Ferro-actinolite

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).

Chemical Group:

  • Silicates

Geological Context:

  • 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.

Key Localities:

  • 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).

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.