Rosasite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper zinc carbonate hydroxide
Chemical Formula: (Cu,Zn)2(CO3)(OH)2
Method(s) of Verification: Frongoch Mine - XRD (Green et al., 1996); Gwaith-yr-afon Mine - XRD (NHM); Hendy Quarry - IR (Plant, 1993); Henfwlch Mine - XRD & EPMA (NMW X-1269); Locks Common - visual; Machen Quarry - IR (Plant & Jones, 1995), XRD (NMW X-911 & X-913)

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Globular aggregates of rosasite from Machen Quarry. I.E. Jones Collection. © National Museum of Wales.
Introduction: rosasite is a secondary carbonate formed as a result of oxidation of primary copper- and zinc-bearing minerals. Hemimorphite and aurichalcite are typically found in association with rosasite.
Occurrence in Wales: rosasite was first recorded in Wales by Fletcher & Young (1988) from calcite-lined vughs in Carboniferous Limestone at Bute Quarry, near Llantrisant and from a borehole at Locks Common, near Porthcawl in South Wales. These occurrences are both considered unconfirmed given the descriptions made, in particular, when the analytical results are carefully considered. It seems most likely that both cases are in fact zincian malachite with highly variable proportions of zinc. However, rosasite is likely to occur at Bute Quarry, as it has been identified from the neighbouring Hendy Quarry (Plant, 1993) where, it forms small bluish-green radiating spherules on calcite. At Machen Quarry, also in South Wales, rosasite is locally common. Blue-green spheroidal and botryoidal aggregates occur associated with altered chalcopyrite within barite-dominated veins. Further north, rosasite is present in small quantities in the Central Wales Orefield. Confirmed occurrences include, Gwaith-yr-Afon (Rust & Mason, 1994) Frongoch (Green et al., 1996) and Henfwlch mines (T.F. Cotterell, unpublished data), while rosasite is thought to be present at Eaglebrook Mine (National Museum of Wales Collection).

Key Localities:

  • Bute Quarry, Llantrisant, South Wales: Fletcher & Young (1988) described rosasite and zincian malachite from Bute Quarry on the basis of electron microprobe analyses. Only one of the four analyses fall within the published 0.40 to 0.68, Zn:Cu ratios for rosasite (Dana, 1951; Jambor, 1976) the other three are zincian malachite. However, Fletcher & Young report that following probe work, X-ray powder diffraction of the rosasite grain gave a pattern of malachite. This occurrence should therefore, be regarded as unconfirmed until further analyses are made.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: Jones (1983) listed sky blue velvet crusts and groups of acicular crystals as possible rosasite. While, a green globular phase associated with linarite on quartz, in the National Museum of Wales Mineral Collection is considered to be rosasite (NMW 95.55G.M.236).
  • Frongoch Mine, Devil's Bridge, Ceredigion: rosasite is rare, but does on occasion, form sizable (for Wales) 1.5 mm, blue-green balls on hemimorphite (Green et al., 1996). This occurrence appears to pre-date the well-documented post-mining assemblage at Frongoch.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: minute groups of green acicular crystals (<0.5 mm) and fragile epimorphs of laths of possibly serpierite, sparsely scattered over drusy malachite are shown to be rosasite (Rust & Mason, 1994).
  • Hendy Quarry, Miskin, South Wales: bluish-green radiating rosasite spherules are recorded by Plant (1993) in association with malachite and aurichalcite on calcite. Rosasite is rare at Hendy, with spherules found covering, areas of calcite to not more than 8 mm in extent.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: rosasite occurs rarely as attractive bright blue-green spherules (to 0.2 mm) on corroded gossan from underground workings (T.F. Cotterell, unpublished data).
  • Locks common, Porthcawl, South Wales: a similar mode of occurrence to Bute Quarry is recorded by Fletcher & Young (1988). No analyses were made and this account was based purely on comparison with alledged rosasite and zincian malachite from Bute Quarry.
  • Machen Quarry, Caerphilly, South Wales: rosasite is uncommon at Machen occurring only occasionally, as typical blue-green spheroidal to botryoidal masses. S.P. Plant first identified rosasite at Machen in 1992 (Plant & Jones, 1995) encrusting massive altered chalcopyrite from level 9. Occurrences tend to be localized and restricted to vein exposures in the upper levels of the quarry. In 1993, relatively rich rosasite was discovered forming blue-green spheroids (to 1 mm) and botryoidal aggregates (to 5 mm) in small cavities within barite gangue (Plant & Jones, 1995). In terms of paragenesis, rosasite has been observed both pre- and post-dating aurichalcite at Machen Quarry.

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. Dana, J.D., Dana, E.S & Palache, C., 1952. A System of Mineralogy. 7th Edition, Vol. 2. New York, John Wiley, 1124p.
  3. Fletcher, C.J.N. & Young, B.R., 1988. Rosasite from Bute Quarry, Mid Glamorgan. The first reported occurrence in Wales. Journal of the Russell Society, 2(1), 19-23.
  4. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  5. Jambor, J.L., 1976. Studies of basic copper and zinc carbonates: three powder X-ray data for zincian malachite, rosasite, and cobalt analogues. Rept. of Activities, Part C, Geological Survey of Canada, Paper 76-1C, 97-105.
  6. Jones, A.D., 1983. Nant-y-Cagl. Mineral Realm, 3, 42-76.
  7. Plant, S., 1993. Rosasite from Hendy Quarry, Miskin, South Wales. British Micromount Society Newsletter, 35, 8.
  8. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.
  9. Rust, S.A. & Mason, J.S., 1994. An unusual occurrence of arsenate minerals at Gwaith-yr-Afon mine, Dyfed, Wales. Journal of the Russell Society, 5(2), 109-113.

There are no references for this specimen.