Actinolite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
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).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
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.

Key Localities:

  • 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.

There are no key localities for this specimen.

References:

  1. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  2. 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.
  3. 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.
  4. 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.
  5. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  6. 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.
  7. Koklaar, P.B., 1977. The igneous history of the Robell Fawr area, Merioneth, North Wales. Unpublished Ph.D. thesis, University of Wales.
  8. 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.
  9. Williams, H., 1922. The igneous rocks of the Capel Curig district (North Wales). Proceedings of the Liverpool Geological Society,13,166-202.

There are no references for this specimen.