Lawsonite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Uncommon
Chemical Composition: Calcium aluminium silicate hydroxide hydrate
Chemical Formula: CaAl2Si2O7(OH)2.H2O
Method(s) of Verification: Hendre - polarizing microscope (Natataj, (1967), polarizing microscope & EMPA (Gibbons & Mann, 1983); Church Island - polarizing microscope & EMPA (Gibbons & Mann, 1983).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic
Photomicrograph of tabular lawsonite crystal, up to 0.8 mm long, in blueschist from Hendrefor, Anglesey. © W. Gibbons.
Introduction: lawsonite, is found typically in a relatively pure state and is produced under conditions of relatively low temperature and moderately high water pressure from sediments and basic igneous rocks of suitable composition. It is indicative of blueschist facies metamorphism and typically occurs with glaucophane, jadeite, chlorite and calcite or aragonite.
Occurrence in Wales: Nataraj (1967) recorded the first UK occurrence of lawsonite from metabasites in southeast Anglesey. This identification was subsequently confirmed by Gibbons & Mann (1983) and additional localities were also identified. The lawsonite occurs within the Anglesey Blueschist Belt (Aethwy Zone) (Gibbons & Horák, 1990), interpreted as having formed by subduction of oceanic crust in the late Precambrian with subsequent uplift and emplacement by synestral transform faulting.

Key Localities:

  • Hendre, Anglesey: lawsonite is found in schists at this locality but can only be identified in thin section by polarizing microscope. It occurs as well-formed, tabular crystals, no larger than 1 mm, and more typically 0.4 x 0.2 mm.

References:

  1. Gibbons, W. & Horák, J.M., 1990. Contrasting metamorphic terranes in NW Wales. In D'Lemos, R.D., Strachan, R.A., Topley, C.G. and Beckinsale, R.D. eds, The Cadomian Orogeny, Geological Society of London, Special Publication No. 51, 315-327.
  2. Gibbons, W. & Mann, A., 1983. Pre-Mesozoic lawsonite in Anglesey, northern Wales: Preservation of ancient blueschists. Geology, 11, 3-6.
  3. Nataraj, T.S., 1967. Glaucophane metamorphism in Anglesey. Unpublished Ph.D. thesis, University of Leeds, 103pp.