Clinochlore

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Magnesium iron aluminium silicate hydroxide
Chemical Formula: (Mg,Fe2+)5Al(Si3Al)O10(OH)8
Method(s) of Verification: alpine-type veins - XRD (Manchester Museum).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Hydrothermal: alpine type veins
Group of pseudohexagonal clinochlore plates on anatase. Small white projection on the left is xenotime. Gloddfa Ganol Quarry, Blaenau Ffestiniog. Scale bar 0.05 mm. SEM photomicrograph, backscatter-mode. © National Museum of Wales.
Introduction: clinochlore, a member of the chlorite group, is found typically as a secondary mineral developed in low-grade metamorphic or hydrothermally-altered intermediate and basic igneous rocks and in veins cutting them.
Occurrence in Wales: clinochlore, as defined by the chlorite group nomenclature of Bayliss (1975), is widely developed in Wales as a component of altered basic igneous rocks (Bevins & Rowbotham, 1983). It is a frequent replacement of primary mafic minerals and also fills vesicles and other cavities. It is also abundantly developed in syn-tectonic 'alpine fissure-type' veins which were formed within basic igneous rock bodies during the regional Caledonian deformation of the area. In the latter association, it tends to form massive, friable green aggregates of microcrystals.

Key Localities:

  • Afon Wen, Gwynedd: clinochlore is present in quantity in alpine fissure-type veins cutting altered dioritic rocks of the Afon Wen Intrusive Complex. It is associated with epidote-clinozoisite, albite, quartz and calcite and was discovered (by J.S. Mason & M.J. Liezers) during recent mineral exploration work.
  • Clogau Mine, Bontddu, Gwynedd: a common component of the 'White Elephant' veins which are lens-shaped alpine fissure-type bodies occupying flat or low-angle fractures cutting mesothermal gold-lodes and adjacent altered basic igneous rocks. Associated minerals are quartz, calcite and rutile. Discovered (by J.S. Mason, M.J. Liezers) during recent mineral exploration work.
  • Dinorwic Quarry, Llanberis, Gwynedd: the details of this occurrence at identical to that at Penrhyn Quarry, with clinochlore common within mineralized boudin neck-zones in basalt dykes cutting Cambrian slates in this area. Associated minerals include epidote, quartz, albite and specular hematite (Bevins & Mason, 1998).
  • Gloddfa Ganol Quarry, Blaenau Ffestiniog, Gwynedd: this mineral occurs here as part of an alpine fissure-type assemblage with anatase, albite, quartz, synchysite and apatite, occupying fractures in dykes and tuff beds and also in localized breccia-zones, discovered during MINESCAN follow-up work, 2001 (National Museum of Wales/Manchester Museum unpublished data).
  • Manod Quarry, Blaenau Ffestiniog, Gwynedd: this locality has provided many examples of alpine fissure-type mineralization (e.g. Green & Middleton, 1996). Clinochlore is common as part of the mineral assemblage and is most frequently associated with anatase, brookite, quartz (within which it frequently forms inclusions) and albite.
  • Penrhyn Quarry, Bethesda, Gwynedd: clinochlore is common within mineralized boudin neck-zones in basalt dykes cutting Cambrian slates in this area. Associated minerals include epidote, quartz, albite and specular hematite (Bevins & Mason, 1998).
  • Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: the occurrence of clinochlore at this locality is identical to that cited for Manod Quarry.

References:

  1. Bayliss, P., 1975. Nomenclature of the trioctahedral chlorites. Canadian Mineralogist, 13, 178-180.
  2. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  3. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  4. Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.