The weird and wonderful World of Rocks & Minerals

This mineralogical ‘christmas tree’ is a thin section of a mineral called ‘inverted pigeonite’. Pigeonite is a calcium-poor clinopyroxene which commonly occurs in basaltic volcanic rocks. Because volcanic rocks undergo rapid cooling, the mineral is ‘frozen’ and preserved as pigeonite. Pigeonite is, however only stable at high temperatures and will undergo transformation during slow cooling. This photo, which represents a thin section of a gabbro, a mafic plutonic rock, viewed under crossed nicols, shows the effects of this transformation. the high-temperature monoclinic pyroxene becomes unstable and exsolves its calcium content in two generations of calcium-rich augite, the broad lamellae (the ‘branches’) and the shorter and narrower lamellae (the ‘needles’ or ‘tinsel’). The remainder of the mineral is ‘inverted’ into a calcium-poor orthorhombic pyroxene (black).
This photo was taken from a gabbro from the 2 billion year old Molopo Farms Complex, Botswana. Image width: 0.74 mm

The spectacular meteorite impact crater ‘Tswaing’ (‘place of salt’), near Pretoria. This structure is approximately 220 000 years old and has a diameter of about 1.2 km. It is one of only 5 meteorite craters in the world which have a museum attached and is readily accessible by road. With its well preserved crater rim and its un-interrupted record of 220 000 years of climatic change, preserved in the crater sediments, Tswaing is of extraordinary interest, both to scientists and tourists. 

Igneous layering in the northeastern Bushveld Complex.
The 2060 million year old Bushveld Complex is the largest known layered intrusion in the world. Measuring more than 400 km from east to west, the complex hosts the largest reserves of vanadium, chromium and platinum-group metals in the world.

Main Magnetite Layer, eastern Bushveld Complex

Here’s looking at you, kid!

Humphrey Bogart’s ancestor, Australopithecus africanus, not from Casablanca, but from Sterkfontein (reconstruction by Dr. F Durand)

Drakensberg basalt overlying Karoo sediments

Charnockite dikes in marble, KwaZulu-Natal

The Hoba iron-nickel meteorite, Namibia

Skull of a Gorgonopsian , a mammal-like reptile from the Karoo