Orthopyroxene in Cheviot igneous rocks
Orthopyroxene is present as a primary mineral in most igneous rocks in the Cheviots often occurring as phenocrysts, but also in the groundmass of porphyritic types.
It forms as colourless to grey, subhedral, prismatic grains with size varying between 0.1 – 1.0 mm and often contains inclusions of magnetite and apatite. It is sometimes altered to fibrous actinolite or chlorite around crystal edges or adjacent to internal cracks.
N.M. Al-Hafdh conducted electron microprobe analysis on two orthopyroxene grains in a Marginal-type sample and on one grain from a porphyritic type. All three gave a composition corresponding to hypersthene, approx. 59% enstatite, 38% ferrosilite, 3% wollastonite. Although hyperthene is no longer a credited mineral name, orthopyroxene being preferred instead, we find it useful to have this name label for a mineral that is common locally.
Al-Hafdh also found low aluminium and titanium levels which he held to be consistent with the view that the Cheviot granitic rocks were formed close to the surface under low pressure.
H.W. Haslam conducted electron microprobe analysis on two orthopyroxene grains, one in a sample from the summit of Dunmoor Hill, the other from an outcrop north of Great Standrop. Both grains showed a very similar composition that averaged at 63% enstatite 35% ferrosilite, 2% wollastonite – a composition close to that determined by Al-Hafdh.
Along with the low calcium levels, Haslam also recorded low aluminium and titanium levels. He too interpreted these levels as evidence for low temperatures of equilibrium in the range of 650-850oC at less than 2Kbars pressure suggesting the orthopyroxene compositions must have been modified after magma consolidation.
Composition Diagram for Cheviot Orthopyroxene
Identification
Hypersthene has moderately high relief, almost 90o cleavage and fairly low, first order birefringence. It can be distinguished from clinopyroxenes by its straight extinction and from enstatite by its pink to green colours, pleochroism, and higher relief and birefringence.
Photomicrographs of Orthopyroxene
The orthopyroxene crystal in the centre of view is 1.0 mm in length, has moderately high relief and has, in this view, cleavage lines at 45o to the vibration direction of polarised light (and to the edge of the photograph). In this position it shows a red tint.
In this view, the same orthopyroxene crystal has been rotated so its cleavage lies at 90o to the direction of light edge. In this position it shows a green tint.
Locality: N. Rig Cairn, Cheviot Hills, Northumberland.
The same orthopyroxene crystal with cleavage lines at 45o showing first order interference colours.
Locality: N. Rig Cairn, Cheviot Hills, Northumberland.
The same orthopyroxene crystal with cleavage lines running almost vertically now showing straight extinction.
Return to list of minerals in Cheviot rocks