Ewaldite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Barium calcium yttrium sodium potassium carbonate
Chemical Formula: Ba(Ca,Y,Na,K)(CO3)2
Method(s) of Verification: Dolyhir Quarry - XRD (Manchester Museum [MANCH:XRD1021] & National Museum of Wales [NMW X-1680]) & EDS (Manchester Museum).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: alpine type veins
Yellow-brown hexagonal ewaldite crystal 2.5 mm long with colourless prismatic harmotome, from Dolyhir Quarry. Photo D.I. Green, © D.I. Green.
A 1 mm tall, equant stepped pyramidal ewaldite crystal from Dolyhir Quarry. Neil Hubbard Collection, photo D.I. Green. © D.I. Green.
Scanning electron micrograph of a ewaldite crystal showing stepped growth patterns, with cruciform-twinned prismatic harmotome crystals, from Dolyhir Quarry. National Museum of Wales Collection (NMW 2007.22G.M.16). © National Museum of Wales.
Introduction: ewaldite occurs typically within evaporite deposits, but in the case of Dolyhir Quarry it is found within a hydrothermal barium carbonate dominated assemblage.
Occurrence in Wales: ewaldite is an extremely rare mineral previously recorded from only three other localities worldwide. In 2003, N. Hubbard found specimens forming extremely rare, creamy, hexagonal, prismatic microcrystals at Dolyhir Quarry in the Welsh Borderlands. A second discovery made, at Dolyhir, by T.F. Cotterell in 2004 revealed, sparse, euhedral, pale yellow prismatic microcrystals, exceptionally to 2.5 mm in length associated with euhedral core-bit twinned harmotome in a loose boulder of Yat Wood Formation sediments, beneath the north face. Both finds were detailed by Green et al. (2005). Several more discoveries have been made at Dolyhir Quarry, including in-situ material, with some handspecimens displaying tens of crystals richly scattered across fractures in Precambrian Yat Wood Formation rocks (T.F. Cotterell, unpublished data).

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: ewaldite was identified initially in 2003, as extremely rare creamy hexagonal prismatic microcrystals on a specimen collected by N. Hubbard. A second discovery (T.F. Cotterell, 2004) has the form of sparse, euhedral, pale yellow prismatic microcrystals, exceptionally up to 2.5 mm in length associated with euhedral core-bit twinned harmotome in a loose boulder of Yat Wood Formation sediments, beneath the north face. EDS analysis showed barium, yttrium, sodium and calcium to be the major elements present with an atomic number greater than 10, while a reaction with dilute mineral acid suggests the mineral is a carbonate (Green et al., 2005). When combined, this data rules out all other possible mineral species including, the visually and chemically similar mineral mckelvyite (this has a different XRD pattern) and the undescribed, but structurally similar, strontium analogue of ewaldite (Green et al., 2005).

References:

  1. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.
  2. Green, D.I., Hubbard, N. and Cotterell, T.F., 2005. The first British occurrence of ewaldite, at Dolyhir Quarry, Old Radnor, Powys. UK Journal of Mines and Minerals, 25, 23-24.