Tschermakite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium magnesium iron aluminium silicate hydroxide
Chemical Formula: •Ca2(Mg3AlFe3+)Si6Al2O22(OH)2
Method(s) of Verification: Rhiw - EMPA (R.E. Bevins, NMW, unpublished data); Parwyd - EMPA (Horak, 1993).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic
Introduction: tschermakite belongs to the hornblende group of calcic amphiboles, and is a rock-forming mineral found in basic igneous rocks and their high-grade metamorphic equivalents. The symbol at the start of the formula indicates that the crystal lattice includes an empty space (that is, not occupied by an atom), known as a vacancy.
Occurrence in Wales: there are two documented occurrences of tschermakite from Wales, both from the Llŷn. The modern amphibole classification scheme (see Leake et al., 1997) relies on chemical analyses of amphiboles rather than optical properties alone, it is therefore likely that other as yet un-analysed occurrences of tschermakite will be added to the localities listed below.

Key Localities:

  • Parwyd, Llŷn, Gwynedd: the Parwyd Gneisses, exposed at Parwyd Bay on the southern tip of Llŷn include highly retrogressed garnet amphibolites. ‘Hornblendic’ amphibole constitutes up to 80 percent of the rock, which are essentially ultrabasic in composition. Microprobe analyses indicate that this amphibole is tschermakitic in composition (Horák, 1993). The origin and affinity of these amphibolites remains unproven.
  • Rhiw, Llŷn, Gwynedd: tschermakite has been identified from gabbroic rocks of the Rhiw Intrusion, where it forms a late stage magmatic mineral (R.E. Bevins, NMW, unpublished data).

References:

  1. Horák, J.M., 1993. The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.
  2. Leake, B.E., et al., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the international mineralogical association, commission on new minerals and mineral names. The Canadian Mineralogist, 35, 219-246.