CSM Rock Collection

Gabbronorite, IG09GB01
Location: Bushveld, South Africa

Description

Gabbronorite is a mafic igneous rock with significant amounts of orthopyroxene. The rock formed by crystallisation of a silicic high-magnesium basalt and is typically found in the evolved parts of layered, mafic-ultramafic intrusions.

Hand specimen

Specimen size: ?? mm
The hand specimen displays an equigranular texture with roughly equial amounts of plagioclase (clear, white) and pyroxene (dark, black).

Field of view: 30 x 22mm
Under a hand lens the lath shaped plagioclase crystals (clear, white) are clearly recognised due to their shape and cleavage. With close inspection, the cleavage planes appear to be striped because of the intersection with albite twins. The dark (mafic) grains are pyroxenes. Individual orthopyroxene and clinopyroxene crystals cannot be directly identified at the hand lens scale.

Thin section

Thin section in plane polarised light (full section)

In plane-polarised light, the dominant plagioclase (white, low relief) and pyroxene (greenish-brownish grey, medium relief) can be recognised along with traces of biotite (brown, low relief) and opaque minerals.
The different pyroxenes require cross polarised light for identification.

Thin section in cross polarised light (full section)

Under cross-polarised light, the characteristic multiple albite twinning of the plagioclase (variable grey-black, low relief) becomes apparent. The dominance of orthopyroxene (brown colours) over clinopyroxene (blue-red-purple) also becomes apparent by the abundance of grains with low interference colours.

Thin section, detail

A close-up (plane-polarised) of the orthopyroxene reveals the medium relief against the plagioclase and the cleavages. The grain with thin exsolution lamellae (top left corner) is clinopyroxene. Small grains of biotite (brown) are associated with fractures and grain boundaries.

Under crossed polarisers the different interference colours become apparent. Orthopyroxene has brown colours, while the clinopyroxene has red-blue-purple interference colours.

In cross polarised light, the straight extinction of the orthopyroxene with reference to the dominant cleavage becomes apparent as the grain is rotated. Note the exsolution lamellae (grey) oriented near to vertical in the image.

This view in cross polarised light displays the cleavage at a position away from extinction.