Tremolite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium magnesium iron silicate hydroxide
Chemical Formula: Ca2(Mg,Fe2+)5Si8O22(OH)2
Method(s) of Verification: localities cited - polarizing microscope

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: serpentinization
  • Metamorphic
Introduction: tremolite is an amphibole and a member of the calcic amphibole group. It forms the magnesium-rich member of a series with actinolite and ferro-actinolite and typically forms during low-grade (greenschist facies) metamorphism of ultrabasic rocks or Mg-rich carbonates
Occurrence in Wales: although not specifically named, tremolite is probably one of the earliest described mineral from Wales being mentioned by Lhuyd in his paper on Linum Asbestinum in 1684. The reported occurrences are all associated with carbonate-rich rocks formed by metamorphism of gabbro and ultrabasic (peridotite) bodies in the Monian Supergroup metasediments on Anglesey. Specific occurrence of tremolite in ultrabasic and basic rocks northern and western Anglesey are provided in the detailed work of Greenly (1919). He also make passing mention of tremolite from Bodrwyn near Cerrigceinwen and Tyfry (Tŷ-fry) near Pentreath in the south east of the island.

Key Localities:

  • Holy Island, Anglesey: Greenly (1919) reported the presence of tremolite in various carbonate-rich rocks formed by serpentinization and carbonization of a suite of ultramafic (peridotite) on Holy Island. In particular he recorded tremolite-schist and tremolite marble from numerous localities, including those near Rhyd-y-Bont; near Fadog; near Pwllpillo; a few metres southeast of Gareg-lwyd House; and in a small quarry c 250 metres north of Bronddel. Maltman (1977) gave further accounts of tremolite in this suite of rocks, although he considered it to be more extensively developed within and at the margins of altered gabbroic bodies, particularly in fault zones.
  • Northern Anglesey: Greenly (1919) also described tremolite from a number of localities in northern Anglesey where the equivalent basic and ultrabasic rocks to those seen on Holy Island are exposed. These include the serpentinite at Mynachdy; tremolite-schist at Cefn-Coch mill; and forming a tremolitic serpentinite in massive ophicalcite at Tyddyn-dai.

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Horák, J.M., 2005. Mona Marble: Characterisation and Usage. Stone in Wales: Materials, Heritage and Conservation. Malcolm Coulson (Ed.), Cardiff: Cadw.
  3. Lhuyd, E., 1684. An account of a sort of paper made of Lineum Asbestium found in Wales. Phil Trans, 14 (no 166), 823.
  4. Phillips, E., 1989. The Geology of the Monian Supergroup, Western Anglesey, North Wales. Unpublished Ph.D. thesis, University of Wales.