Sillimanite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Aluminium silicate
Chemical Formula: Al2SiO5
Method(s) of Verification: Anglesey localities - EMPA and optical identification (Horák, 1993).

Geological Context:

  • Metamorphic: medium-grade
Photomicrograph, sheafs of sillimanite needles in high-grade metamorphic rocks from north east Anglesey (Coedana Complex gneisses). Specimen & © National Museum of Wales.
Introduction: sillimanite is a metamorphic mineral found in high-grade rocks (upper amphibolite/lower granulite facies) most typically in aluminium-rich pelites. Sillimanite has two polymorphs, kyanite and andalusite, which develop under conditions of lower temperature and different pressure to sillimanite.
Occurrence in Wales: the only Welsh occurrence of sillimanite is from pelitic gneisses of the Coedana Complex, Anglesey. These are some of the highest grade metamorphic rocks in Southern Britain and the highest grade metamorphic rocks in Wales. Their age is unproven, but is considered to be Neoproterozoic (late Precambrian). The general assemblage within these gneisses is quartz + oligoclase + biotite + garnet + sillimanite ± K-feldspar with the compositional layering enhanced by the presence of melt layers (leucosomes). Sillimanite has been identified in hand specimens from both Llechynfarwy in central Anglesey and from Porth Heligan, to the north-east of the island (Greenly 1896, 1919). The mineral is probably much more extensively developed throughout the gneisses but its presence is obscured by shearing and retrogression.

Key Localities:

  • Llechynfarwy, Anglesey: sillimanite forms silky aggregates of pale fibres a few mm long within finely banded garnet-biotite-sillimanite gneiss. In thin section both prismatic sillimanite and the fine fibrous form, known as 'fibrolite', are present (Horák, 1993). It should be noted that this is a scheduled site and permission from CCW is required to collect. Samples for study are held in the National Museum of Wales Mineral Collection under accession number 93.12G.
  • Porth Heligan, Anglesey: the coastal exposures of the Coedana Complex gneisses contain layers rich in pale yellow prismatic sillimanite up to 4 mm in length, in addition to fibrolite. In thin section biotite and sillimanite show complex textures with the sillimanite intergrown with biotite but in other places appearing to consume it. These textures record mineral reactions reflecting increasing metamorphic grade (Horák, 1993).

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Greenly, E., 1896. On the occurrence of sillimanite gneisses in central Anglesey. Geological Magazine, 3, 494-496.
  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.