Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence - 2nd UK recording
Chemical Composition: Copper sulphide (non-stoichiometric)
Chemical Formula: Cu9S8
Method(s) of Verification: high-resolution ore petrology
- Supergene: in situ natural oxidation & weathering deposits
Introduction: yarrowite is one of several non-stoichiometric copper sulphide species. By this it is meant that they have compositions which fit in between copper1+ sulphide (chalcocite, Cu2S), and copper2+ sulphide (covelline, CuS). Many of these minerals are recent discoveries as they occur in complex, microscopic intergrowths which have required modern analytical techniques to unravel their detailed mineralogy. The combined techniques of high-resolution ore petrology, X-ray diffraction and electron microprobe analyses are required in order to identify yarrowite and related minerals.
Yarrowite occurs, along with the other non-stoichiometric copper sulphides and covelline, in supergene alteration assemblages derived from primary sulphide mineralisation in which copper is an important component. It typically occurs as replacements of the primary minerals, forming rims and veinlets along cleavage-planes and/or grain boundaries
Occurrence in Wales: yarrowite has to date been found at one locality in Central Wales during a high-resolution petrological examination of its ore mineralogy (Ixer, 2003). However, many previously studied assemblages containing 'supergene chalcocite' or covellite-like minerals have only been studied using more basic ore petrology: hence, the current single-locality status of yarrowite in Wales is likely to alter with further examination.
There are no key localities for this specimen.
- Ixer, R.A., 2003. Ore Mineralogy. In: Excavations on Copa Hill, Cwmystwyth (1986 - 1999). An early Bronze Age copper mine within the uplands of Central Wales. S. Timberlake (ed.) BAR British Series, 348, 99.