Magnesiohornblende

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: calcium magnesium iron aluminium silicate hydroxide
Chemical Formula: •Ca2(Mg,Fe2+)4(Al,Fe3+)(Si7Al)O22(OH,F)2
Method(s) of Verification: Aberbach, Pembrokeshire – EMPA (Bevins, 1979); Llŷn – EMPA (Horák, 1993).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic
Magnesiohornblende crystals from altered diorite from the Sarn Complex, Ll?n. Viewed under the microscope in crossed polarized light. National Museum of Wales specimen (NMW 93.12G). © National Museum of Wales.
Introduction: magnesiohornblende is an amphibole, more specifically a calcic amphibole which occurs as a rock-forming mineral in basic igneous rocks and as amphibolite grade metamorphic rocks.
Occurrence in Wales: as with many other ferro-magnesian minerals, magnesiohorneblende has only been confirmed from a few localities in Wales, and there are even fewer analyses presented in the literature. It is quite likely that it is more abundant than these reports reflect.

Key Localities:

  • Aberbach, Pembrokeshire: the Llech Dafad Intrusion, of Ordovician age, exposed to the northeast of Aberbach, Pembrokeshire contains green pleochroic magnesiohornblende forming needle-like crystals. These crystals over print clinopyroxene and replace the groundmass and are therefore interpreted as sub-solidus in origin (Bevins, 1979).
  • Llŷn, Gwynedd: magnesiohornblende occurs within the basic rocks of the Precambrian Sarn Complex, exposed in scattered outcrops on Llŷn. The amphibole has several textural forms; as small anhedral crystals interstitial to plagioclase; as thin rims around clinopyroxene; and as subhedral clinopyroxene-free crystals. Whereas the amphibole in the sheared rocks clearly has a subsolidus origin that in the unsheared dioritic rocks may be either post-magmatic or late-stage magmatic (Horák, 1993).

References:

  1. Bevins, R.E., 1979. The geology of the Strumble Head-Fishguard region, Dyfed, Wales. Unpublished Ph.D. thesis, University of Keele, 256pp.
  2. 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 Unpublished Ph.D. thesis, University of Wales, 415pp.
  3. Rock, N.M.S. & Leake, B.E., 1979. A FORTRAN program for the classification of amphiboles according to IMA (1978). IGS Petrographic Report 560124 (unpublished).