Almandine

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron aluminium silicate
Chemical Formula: Fe2+3Al2(SiO4)3
Method(s) of Verification: Blaen-y-nant, Nant Ffrancon - optical (NMW collections); all Anglesey localities - EMPA (Horák, 1993).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic: medium-grade
Photomicrograph, taken in crossed polarized light, of almandine crystals (black) from biotite sillimanite schist from central Anglesey. NMW specimen. © National Museum of Wales.
Introduction: almandine belongs to the garnet group, but does not occur in a pure form but as a mixture with pyrope, spessartine and grossular. Almandine-rich garnet is found in intermediate to acid igneous (andesite to rhyolite) rocks, but is more commonly generated by medium to high-grade metamorphism of basic igneous rocks or sediments.
Occurrence in Wales: both igneous and metamorphic almandine-rich garnet is found in Wales. Williams (1922) described garnet aggregates associated with secondary quartz in rhyolitic lava in the Capel Curig district of Snowdonia and Horák (1993) identified both pseudomorphs and unaltered garnet from the Coedana Granite in Anglesey. Both of these occurrences are indicative of formation or inclusion in calc-alkaline acid magmas. Metamorphic almandine-rich garnet occurs with biotite at Blaen-y-nant, North Wales, in the contact-zone surrounding the Bwlch y Cywion Intrusion (Williams, 1930), and in both pelitic and calc-silicate gneisses of the Coedana Complex (Greenly, 1919). Claxton (1963) also recorded garnet from the ‘Dutch Gin Schists’ in Pembrokeshire, once thought to be metamorphosed Precambrian sediments but now considered to be metamorphosed and sheared igneous rocks.

Key Localities:

  • Blaen-y-Nant, Nant Ffrancon, Gwynedd: at this locality, at the north end of the Nant Ffrancon pass, mudstone from the Nant Ffrancon Formation is contact metamorphosed by the Bwlch y Cywion intrusion. In hand specimen the contact zone rocks (also referred to as a skarn) show little, being, dark, dense and fine-grained. In thin section they are seen to be composed of an unusual green biotite, topaz and crammed with pinhead garnets, typically less than 1 mm across.
  • Central Anglesey: Horák (1993) identified Mn-rich almandine garnet in aplite and the non-porphyritic muscovite facies of the Coedana Granite (see Horák, 1993 for analyses). The garnets are, colourless, up to 0.75 mm in size, and subhedral in shape. They show no indication of zoning or a reaction rim on their margins. These features and their Mn-rich composition have been used to indicate that the garnet crystallized from a calc-alkaline granite magma at a depth of less than 11 km. Almandine-rich garnet is also found in the biotite gneisses of the Coedana Complex in central Anglesey. With the naked eye it can be seen as glassy, red crystals a few mm across. In thin section the centres of the garnet are seen to be crammed with minute crystals (inclusions) of quartz, biotite and plagioclase crystals. These formed before the garnet grew and were overgrown by the garnet as it formed. In many localities the garnet is retrogressed. The best exposures are at Llechynfarwy (see sillimanite entry). It should be noted that this is scheduled site and permission from CCW is required to collect. Samples for study are held in the NMW collection under accession number 93.12G.
  • Northeast Anglesey: the Coedana Complex gneisses in the north-east of Anglesey contain minor, but very poorly exposed, calcareous bands (near Bryn Fuches and Rhosmynach Isaf), formed by amphibolite facies metamorphism of calcareous mudstone horizons. The calc-silicate gneiss contains abundant almandine-grossular garnet, in addition to hornblende, clinopyroxene, and biotite. The garnets have an irregular form and are up to 4 mm across. In thin section the garnets are seen to have formed in two stages, the cores containing inclusions but the rims and smaller well-formed crystals are inclusions-free (Horák, 1993).

References:

  1. Claxton, C.W., 1963. An occurrence of regionally metamorphosed Pre-Cambrian schist in South-West Pembrokeshire. Geological magazine, 100, 219-223.
  2. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  3. 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.
  4. Williams, H., 1922. The igneous rocks of the Capel Curig district (North Wales). Proceedings of the Liverpool Geological Society,13,166-202.
  5. Williams, H., 1930. The Snowdon District: Report of the Easter field meeting of the Geologists’ Association. Proceedings of the Geologists’ Association, 41,190-205.