Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Chemical Composition: Potassium calcium manganese iron zinc magnesium aluminium silicate hydroxide hydrate
Chemical Formula: KCa(Mn,Fe2+,Zn,Mg)20(Si,Al)32O76(OH)16 •4-12H2O
Method(s) of Verification: Benallt Mine - XRD analysis (Smith & Frondel, 1968).
- Metamorphic: low-grade
Pale brown bannisterite intergrown with white calcite from Benallt Mine, Pen Ll?n. Specimen 5.5 cm across. National Museum of Wales specimen no. NMW 2007.7G.M.1. © National Museum of Wales.
Introduction: bannisterite is a rare mineral typically found in metamorphosed manganese and zinc deposits. It is named after Dr. F.A. Bannister, former Keeper of Minerals at the British Museum (Natural History), London who researched the mineralogy of Benallt Mine, where bannisterite was first discovered.
Occurrence in Wales: the only occurrence of bannisterite known from Wales is from the type locality at Benallt Mine. Type material is held at the Natural History Museum, London, registered as B.M. 1967, 321.
- Benallt Mine, Llŷn, Gwynedd: Campbell Smith (1948) recorded ganophyllite in veins cutting manganese ore from Tŷ Canol incline of Benallt Mine, Llŷn but noted that the six-sided brown crystals (<1 mm) had two optical forms. Smith & Frondel (1968) later demonstrated that these cinnamon-brown crystals could be separated into two different minerals; ganophyllite and a new mineral bannisterite. The bannisterite is associated with calcite and is found on fractures surfaces of the manganese ore.
- Campbell Smith, W., 1948. Ganophyllite from the Benallt Mine, Rhiw, Caernarvonshire. Mineralogical Magazine, 28, 343-352.
- Smith, M.L. & Frondel, C., 1968. The related layered minerals ganophyllite, bannisterite, and stilpnomelane. Mineralogical Magazine, 36, 893-913.