Mineral Database

Mineral Database
Mineralogy of Wales

Ramsdellite

Crystal System: Orthorhombic
Formula: MnO2
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Manganese dioxide
Method(s) of Verification: Drosgol Mine - XRD at the National Museum of Wales (specimen no. NMW 2003.22G.M.4, XRD no. NMW X-1353).

Prismatic ramsdellite in manganese ore from Drosgol Mine, Central Wales Orefield. National Museum of Wales Collection. Photo T.F. Cotterell, © National Museum of Wales.

Geological Context:
  • Supergene : in situ natural oxidation & weathering deposits
Introduction: ramsdellite is a secondary manganese mineral polymorphous with pyrolusite and akhtenskite. Ramsdellite typically forms through the alteration of pyrolusite or groutite in manganese deposits.
Occurrence in Wales: ramsdellite occurs within rich secondary manganese mineralization derived from an intensely altered section of the base metal dominated Camdwr Lode at Drosgol in the Central Wales Orefield (Cotterell, 2009). Complete alteration of primary sulphides, namely sphalerite, and leaching of surrounding host rocks has resulted in the formation of a strong manganese-zinc dominated assemblage (Cotterell & Mason, 2003).
Key Localities:
  • Drosgol Mine, Drosgol, Ponterwyd, Ceredigion: thin (2 mm) layers of prismatic ramsdellite crystals occur associated with massive cryptomelane and small, platy, chalcophanite crystals cementing extensively bleached brecciated mudstone clasts within the upper portion of the Camdwr Lode (Cotterell, 2009).
References:
  • Cotterell, T.F., 2009 Supergene manganese mineralization associated with the Camdwr Fault in the Central Wales Orefield.  Journal of the Russell Society, 12, 15-25.
  • Cotterell, T.F., & Mason, J.S., 2003 Chalcophanite from Drosgol Mine, Ponterwyd, Ceredigion: The First Welsh Occurrence.  UK Journal of Mines and Minerals, 23, 39-40.