Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium magnesium silicate
Chemical Formula: CaMgSi2O6
Method(s) of Verification: Holy Island - polarizing microscope (Greenly, 1919); NE Anglesey - EMPA (Horák, 1993); Rhiw - EMPA (Cattermole, 1976); St David’s Head - EMPA (Bevins et al., 1994).
Introduction: diopside belongs to the pyroxene group of minerals and forms a series with both hedenbergite (CaFe2+Si2O6) and with the manganese-rich pyroxene johannsenite (CaMn2+Si2O6). It is found as a primary mineral in ultrabasic to basic igneous rocks and also forms by high-grade metamorphism of calcareous rocks.
Occurrence in Wales: diopside is present in various ultrabasic and basic igneous rocks throughout Wales, although it has just one reported metamorphic occurrence, from amphibolite grade impure limestone in NE Anglesey (Greenly, 1919). As with other ferro-magnesian silicate minerals, many occurrences of primary diopside have been totally or partially altered to produce secondary hydrous silicate minerals such as chlorite, actinolite or pumpellyite.
- Great House, Usk, Monmouthshire: the Great House diatreme contains xenoliths of magnesian peridotite composed of emerald-green chrome diopside (D.T. Moffat, unpublished data). This locality is a protected SSSI site and permission is needed for access.
- Holy Island, Anglesey: Greenly (1919) identified diopside in ophicalcite rocks associated with serpentinite. Maltman (1977) reported diopside-bearing altered gabbros near Mynachdy.
- North East Anglesey: Greenly (1919) reported the presence of diopside in Precambrian calc-silicate rocks of the Nebo Inlier (previously referred to as the Mona Complex); part of the Coedana Complex (Horák, 1993). He described forsterite marbles and crystalline limestone. More detailed investigation has shown there to be three different diopside-bearing lithologies of which the calc-silicate marble is the most abundant. In all lithologies diopside occurs as stubby equant crystals, in the calc-silicate marble it is associated with calcite, plagioclase and quartz and it may be partially retrogressed to actinolite, and in the calc-silicate gneisses it is associated with grossular-rich garnet and hornblendic amphibole. The pyroxene compositions within the gneisses span the diopside-hedenbergite boundary whereas those in the marbles lie entirely within the diopside field (Horák, 1993).
- Rhiw, Llŷn, Gwynedd: Hawkins (1970) and Cattermole (1976) record diopside from the Rhiw Intrusion, in particular from the Lower Ultrabasic Zone and the Lower Gabbro Zone.
- St. David's Head, Pembrokeshire: detailed mapping and petrology of the St. David’s Head Intrusion in North Pembrokeshire, documents variations in pyroxene composition within the various igneous layers (Roach, 1969, Bevins et al., 1994). Electron microprobe analyses show that the pyroxene compositions span the augite and diopside fields, with diopside occurring most frequently in the leucogabbro and granophyric gabbro. Diopside from the leucogabbro may enclose olivine crystals and individual crystals are strongly zoned with iron-poor cores and iron-rich rims (Bevins et al., 1994).
- Bevins, R.E., Lees, G.J. Roach, R.A. Rowbotham, G. & Floyd, P.A., 1994. Petrogenesis of the St David’s Head Layered Intrusion, Wales: a complex history of multiple magma injection and in situ crystallisation. Transactions of the Royal Society of Edinburgh: Earth Sciences, 85, 91-121
- Cattermole, P.J., 1976. The crystallization and differentiation of a layered intrusion of hydrated alkali olivine-basalt parentage at Rhiw, North Wales. Geological Journal,11,45-70.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Hawkins, T.R.W., 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geological Journal, 7, 1-24.
- 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.
- Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.
- Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.