Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Iron magnesium aluminium silicate oxyhydroxide
Chemical Formula: (Fe2+,Mg,Fe3+)5Al(Si3Al)O10(OH,O)8
Method(s) of Verification: Moelgolomen Mine - XRD (National Museum of Wales, NMW X-1265).
Introduction: chamosite is a member of the chlorite group of minerals, it is dimorphous with orthochamosite (it has the same chemical composition but a different crystal lattice structure) and forms a series with clinochlore.
Occurrence in Wales: following the nomenclature for dioctahedral chlorites, provided by Bayliss (1975), chamosite can be considered a common mineral in Wales. The first recording of chamosite in Wales is provided by Hallimond (1924) in his description of the first occurrence of stilpnomelane in Great Britain at Pen-yr-allt Mine, near Penrhyndeudraeth, Gwynedd. He noted that oolites in the oolitic iron ores of Ordovician age exposed across North Wales, from Anglesey in the north, to the Cader Idris area in the south, were composed of concentric layers of magnetite set in a chamosite matrix. Further reference to these iron ores is provided by Hallimond (1925), and by Fernside (in Williams, 1930), while full locality information is given in Pulfrey (1933). A brief mention of the presence of chamosite is also made by Matthews & Scoon (1964) in a description of a new occurrence of stilpnomelane at Tyllau Mwn, near Drws-y-nant, Dolgellau. This is found within chamosite-bearing oolitic ironstones at the same stratigraphic horizon as that described by Hallimond and Pulfrey. Chamosite is also widely developed as a secondary mineral in altered igneous rocks throughout Wales, forming pseudomorphs after primary igneous mafic minerals such as olivine, or pyroxene, as well as occurring in vesicles, veins and replacing the groundmass. Many workers use the chlorite nomenclature of Hey (1954) and therefore do not refer to chamosite but minerals such as brunsvigite, ripidolite and diabantite. For example Bevins & Rowbotham (1983) mention all three of these minerals which equate to magnesian chamosite, magnesian aluminium chamosite and magnesian siliconian chamosite, respectively, following Bayliss' (1975) trioctahedral chlorite classification. As chamosite is widely distributed only selected locality information is provided below, referenced to specimens in the National Museum of Wales Collection.
- Brynycastell Mine, Cross Foxes, Dolgellau, Gwynedd: specimens in the National Museum of Wales collections contain chamosite as part of a magnetite-chamosite assemblage from altered oolitic iron ore (e.g. NMW 98.16G.M.73 and NMW 98.16G.M.74).
- Fforchaman Colliery, Cwmaman, South Wales: chamosite associated with kaolinite has been identified, on the basis of X-ray diffraction, in feldspathic sandstone of Carboniferous age from Fforchaman Colliery (National Museum of Wales specimens NMW 39.181.GR.8 and NMW 39.232.GR.15). Bevins (1994) also notes chamosite from this locality in carbonaceous shales.
- Moelgolomen Mine, Tal-y-bont, Ceredigion: a green glassy mineral found within brecciated metasediment, cemented by quartz and cut by quartz and sulphide veins, has been identified as chamosite on the basis of X-ray diffraction (specimen NMW 98.35G.M.162).
- Bayliss, P., 1975. Nomenclature of the trioctahedral chlorites. Canadian Mineralogist, 13, 178-180.
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- Hallimond, A.F., 1924. On stilpnomelane from North Wales. Mineralogical Magazine, 20, 193-197.
- Hallimond, A.F., 1925. Iron ores: Bedded Ores of England and Wales: Petrography and Chemistry. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 29.
- Hey, M.H., 1954. A new review of the chlorites. Mineralogical Magazine, 30, 277-292.
- Matthews, D.W. & Scoon, J.H., 1964. Notes on a new occurrence of stilpnomelane from North Wales. Mineralogical Magazine, 33, 1032-1037.
- Pulfrey, W., 1933. The iron-ore oolites and pisolites of North Wales. Quarterly Journal of the Geological Society of London, 89, 401-430.
- Williams, D., 1930. The geology of the country between Nant Peris and Nant Ffrancon (Snowdonia). Quarterly Journal of the Geological Society of London, 86, 191-232.