Clinopyroxene in Cheviot igneous rocks
Being 2 more resistant to the alteration processes at work in the Cheviots, clinopyroxene is more commonly seen than orthopyroxene.
Clinopyroxene crystals often appear as pale green-grey to brown, well formed but stumpy crystals between 0.1 – 2.6 mm sometimes showing simple twinning.
Al-Hafdh made eight electron microprobe analyses of clinopyroxene crystals in samples of ‘Marginal’, course-grained porphyritic, and medium grained porphyritic granitic rock. Analysis revealed that the clinopyroxene in all samples shared a composition close to 46% wollastonite, 39% enstatite, and 14% ferrosilite making them all augite.
The high wollastonite content indicates a low final equilibrium temperature. In keeping with the Cheviot orthopyroxene, the clinopyroxene also showed a low aluminium and titanium content.
H.W. Haslam conducted electron microprobe analysis on two clinopyroxene grains, one in a sample from the summit of Dunmoor Hill, the other from a sample from near Great Standrop. Both grains showed a very similar composition that averaged at 44% wollastonite, 41% enstatite, and 15% ferrosilite – a composition close to that determined by Al-Hafdh.
Haslam interpreted the high calcium levels together with the low aluminium and titanium levels as evidence for low temperatures of equilibrium in the range of 650-850oC at less than 2Kbars pressure. This would be in keeping with the modification of original clinopyroxene compositions post magma consolidation.
Composition Diagram for Cheviot Clinopyroxene
Identification
Augite is identified by its high relief, almost 90o cleavage, moderately high birefringence giving up to second order interference colours, and inclined extinction (35o-48o).
Augite may contain exsolution lamellae of orthopyroxene and the intermediate mineral between augite and hypersthene, pigeonite. In Cheviot rocks it is often partially or wholly altered to actinolite and biotite.
Photomicrographs of Cheviot Clinopyroxene
Location: Hedgehope Hill, Nr. Great Standrop, Northumberland
Viewed in plane polarised light (FoV 2.5 x 1.7 mm)
Location: Hedgehope Hill, Nr. Great Standrop, Northumberland
Viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)
Return to list of minerals in Cheviot rocks