Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium iron aluminium silicate hydroxide hydrate
Chemical Formula: (Mg,Fe,Al)9(Si,Al)8O20(OH)10.nH2O
Method(s) of Verification: all cited occurrences - XRD (Garvie & Metcalfe, 1997; Merriman & Roberts, 1985; Roberts & Merriman, 1990).
- Metamorphic: low-grade
Introduction: corrensite is a clay mineral consisting of regular interstratifications of trioctahedral chlorite and trioctahedral smectite in equal proportions. It most typically develops as an authigenic mineral (one that forms after deposition) in sedimentary rocks such as mudstone and shales, or as an alteration product in hydrothermally altered rocks, particularly those of igneous origin.
Occurrence in Wales: Merriman & Roberts (1985) described the occurrence of corrensite in rocks of Arenig and Llanvirn (Ordovician) age from Snowdonia and Llŷn, in mudstones close to intrusions and which had been affected by contact metamorphism prior to regional metamorphism. On Llŷn, chlorite/vermiculite is associated with basic intrusions in the Rhiw area, and chlorite/smectite is associated with intrusions in the Llanbedrog and Yr Eifl areas. Roberts & Merriman (1990) have also identified corrensite as a component of metabentonites of Middle Cambrian age from the St. Tudwal's Peninsula, also on Llŷn. More recently Garvie & Metcalfe (1997) recorded corrensite from veins cutting Ordovician volcanic rocks at Builth Wells.
- Llanelwedd Quarry, Builth Wells, Powys: corrensite from all other localities in Wales occurs as a rock-forming mineral. However, quartz-calcite veins cutting altered andesite contain both talc and a saponite/corrensite/chlorite-dominated assemblage nearer to the walls of the vein (Garvie & Metcalfe, 1997).
There are no key localities for this specimen.
- Garvie, L.A.J. & Metcalfe, R., 1997. A vein occurrence of co-existing talc, saponite, and corrensite, Builth Wells, Wales. Clay Minerals, 32, 223-240.
- Merriman, R.J. & Roberts, B., 1985. A survey of white mica crystallinity and polytypes in pelitic rocks of Snowdonia and Llŷn, North Wales. Mineralogical Magazine, 49, 305-319
- Roberts, B. & Merriman, R.J., 1990. Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin. Geological Magazine, 127, 31-43.
There are no references for this specimen.