Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium aluminium silicate hydroxide
Chemical Formula: Ca2Al3(SiO4)3(OH)
Method(s) of Verification: Dolgellau Gold-belt - XRD (Gilbey, 1968); Gellilydan - XRD (National Museum of Wales).
- Metamorphic: low-grade
- Hydrothermal: alpine type veins
Slightly deformed prismatic clinzoisite within calcite-quartz vein cutting metabasite. Gellilydan, Ffestiniog. Specimen National Museum of Wales (NMW 2005.3G.M.2), photo D.I. Green, © National Musuem of Wales.
Introduction: a member of the epidote group, clinozoisite and epidote (with which it forms a solid solution) can be difficult to distinguish without employing analytical methods. Clinozoisite typically occurs in basic igneous rocks that have undergone low and medium-grade (greenschist and amphibolite facies) metamorphism. It also occurs in alpine fissure-type veins cutting low-grade metamorphic rocks. In such associations, coarsely-crystalline clinozoisite is typically a brownish or buff colour, in contrast to the distinctive, bright pistachio-green of epidote.
Occurrence in Wales: many early descriptions of altered basic igneous rocks from Wales refer to the presence of epidote: however, Roberts (1981) reported the widespread presence of clinozoisite in rocks from Central Snowdonia and Llŷn. Similarly, many so-called epidotes from the Mynydd Preseli region of SW Wales are clinozoisite (R.E. Bevins, unpublished data). More recently, the presence of locally abundant and coarse-grained clinozoisite in 'alpine fissure-type' veins has been noted from a number of sites within the Dolgellau Gold-belt.
- Dolgellau Gold-belt, Gwynedd: clinozoisite occurs, together with epidote, quartz, calcite and albite, in alpine-fissure type assemblages at a number of localities, including the Afon Wen catchment (discovered by J.S. Mason & M.J. Liezers during recent mineral exploration work) and at several old gold mines, notably Cae-Mawr (Gilbey, 1968). It forms acicular brownish crystals to 2-3 cm.
- Gellilydan, Gwynedd: during the construction of the Gellilydan road cutting, in the late 1990s, a number of alpine fissure-type veins were exposed, hosted in altered basic intrusive bodies occurring in Cambrian sedimentary rocks. Clinozoisite, as buff crystals, up to several centimetres in length, were abundant in some of the veins, in association with quartz and calcite. Discovered during MINESCAN follow-up work, 2000 (NMW/Manchester Museum unpublished data).
- Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
- Roberts, B., 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llyn and Snowdonia, Gwynedd, North Wales. Geological Magazine, 118, 189-200.