Vesuvianite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium sodium magnesium iron aluminium silicate hydroxide
Chemical Formula: (Ca,Na)19(Al,Mg,Fe)13(SiO4)10(Si2O7)4(OH,F,O)10
Method(s) of Verification: Plas Newydd - EMPA (National Museum of Wales, unpublished data)

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: skarn mineralization
Photomicrograph of zoned vesuvianite crystal with calcite, from the Plas Newydd dyke skarn, Anglesey. Field of view 2.5 mm wide. © National Museum of Wales.
Introduction: vesuvianite, synonymous with the obsolete term idocrase, is a mineral typical of contact metamorphism of impure limestones or skarns. It occurs in association with garnet, wollastonite and diopside and is thought to form at temperatures as low as 350°C, below which hydrogrossular takes its place (Deer et al. 1992). In thin section vesuvianite is identified by anomalous 'Berlin blue' colours under cross polarised light.
Occurrence in Wales: two occurrence of vesuvianite are known from Wales; although only one, at Plas Newydd, Anglesey, has been confirmed by optical and analytical data (see below). The second occurrence is reported from calc-silicate gneisses of the Coedana Complex at Porth Helygen, north-east Anglesey. Greenly (1919) described clear, glassy, grass-green porphyroblasts (up to 4 mm in diameter) and interpreted these to be vesuvianite. However, more recent work has failed to confirm this occurrence (J.M. Horák, 1993).

Key Localities:

  • Plas Newydd, Anglesey: a thin skarn zone occurs at the junction between the wide, Plas Newydd dyke and the host rock limestones. Vesuvianite is present along the contact zone. In thin section the vesuivianite can be seen to form anhedral crystals, up to 4 mm across, showing zoning and typical anomalous 'Berlin' blue birefingence in crossed polarised light. The outer zone of the crystal commonly lacks these colours, suggesting a variation in chemical composition.

References:

  1. Deer, W.A., Howie, R.A. & Zussman, J., 1992. An Introduction to Rock-Forming Minerals. Longman Scientific & Technical, 696pp.
  2. Deer, W.A., Howie, R.A. & Zussman, J., 1986. Rock-Forming Minerals, Vol. 1A, Orthosilicates, 2nd Ed. Longman Group Ltd, 918pp.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. 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.