CSM Rock Collection

Anorthosite, IG09AN01
Location: Bushveld, South Africa

Description

Anorthosite from the Rustenburg Layered Suite of the Bushveld Igneous Complex in South Africa. The rock is composed of more than 90% plagioclase with minor orthopyroxene and chromite. The rock represents a cumulate where plagioclase has been preferentially concentrated from basaltic magma.

Hand specimen

Specimen size: ?? mm
The hand specimen is dominated by plagioclase (white/grey) with some chromite (black) that defines distinct banding (vertical in the image). This banding would have been subhorizontal within the intrusion. Orthopyroxene is not visible at this scale.

Field of view: 30 x 22mm
Under a hand lens the rock displays an equigranular texture. Plagioclase (white) occupies most of the rock, and chromite (black, metallic) can be seen to define a banding (horizontal). Orthopyroxene is not discernible in the image. Please ignore the fibrous material on the surface.

Thin section

Thin section in plane polarised light (full section)

In plane polarised light, the dominance of plagioclase (clear, appearing white) is very apparent. Bands of chromite grains (opaque, appearing black) define the banding within the rock. Close inspection reveals very small amounts of orthopyroxene (light grey) that is typically concentrated in the areas with chromite.

Thin section in cross polarised light (full section)

Under crossed polarisers, the individual plagioclase grains can be recognised with the typical multiple twinning that is characteristic for the mineral in the plutonic environment. Chromite appears as small, opaque crystals that are concentrated into bands. Orthopyroxene is hardly visible in this image. Plagioclase grains display a preferred orientation that is parallel to these bands.

Thin section, detail

In this higher magnification view under plane polarised light, the opthopyroxene becomes apparent. The dominant plagioclase (low relief, colourless - white in the image) dominates with chromite (black) and small interstitial grains of orthopyroxene (medium relief, greyish brown). A few grains of biotite (low relief, strongly brown) can also be observed.

IG09AN01

In this higher magnification view under crossed polarised light, the low interference colours of the opthopyroxene becomes apparent. The multiple twinning of plagioclase is also visible. Note that the plagioclase crystals have relatively straight grain boundaries that meet in 120º triple junctions indicating textural maturity.

In this high magnification view under plane polarised light, the medium relief and cleavage of the orthopyroxene is particularly visible.

This high magnification view under cross polarised light, illustrates the the medium relief and cleavage as well as the characteristic low interference colours of the orthopyroxene.