Banalsite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Locally Abundant
Chemical Composition: Barium sodium aluminium silicate
Chemical Formula: BaNa2Al4Si4O16
Method(s) of Verification: Benallt Mine - X-ray analysis (Campbell Smith et al., 1944a&b & at the National Museum of Wales, reference numbers NMW X-1495, 1515, 1585 & 1623).

Chemical Group:

  • Silicates
Massive cream-coloured banalsite from Benallt Mine. National Museum of Wales specimen (NMW 2002.54G.M.1). Photo D.I. Green, © National Museum of Wales.
Introduction: banalsite is a rare mineral which belongs to the feldspar family and may form a solid solution with stronalsite (Na2SrAl4Si4O16). It was the first barium aluminium silicate to be described with sodium as the dominant alkali. Banalsite forms under hydrothermal or metamorphic conditions. The name banalsite was formulated from its constituent chemical elements (Ba, Na, Al and Si).
Occurrence in Wales: banalsite was described as a new species by Campbell Smith et al. (1944a&b) from Benallt Mine during the Second World War, when the mine was re-opened by the Home Ore Department of Iron and Steel Control of the Ministry of Defence. It is only known in Wales from the type locality. Initially only massive material was described (Campbell Smith et al., 1944a&b) but later finds revealed crystalline material enabling crystallographic data to be published (Campbell Smith, 1945). Banalsite from the type locality is found in veins and vugs where it is associated with a suite of minerals believed to have been derived from hydrothermal activity involving manganese-rich fluids exhaled onto the Ordovician sea floor (Brown & Evans, 1989).

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: this rare barium feldspar mineral was discovered in July 1943 in ore ‘on the footwall side of no. 1 ore-body by no. 1 Chute, 50-60 feet west of the main shaft, some 10-20 feet above the 130-foot level’. It was found subsequently in the no. 2 ore-body. The banalsite typically forms thin bands of coarsely crystalline material cutting dark spotted manganese ore. Crystals of banalsite were found subsequently, occurring as small, colourless crystals, up to 2 mm in size, within small cavities in the no. 5 ore body. The crystals displayed a pearly to vitreous lustre and were associated with harmotome. In all occurrences in the mine banalsite is associated with calcite, baryte, tephroite, alleghanyite, jacobsite, biotite, natrolite and harmotome.

References:

  1. Brown, M.J. & Evans, A.D., 1989. Geophysical and geochemical investigations of the manganese deposits of Rhiw, western Llyn, North Wales. British Geological Survey Technical Report WF/89/14 (BGS Mineral Reconnaissance Programme Report No.102).
  2. Campbell Smith, W., 1945. Banalsite crystals from Wales. Mineralogical Magazine, 27, 63-64.
  3. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944a. A new barium-feldspar from Wales. Nature, 154, 336-337.
  4. Campbell Smith, W., Bannister, F.A. & Hey, M.H., 1944b. Banalsite, a new barium-feldspar from Wales. Mineralogical Magazine 27, 33-47