Grossular

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium aluminium silicate
Chemical Formula: Ca3Al2(SiO4)3
Method(s) of Verification: Holy Island & NW Anglesey - EMPA (Maltman, 1977); NE Anglesey - EMPA (Horák, 1993); Plas Newydd - XRD (Natural History Museum, 2501F-1979, 2499F-1979, P741), EMPA (National Museum of Wales, unpublished data).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic
Introduction: grossular belongs to the garnet group and is part of the ugrandite series (uvarovite-grossular-andradite) but also forms a second series with the hydrous versions of grossular (hibschite, Ca3Al2(SiO4)2(OH)4 and an intermediate member katoite, Ca3Al2(SiO4)1.5(OH)6. This is generally referred to as the grossular-hydrogrossular series. Although often occurring in a purer form than some of the other garnet members (e.g. almandine) grossular can form a solid solution series with both almandine and pyrope. Grossular-rich garnet typically forms by the metamorphism of calcareous rocks.
Occurrence in Wales: several occurrences of grossular-rich garnet are known from Wales, formed by both contact and regional metamorphism. Henslow (1822) described garnet from contact metamorphosed calcareous shale on the margin of the Plas Newydd dyke, Anglesey, and this was augmented by Harker (1887). Greenly (1919) noted that the exposures were no longer available for inspection, but quoted the findings of the previous reports. Garnet from this locality was confirmed as grossular by XRD. Grossular is also known from two other areas in Anglesey. Maltman (1977) identified it in ‘altered gabbro’ and rodingite (Ca-rich rocks formed during serpentinization) from a number of localities on Holy Island and from near Mynachdy in NW Anglesey. Horák (1993) described grossular-rich garnet from the impure limestone horizons within the Coedana Complex gneisses, in the NE of the Island.

Key Localities:

  • Holy Island, Anglesey: ‘altered gabbros’ associated with serpentine from Holy Island typically consist of chlorite, tremolite, epidote and garnet, although they do not show many features that suggest an igneous origin (Maltman, 1977). The garnet forms ‘clusters and trails up to 2 mm in width’ and in the ‘altered gabbro’ near Mynachdy constitutes up to 50% of the rock. Such garnet-bearing rocks are more accurately referred to as rodingites, which formed by serpentinization processes, rather than alteration of gabbro. It should be noted that garnet from rodingite is more typically hydrogrossular in composition, although this cannot be confirmed from the published probe data.
  • North East Anglesey: the Coedana Complex gneisses in the NE of the island contain minor, but very poorly exposed, calcareous bands (near Bryn Fuches and Rhosmynach Isaf), formed by amphibolite facies metamorphism of impure limestone horizons. One of these, the calc-silicate marble, described by Horák (1993) contains garnet with up to 88% grossular component, associated with calcite, clinopyroxene and plagioclase. The anhedral crystals, up to 5 mm in diameter, contain inclusions of clinopyroxene, plagioclase, calcite, magnetite and titanite. Larger (up to 10 mm), more euhedral crystals of a similar composition occur in quartz-plagioclase veins that cut the calc-silicate marble.
  • North-west Anglesey: this occurrence of grossular is identical to that listed under Holy Island.
  • Plas Newydd, Anglesey: both well-formed garnet showing crystals faces and less well-formed crystals are found with calcite, wollastonite and vesuvianite in calcareous mudstones/limestones in the contact metamorphism zone of the Plas Newydd dolerite dyke. Henslow (1822) described olive-brown, rhomboidal dodecahedron up to ‘seven-tenths of an inch in diameter’ but crystals in the National Museum of Wales' collection (e.g. NMW 2003.32.M.1-3) do not exceed 9 mm. These form a dense coating on a bedding surface. In thin section the crystals show pronounced sector zoning and anisotropy characteristic of grossular garnet. Electron microprobe analyses show very slight variation from the core of the crystals to the rims reflected by a slight decline in grossular and pyrope contents and an increase in that of almandine. It should be noted that this site is now protected and no attempt should be made to remove in-situ material.

There are no key localities for this specimen.

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Harker, A., 1887. On some Anglesey dykes. Geological Magazine, New Series, Decade 3, 4, 409-416.
  3. Henslow, J.S., 1822. Geological description of Anglesea. Transactions of the Cambridge Philisophical Society, 1, 155-156.
  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.
  5. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.

There are no references for this specimen.