Andradite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium iron silicate
Chemical Formula: Ca3Fe2(SiO4)3
Method(s) of Verification: Parc Mine - XRD; Afon Mawddach - XRD at the National Museum of Wales (NMW X-1305).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic
Andradite crystal (< 1mm) on joint surface in dolerite, associated with magnetite, hematite, chalcopyrite and epidote. Afon Mawddach. Specimen J.S. Mason (JM2525). Photo D.I. Green, © National Museum of Wales.
Introduction: andradite is most frequently encountered in skarn-type assemblages developed by contact-metamorphism of calcium-rich rocks. Typically associated minerals include other calcium silicates, epidote, hematite, magnetite and sulphides.
Occurrence in Wales: two occurrences of andradite are known from Wales, both in geological settings consistent with the contact-metamorphic setting. Archer & Elliott (1965) first identified the mineral close to post-mineralization dykes in the Parc Mine, Llanrwst. More recently, andradite has been found within thermally metamorphosed altered intrusive rocks along the Afon Mawddach, Gwynedd. (M.J. Liezers/J.S. Mason, unpublished data).

Key Localities:

  • Afon Mawddach, Ganllwyd, Gwynedd: andradite is found on joints surfaces in a contact-metamorphosed, highly altered, carbonate-rich, sill exposed in the bed of the Afon Mawddach, Gwynedd. It is occurs as scattered euhedral crystals typically 0.5-1.5 mm in size. Associated minerals are magnetite, hematite, chalcopyrite and epidote. The locality is generally inaccessible as it is usually deep underwater. Andradite was also found in heavy mineral concentrates collected during recent mineral exploration activity in the same area (M. J. Liezers/J.S. Mason, unpublished data).
  • Parc Mine, Llanwrst, Gwynedd: honey-yellow crystals on joint-faces occur in calcite and base-metal sulphide dominated lodes adjacent to two later dolerite dykes of Tertiary age (Archer & Elliott, 1965).

There are no key localities for this specimen.

References:

  1. Archer, A.A.& Elliot, R.W., 1965. The occurrence of olivine-dolerite dykes near Llanrwst, North Wales. Bulletin of the Geological Survey of Great Britain, 23, 145-152.

There are no references for this specimen.