Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: CaCO3.H2O
Chemical Formula: Calcium carbonate hydrate
Method(s) of Verification: Bontddu - XRD (Natural History Museum, X-ray no. 6257F).

Chemical Group:

  • Carbonates

Geological Context:

  • Hydrothermal
  • Sedimentary: chemical deposit (evaporites & carbonates)
Introduction: monohydrocalcite is a rare hydrated carbonate known to occur in a diverse range of environments. Examples include, from the bottom sediments of lakes, in beach rocks surrounding lakes, and in polymetallic veins.
Occurrence in Wales: the only account of monohydrocalcite from Wales was by Saich & Rust (1987) describing, minute crystals found on a single specimen collected from a small trial level 1.5 km NW of Bontddu, near Dolgellau in North Wales.

Key Localities:

  • Bontddu, Dolgellau, Gwynedd: monohydrocalcite was recorded by Saich & Rust (1987), from a trial level at Bontddu, from just one specimen. It forms minute, colourless, pyramidal, triangular-section crystals, covering a 1.5 x 1 mm vugh in chalcopyrite-quartz veinstone. It is associated with aragonite and corroded calcite.


  1. Saich, D.A. & Rust, S.A., 1987. Micro-minerals from a trial level in Wales. U.K. Journal of Mines and Minerals, No. 3, 3-4.