The natural lake Marchlyn Mawr has been dammed to form the upper reservoir of the Dinorwig Pumped Storage Scheme, the largest pumped storage hydroelectric scheme in Europe at the time of its construction in 1984 (see also Dinorwig Quarry, the site of the lower reservoir).

Fibrous, radiating aggregates of thomsonite occur here infilling vesicles to 20 mm in diameter in the Marchlyn Dyke; an olivine dolerite of Palaeogene age.

Refs:
Williams, D., 1930. The geology of the country between Nant Peris and Nant Ffrancon (Snowdonia). Quarterly Journal of the Geological Society of London, 86, 191-232.
Williams, D., 1924. On two olivine-dolerite dykes in Snowdonia. Proceedings of the Liverpool Geological Society, 14, 38-47.

Select Mineral List Type

Standard Detailed Gallery Strunz Chemical Elements

Mineral List

ⓘ Thomsonite-Ca

1 valid mineral.

Detailed Mineral List:

ⓘ Thomsonite-Ca

Formula: NaCa₂[Al₅Si₅O₂₀] · 6H₂O

References:

Williams, D., 1924. On two olivine-dolerite dykes in Snowdonia. Proceedings of the Liverpool Geological Society, 14, 38-47.

Gallery:

List of minerals arranged by Strunz 10th Edition classification

Group 9 - Silicates
ⓘ	Thomsonite-Ca	9.GA.10	NaCa₂[Al₅Si₅O₂₀] · 6H₂O

List of minerals for each chemical element

H	Hydrogen
H	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O
O	Oxygen
O	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O
Na	Sodium
Na	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O
Al	Aluminium
Al	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O
Si	Silicon
Si	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O
Ca	Calcium
Ca	ⓘ Thomsonite-Ca	NaCa₂[Al₅Si₅O₂₀] · 6H₂O

Other Regions, Features and Areas containing this locality

British and Irish IslesGroup of Islands

Eurasian PlateTectonic Plate

AvaloniaVolcanic Arc

EuropeContinent

Wales
- Cambrian MountainsMountain Range
- Snowdonia National ParkNational Park

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