Field Guide to Rocks and Minerals of Southern Africa, by Bruce Cairncross

Field Guide to Rocks and Minerals of Southern Africa, by Bruce Cairncross.Penguin Random House. Imprint: Struik Nature. Cape Town, South Africa 2004. ISBN 9781868729852 / ISBN 978-1-86-872985-2

Field Guide to Rocks and Minerals of Southern Africa, by Bruce Cairncross.Penguin Random House. Imprint: Struik Nature. Cape Town, South Africa 2004. ISBN 9781868729852 / ISBN 978-1-86-872985-2

Images as shown in the Field Guide to Rocks and Minerals of Southern Africa by Bruce Cairncross.

Images as shown in the Field Guide to Rocks and Minerals of Southern Africa by Bruce Cairncross.

Field Guide to Rocks and Minerals of Southern Africa, by Bruce Cairncross: Southern Africa boasts many spectacular examples of rocks and minerals; volcanic lavas make up the dramatic Drakensberg mountain range, granite mountains form stunning scenery in Namibia and Zimbabwe, and some of the most ancient rocks on Earth, over 3000 million years old, are to be found on the subcontinent.

Bruce Cairncross  

Hematite

Composition: oxide
Crystal system: hexagonal
Hardness: 5-6
Specific gravity: 5.26
Streak: red to brownish-red
Lustre: metallic, submetallic

Named from the Greek 'aima' meaning 'blood', a reference to the colour of this iron oxide mineral, Hematite is a relatively common mineral that can occur as very finely disseminated grains, forming banded-iron formation rocks composed purely of hematite. It sometimes forms beautiful bright silver crystals, and may also occur in a platy or micaceous habit referred to as specular hematite or as specularite or 'Blinkklip' (glittering stone), a black iron ore with a brilliant glitter. Iron from hematite is used to make steels and alloys such as ferroalloys, ferromanganese and ferrosilicon. Historically, hematite in the form of specularite and red ochre was prized as a cosmetic by southern Africa's indigenous peoples. Specularite was rubbed into the hair to make it sparkle and red ochre was commonly smeared on the body or sprinkled on corpses in the hope that they would revive. Where ochre - a powdery, greasy mixture of hydrated iron oxides - was not available, massive hematite was pounded into a powder and mixed with fat. Specularite was much more difficult to obtain and, therefore, more valuable.

Occurrence: Hematite is found in a variety of rocks. However, it is most abundant in sedimentary banded-iron formations. Iron and manganese tend to be concentrated in the linear belt north of Sishen-Kuruman-Postmasburg in South Africa, and vanadium is found in the Bushveld Complex. South Africa Underground caverns in the Northern Cape were the richest source of specularite and ochre. The most extensive workings were found on the farm Doornfontein, north-west of Postmasburg. In the 1960s these were shown to consist of at least four separate chambers extending over 100 m underground. Hematite is a common constituent in all orebodies in the Kalahari manganese field. Very attractive, highly lustrous and large hematite crystals were found at the Black Rock and Wessels, as well as the N'Chwaning I and II mines. Complex crystals, frequently twinned, up to 7 cm in diameter, came from the Wessels mine, which produced tabular crystals in May 1988, up to 30 cm in diameter and 8 cm thick. These were often coated on one side with andradite garnet crystals and associated with barite and calcite jet-black, very bright crystal groups with crystals up to 8 cm on edge were found in N'Chwaning II. Specularite is a common constituent of iron deposits in the Postmasburg district. The iron ore mines at Sishen and Beeshoek exploit hematite ore. At the old Messina mine, hematite occurred as specularite and small 'eisen rosen' (iron roses). These occur as individual rosette-like balls included in quartz crystals or as free-standing groups. Hematite in Messina ore lodes imparts a red coloration to many associated minerals, particularly as inclusions in quartz crystals. Specularite is commonly associated with quartz as masses of fine platelets either coating the crystal faces or as inclusions. The Harper and Messina No. 5 Shaft mines contained specularite Iron oxide is scarce at the Campbell mine although tabular specular hematite crystals 3 cm in diameter have been found in some lodes. Similar crystals, 3 cm in diameter and 8 mm thick, came from a vug in the 21-760 stope in the Messina mine, where aggregates of flaky specularite locally fill vugs up to 2 m in length. [...]

This is an extract from Field Guide to Rocks and Minerals of Southern Africa, by Bruce Cairncross.

Title: Field Guide to Rocks and Minerals of Southern Africa
Author: Bruce Cairncross
Publisher: Penguin Random House
Imprint: Struik Nature
Cape Town, South Africa 2004
ISBN 9781868729852 / ISBN 978-1-86-872985-2
Softcover, 15 x 21 cm, 288 pages, throughout colour photos

Cairncross, Bruce im Namibiana-Buchangebot

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