Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Calcium iron magnesium silicate hydroxide
Chemical Formula: Ca2(Fe2+,Mg)5Si8O22(OH)2
Method(s) of Verification: Tal y Fan - EMPA (Bevins & Merriman, 1988).
- Metamorphic: low-grade
Introduction: ferro-actinolite is a calcic amphibole and is the most iron-rich member of the tremolite-ferro-actinolite series. Ferro-actinolite has a magnesium to iron (Fe2+) ratio of less than one. With increasing magnesium content, ferro-actinolite gives way successively to actinolite and then tremolite. Ferro-actinolite typically forms by metamorphism of iron-rich rocks such as iron formations or greenschist or blueschist grade basic igneous rocks.
Occurrence in Wales: greenschist facies metamorphic rocks are recorded from several areas of Wales (Bevins & Rowbotham, 1983). It is therefore likely that ferro-actinolite occurs in basic rocks within these areas, although it has been poorly documented in the literature. The Tal y Fan occurrence described below represented the first occurrence confirmed by electron microprobe data in Wales and is also thought to be the first reported occurrence in the British Isles.
- Tal y Fan, Conway: ferro-actinolite has been described as part of the low-grade metamorphic assemblage in the Tal y Fan Intrusion, North Wales, where it forms colourless to pale green pleochroic needle-like crystals rimming pyroxene (Bevins & Merriman, 1988).
- 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.