasomatized Ordovician slates close to the contact with intrusive rhyolite and near the old Nant Ffrancon road, north-west of Blaen-y-Nant. Pyrosmalite forms clusters of radiating crystals up to 0.5 mm in length.
References:
  • Brown, P.E., 1959 A note on pyrosmalite from Nant Francon, North Wales.  Mineralogical Magazine, 32, 242-244.