Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium magnesium iron silicate hydroxide
Chemical Formula: Ca2(Mg,Fe2+)5Si8O22(OH)2
Method(s) of Verification: Moel Hebog - X-ray diffraction (Langford, 1973); various other localities - EMPA (Bevins & Rowbotham, 1983; Gibbons & Gyopari, 1986; Bevins & Merriman, 1988).
Silky-white, fibrous, asbestiform actinolite from Moel yr Ogof, nr. Beddgelert, Snowdonia. Specimen 22 cm long. National Museum of Wales Collection (NMW 27.111.GR.270) ex G.J. Williams Collection. Photo T.F. Cotterell, © National Museum of Wales
Photomicrograph of zoned amphibole (0.45 mm across) with inner pale core of actinolite and outer rims of barroisite and glaucophane, Llanfairpwllgwyngyll, Anglesey. © Dr W. Gibbons.
Introduction: actinolite is a member of the amphibole group. It forms a series with tremolite and ferro-actinolite being more iron-rich than the former. It occurs in basic rocks metamorphosed under low-grade metamorphic conditions (greenschist and blueschist facies).
Occurrence in Wales: actinolite has a widespread distribution across Wales in altered basic igneous rocks, although in most instances it can only be identified with the aid of a petrological microscope. Greater access to electron microscope analyses has made it easier to differentiate actinolite from ferro-actinolite.
Early reported occurrences include those by Greenly (1919) from Anglesey, Williams (1922) from Snowdonia and Cox & Wells (1920) who recorded ‘wisps of hornblende’ (now known to be actinolite) in dolerites from the Cader Idris region (see below).
More recent studies citing actinolite occurrences include: metabasites from Mynydd Presili (Dyfed), the Aran Mountains, Cadar Idris and central and southern Snowdonia (Bevins & Rowbotham, 1983), where actinolite overgrows chlorite or forms epitaxial overgrowths on igneous pyroxene; actinolite associated with pargasite-bearing mafic cummulates from north-east of Eglwys Rhobell Formation, Rhobell Volcanic Group, north-east of Dolgellau (Koklaar, 1977, 1986) and from a detailed study of the low-grade metamorphism of basic igneous rocks of the Tal y Fan intrusion, Gwynedd (Bevins & Merriman, 1988).
A more unusual occurrence of actinolite is where it is associated with blueschist facies metabasites around Llanfairpwllgwyngyll, Anglesey (Gibbons & Gyopari, 1986). The actinolite has two forms: augen, 1-2 mm long in greenshist metabasite, itself enclosed within blueschist metabasite, and within the blueschist metabasite where remnant cores of actinolite are rimmed by barroisite and/or blue amphibole. The actinolite formed during sea floor metamorphism which converted the original basaltic rocks to greenschist which was subsequently overprinted by a blueschist mineralogy.
The only hand specimen samples of actinolite known are from veins in the vicinity of Moel Hebog, near Beddgelert, North Wales.
- Moel yr Ogof, near Beddgelert, Gwynedd: silky cream to very pale green asbestiform fibres associated with calcite in a vein at Ogof Owain Glyndwr north of Moel Hebog (National Museum of Wales specimen NMW 72.36G.M.1) were confirmed as actinolite by Langford (1973) using XRD. It is thought that a Natural History Museum analysis (X-ray ref. no. 7349F) of an actinolite-tremolite mineral from ‘Moel Hebog’ is probably from the same occurrence.
- Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- Bevins,R.E. & Merriman, R. J., 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphism field gradients. Journal of Metamorphic Geology, 6, 17-39.
- Cox, A.H. & Wells, A.K., 1920. The Lower Palaeozoic rocks of the Arthog-Dolgelly district. Quarterly Journal of the Geological Society of London, 76, 254-324.
- Gibbons, W. & Gyopari, M., 1986. A greenschist protolith for blueschist on Anglesey, U.K. In: Evans, B.W. & Brown, E.H. (eds), Blueschist and Eclogites. Geological Society of American Memoir, 164, 217-228.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Koklaar, B.P., 1986. Petrology and geochemistry of the Rhobell Volcanic Comples: Amphibole-dominated fractionation at an early Ordovician arc volcano in North Wales. Journal of Petrology, 27, 887-914.
- Koklaar, P.B., 1977. The igneous history of the Robell Fawr area, Merioneth, North Wales. Unpublished Ph.D. thesis, University of Wales.
- Langford, J.I., 1973. The accuracy of cell dimensions determined by Cohen’s method of least squares and systematic indexing of powder data. Journal of Applied Crystallography,6,190-196.
- Williams, H., 1922. The igneous rocks of the Capel Curig district (North Wales). Proceedings of the Liverpool Geological Society,13,166-202.