Antigorite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium iron silicate hydroxide
Chemical Formula: (Mg,Fe2+)3Si2O5(OH)4
Method(s) of Verification: polarizing microscope.

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal: serpentinization
  • Metamorphic
Introduction: antigorite belongs to the kaolinite-serpentinite group of minerals and is one of three minerals (antigorite, lizardite and chrysotile) commonly referred to as ‘serpentine’. Antigorite and lizardite are soft green platy minerals, whereas chrysotile is fibrous. These minerals commonly result from the hydrothermal or retrograde metamorphism of mafic minerals such as olivine, pyroxene or amphibole, usually occurring in basic and ultrabasic igneous rocks. In general antigorite is less common than lizardite.
Occurrence in Wales: all recorded occurrences of antigorite from Wales are associated with exposures of serpentinite, or rocks linked to the serpentinization of ultrabasic and basic rocks, within the New Harbour Group of the Monian Supergroup in Anglesey. These rocks typically form scattered exposures, but they can be grouped into two main occurrences, Holy Island and NW Anglesey. The mineral associations are broadly similar in all occurrences although the abundance and nature of associated minerals may vary locally. It is these serpentinized rocks that were once referred to as ‘Mona Marble’ and worked as an ornamental stone (see Horák, 2005 for further details).

Key Localities:

  • Holy Island, Anglesey: Greenly (1919) described antigorite from various localities on Holy Island, Anglesey. He makes specific mention of bright green antigorite just west of Four Mile Bridge and Maltman (1977) mentions antigorite associated with lizardite in serpentinite from Holy Island.
  • North-west Anglesey: Greenly (1919) notes three main occurrences of antigorite in this region; radiating antigorite sprays ‘stars’ associated with carbonate and green and purple serpentinite, from south-west of Llanfechell; antigorite ‘stars’ associated with clusters and veinlets of minute garnet crystals in serpentinite at Mynachdy; and antigorite in carbonate associated with purple-red serpentinite at Tregele. The carbonate is probably an ophicalcite (altered and carbonated peridotite).

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. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.