Chlorite in Cheviot igneous rocks
Chlorite occurs as a secondary mineral related to the sericitisation and propylitisation of rocks in the Cheviot’s alteration zones. It is also seen in association with calcite infilling and veining.
Al-Hafdh carried out electron microprobe analysis on five grains of chlorite selected from samples of fine, medium, and large grained porphyritic rocks (corresponding to his Hedgehope, Dunmoor and Standrop types).
Four analyses gave a composition corresponding to pycnochlorite using the nomenclature of chlorites and oxidized chlorites (Hey, 1954).
The fifth analysis, on a grain in a sample of fresh fine-grained porphyritic rock from Kitty Crags (his Hedgehope type), revealed an apparently very silica-rich chlorite with a high K2O content. Al-Hafdh suggested this might be a mixed analysis due to the grain’s close association with a relict biotite.
Composition Diagram for Cheviot Chlorite
Identification
In hand specimens chlorite appears very dark green and is brittle and flaky in texture.
In thin section it appears very pale-green in plane polarised light, sometimes with pleochroism in shades of green. With crossed polars, it shows low first order and anomalously dark blue interference colours sometimes with straight extinction and sometimes with radial extinction.
Chlorite has one set of fine micaceous cleavage and a low to moderate refractive index.
Photomicrographs of Cheviot Chlorite
The chlorite crystal in the centre of the picture is positioned so that it’s cleavage lies parallel to the vibration direction of the plane polarised light. In this position it reveals its weakest absorption colour, a very light yellow-green. Opaque magnetite is included along the crystal cleavages.
Locality: Hedgehope Hill, Northumberland.
The same chlorite crystal is rotated so that its cleavage lies at right angles to the vibration direction of the plane polarised light. In this position the crystal reveals its pleochroism with a slightly darker green absorbtion colour.
Locality: Hedgehope Hill, Northumberland.
Here, with crossed polarising filters, the mineral’s low and anomalous interference colours are evident.
Locality: Cheviot Hill, Northumberland.
Return to list of minerals in Cheviot rocks