Rocks and Minerals

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Minerals

Minerals are commonly described as homogenous, naturally occurring solids with a definite chemical composition and a distinct arrangement of atoms. Minerals contain a variety of elements such as silicon, calcium, oxygen etc. Minerals that are most common in rocks are called rock forming minerals such as quartz while other types of minerals such as diamonds (gemstones) can be described as commercially valuable minerals. Iziko Museums of South Africa is home to a mineral collection of approximately 10,000 specimens of many different minerals.

Rocks

Rocks are composed of individual or an assemblage of minerals. Rocks are divided into three basic groups, namely igneous, sedimentary and metamorphic rocks.

Igneous rocks are rocks that form when magma (molten rock) rises to the Earth’s surface and solidifies.

The Cape Granite Suite and the Karoo dolerite intrusions are examples of igneous rocks. Sedimentary rocks form through the coalescence, cementation and subsequent hardening of pre-existing rock and sediment material and are the most common rock type.

Table Mountain is made up of Malmesbury Shale below and Graafwater Formation mudstones and sandstones, and Peninsula Sandstone above it; which are examples of sedimentary rocks.

Metamorphic rocks form as a result of the transformation of existing rocks into new rocks as a result of increased temperature and pressure.

Iziko Museums of South Africa houses a wide variety of rock samples providing insight into the Earth’s composition and processes; both past and present.

The Collection

The Rocks and Minerals Collections at the Iziko Museum of South Africa includes a fine collection of calcite, a large collection from the former Tsumeb Mines in Namibia, rhodochrosite from Hotazel in the Northern Cape and numerous other rock and mineral specimens. There is also a unique meteorite collection, including both iron and stony chondrites and a rare carbonaceous chondrite.

Keywords

Carbonaceous chondrite: A broad group of chondrite meteorites that include some of the most primitive known meteorites.
Chondrite: Stony (non-metallic) meteorites created when various types of dust and small grains in the early solar system massed or pulled together to form primitive asteroids. They are also the most common type of meteorite that falls to Earth.
Comet: A celestial (from space) object with a core of ice and dust that forms a ‘tail’ of gas and dust particles when near the Sun.
Meteorite: A solid piece of debris (i.e. made from a variety of rock fragments in space) from such sources as asteroids or comets; it is formed in outer space and survives its impact with the Earth’s surface.

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