Synchysite-(Ce)

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Calcium rare earth fluorocarbonate
Chemical Formula: Ca(Ce,La,Nd,Y)(CO3)2F
Method(s) of Verification: occurrences listed - XRD (Manchester Museum) & EMPA (National Museum of Wales & Manchester Museum)

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal: alpine type veins
Pseudohexagonal prismatic synchysite-(Ce) crystal with anatase and minor xenotime. Cwmorthin Quarry, Gwynedd. Photo D.I. Green, © D.I. Green.
Partially altered (orange colour) synchysite-(Ce) rosette (2 mm across) from Gloddfa Ganol Quarry, Gwynedd. Photo T.F. Cotterell, © National Museum of Wales.
A 0.5 mm diameter rosette of synchysite-(Ce) with minor pyrite, quartz and anatase in association. Dolyhir Quarry. Specimen and photo D.I. Green. © D.I. Green.
Introduction: synchysite-(Ce) is limited in its occurrence to a relatively small number of paragenetic situations. It occurs in pegmatites and in carbonatites: however it is perhaps best known from alpine fissure-type veins, where it is typically accompanied by other REE-bearing minerals and quartz, albite, the TiO2 polymorphs and chlorite. Synchysite-(Ce) is rare in known occurrences, the platy to prismatic pseudohexagonal crystal habit of this mineral is as distinctive aid to identification. However, caution needs to be exercised because synchysite is virtually identical visually to the closely related mineral, parisite (in which there is more fluoride and less carbonate), and can only be distinguished from it by analytical means.
Occurrence in Wales: synchysite-(Ce) was first discovered in Wales, during the 1980s and 1990s when a small number of finds were made, all from alpine fissure-type veins, in the Blaenau Ffestiniog area of North Wales. Only two accounts have so far been published of these occurrences and only one in any detail (Bevins, 1994; Green & Middleton, 1996). The most spectacular examples found to date were discovered during fieldwork by National Museum of Wales and Manchester Museum staff at the slate quarries to the north of Blaenau Ffestiniog in 2001: further research is required on this material prior to full descriptions being published. An unexpected, but quite rich occurrence of synchysite-(Ce) was discovered at Dolyhir Quarry in the Welsh borderland, in 2005 (Cotterell et al., 2011), far removed from the earlier discoveries in the mountains of Snowdonia.

Key Localities:

  • Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd: synchysite-(Ce) is rare at this locality but a small number of unusually rich specimens have been recovered from thin alpine fissure-type veins hosted by notoriously tough and splintery banded tuff-turbidite. The finest specimens (including the top illustration above) all came from a single, planar cavity discovered in 2001: these revealed richly scattered buff to orange-brown pseudohexagonal tabular and prismatic crystals in the 0.5 to (exceptionally) 4 mm size range, associated with quartz, black bipyramidal anatase and microscopic crystals of xenotime.
  • Dolyhir Quarry, Old Radnor, Powys: identified initially, in 2005, as pale pink plates forming rosettes to 0.5 mm across, in large scale alpine-type fractures cutting dark coloured Yat Wood Formation shales immediately below an unconformity (Cotterell et al., 2011). Richer specimens, again in dark coloured Yat Wood Formation shales, have been discovered as the quarry progresses deeper. Particularly good examples were collected in 2007.
  • Gloddfa Ganol Quarry, Blaenau Ffestiniog, Gwynedd: although geographically very close to Cwmorthin, the habit of synchysite-(Ce) at this locality is quite different to that at the latter locality, typically forming rosettes 1-5 mm across of pinkish-white thin, platy crystals, often sparingly coated in microscopic crystals of pyrite. Again, it is very rare, occurring in alpine fissure-type quartz veins and, with albite and anatase, in occasional blocks of vuggy brecciated slate, the bedrock source for which has not been discovered (it may have been quarried away).
  • Manod Quarry, Blaenau Ffestiniog, Gwynedd: well-known for small synchysite specimens for some time, but the only published record is that in Bevins (1994). Pinkish to white pseudohexagonal plates up to 1 mm in size, aggregated into rosettes, occur in association with quartz and albite in thin alpine fissure-type veins cutting an altered intermediate intrusive rock.
  • Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: recorded by Green & Middleton (1996) as a very rare late-stage mineral in the alpine fissure-type veins that were exposed by blasting in the mid-1970s at this locality. Synchysite-(Ce) forms white tabular crystals, up to 0.5 mm acoss, in association with quartz, albite, anatase and clinochlore.

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. 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.
  3. Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.

There are no references for this specimen.