Mineral Database

Mineral Database
Mineralogy of Wales

Magnesiohornblende

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

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.

Chemical Group:
  • Silicates
Geological Context:
  • Hydrothermal
  • Metamorphic
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:
  • Bevins, R.E., 1979 The geology of the Strumble Head-Fishguard region, Dyfed, Wales.  Unpublished Ph.D. thesis, University of Keele, 256pp.
  • 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  Unpublished Ph.D. thesis, University of Wales, 415pp.
  • 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).