Mackinawite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Iron nickel sulphide
Chemical Formula: (Fe,Ni)9S8
Method(s) of Verification: Dolgellau Gold-belt & Mynydd Preseli - identification on optical properties in polished section.

Chemical Group:

  • Sulphides

Geological Context:

  • Igneous
  • Hydrothermal: mesothermal polymetallic veins
Introduction: mackinawite occurs principally in magmatic sulphide deposits and as part of the opaques assemblage in basic and ultrabasic intrusive rocks, but can also be found in medium or high-temperature hydrothermal ore deposits, where it tends to occur as exsolution bodies in chalcopyrite.
Occurrence in Wales: mackinawite is present in Wales in both the igneous and hydrothermal associations. It is nowhere coarse enough to be identified by the naked eye and the accounts of its occurrence described here are all based on reflected-light petrography using polished sections.

Key Localities:

  • Dolgellau Gold-belt, Gwynedd: Gilbey (1968) described aligned lath-shaped inclusions of mackinawite in chalcopyrite from Cefndeuddwr Mine and irregular, vermicular inclusions in chalcopyrite (associated with pyrrhotite) from one of the lodes worked at the Foel Ispri mines. These occurrences were considered to represent exsolution of excess iron and nickel from high-temperature chalcopyrite. Given the regional nature of the Gold-belt mineralization, further, similar occurrences at other mines in the area are to be expected.
  • Preseli Mountains (Mynydd Preseli), Pembrokeshire: mixed pyrrhotite, chalcopyrite and mackinawite grains have been identified in altered basic intrusive rocks outcropping at Carn Menyn in the Preseli hills (Ixer, in Thorpe et al., 1991). Studies of opaque minerals in the igneous rocks of Wales are few and it is again likely that further occurrences exist.

References:

  1. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  2. Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G. & Watson, J.S., 1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, U.K. Proceedings of the Prehistoric Society, 57, 103-157.