Clintonite

Crystal System: Monoclinic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Uncommon
Chemical Composition: Calcium magnesium aluminium silicate hydroxide
Chemical Formula: Ca(Mg,Al)3(Al,Si)4O10(OH,F)2
Method(s) of Verification: all cited occurrences optical microscopy.

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Metamorphic: skarn mineralization
Introduction: clintonite belongs to the mica family of minerals and has a restricted composition. It is found in metamorphic rocks, most commonly in metasomatised limestones and in siliceous skarns associated with contact metamorphism. It is associated commonly with talc in chlorite schists, and with grossular, vesuvianite, clinopyroxene, monticellite, and chondrodite in skarns.
Occurrence in Wales: Greenly (1919) noted a strongly pleochroic (ruddy-brown to dull olive-green) partly chloritized mica, which he considered to be clintonite, from two localities within different stratigraphic units, in Anglesey. Although Greenly’s description represents the first UK recording of this mineral, these occurrences are considered to be of dubious status as neither has been substantiated by subsequent work (e.g. Mann, 1986; Horák, 1993).

Key Localities:

  • Central Anglesey: Greenly (1919) reported clintonite from hornfels forming the country rocks to the Coedana Granite in central Anglesey. More specific localities are cited as approximately 400 metres north of Bryn-ala and also in the vicinity of Llanfaelog Church. However clintonite was not detected during a re-evaluation of the hornfels (Horák, 1993) and its presence must be considered dubious.
  • Llanfaelog, Anglesey: pleochroic mica was also recorded from graphite-schist within the ‘Penmynydd Zone’, exposed on the coast to the SW of Llanfaelog (Greenly, 1919). These rocks represent variably retrogressed mylonites and mylonitic schist formed within a major shear zone (now known as the ‘Central Anglesey Shear Zone’, see Gibbons & Horák (1990) for details. The occurrence of clintonite has not been confirmed and should be considered dubious.

References:

  1. Gibbons, W. & Horák, J.M., 1990. Contrasting metamorphic terranes in NW Wales. In D'Lemos, R.D., Strachan, R.A., Topley, C.G. and Beckinsale, R.D. eds, The Cadomian Orogeny, Geological Society of London, Special Publication No. 51, 315-327.
  2. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  3. 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.
  4. Mann, A., 1986. Geological studies within the Mona Complex of Central Anglesey, North Wales. Ph.D. thesis, University of Wales (Cardiff).