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

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

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

Foreword 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  

The region is home to many important and interesting minerals, as well as a wide array of beautiful gemstones, diamonds from South Africa, emeralds from Zimbabwe and tourmalines from Namibia. Field Guide to Rocks and Minerals of Southern Africa is the first definitive field guide to document the common, rare and unusual rocks and minerals of South Africa, Namibia, Botswana, Zimbabwe, Lesotho, Swaziland and southern Mozambique.

Supported by more than 500 full-colour photographs, the text describes in detail a wide range of the region’s common and attractive rocks and minerals. Authoritative information is presented in a user-friendly manner and is enlivened by box and panel features. It will appeal to mineral collectors, geologists, gemologists, students, and anyone with an interest in the natural environment. Professor Bruce Cairncross is chairman of the geology department at the Rand Afrikaans University in Johannesburg. He has an inherent interest in Southern African mineralogy and geology, and his research efforts are aimed at documenting important geological sites and promoting the preservation of the region’s mineralogical heritage. He is the author of four books, The Minerals of South Africa, The Manganese Adventure, The Desmond Sacco Collection – Focus on Southern Africa and the First Field Guide to Gemstones of Southern Africa. He is consulting editor for an American journal, Rocks & Minerals, and is an accomplished photographer of minerals and gemstones.

Content of Field Guide to Rocks and Minerals of Southern Africa

Introduction
What is a mineral?
What is a rock?
Collecting rocks and minerals
Minerals
A-Z of minerals
Minerals: a summary
Rocks
Geological map of Southern Africa
Glossary
Bibliography
Index
Photographic credits
Acknowledgements

Example of how Bruce Cairncross describes rocks and minerals: Hematite

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. Attractive reniform masses of hematite are found at the Verge-noeg fluorspar mine north-east of Pretoria. Hematite-included quartz and hematite-coated quartz come from several pegmatites along the Orange River in the Northern Cape. Beautiful specimens of a vibrant blood-red colour, some sceptered and others doubly terminated, were collected at Onseepkans in 2001 and 2002.

Namibia

Hematite-rich iron ore has been mined from syenites and carbonatites in the Kalkveld Complex on the farm Eisenberg 78 in the Otjiwarongo district, and both hematite and goethite are abundant in the Otjosondu manganese field. Basalt cavities in the Cobosobeb Mountains west of the Brandberg have produced water-clear quartz crystals with inclusions of tiny bright red hematite flakes. In the Erongo mountain pegmatites, tiny bright silver crystals, are sometimes found with quartz and feldspar. Large beds of banded-iron formation that out-crop south and south-east of Namibia consist primarily of hematite and siderite. In southern Namibia, hematite is found included as micro-scopic red crystals inside quartz from pegmatites north of the Orange River. These colour the quartz a vibrant red and orange, producing visually stunning specimens. Alternate bands of jasper and iron-poor chert form rocks known as banded-iron formations.

Zimbabwe

Iron ore has been mined for centuries in Zimbabwe and virtually every substantial outcrop that had potential for iron production was exploited. The Mashona produced a variety of iron tools, weapons and implements from smelting in clay furnaces. Hematite occurs in the major iron-ore deposits distributed in the Archaean greenstone belts. It has been mined in the Kwekwe, Gweru, Chiredzi and Masvingo districts, among others. Attractive specimens of reniform hematite came from the Yank mine in the Kadoma district where banded-iron formations outcrop. Specularite often occurs as minute inclusions in quartz crystals.

Botswana

Most of Botswana's hematite is found in banded-iron formations. These outcrop either in the Tati schist belt region in the north-east, or in the somewhat younger sedimentary rocks in the southern extremity of the country. The latter occur south of Kanye and south-east of Lobatse where there are extensive deposits of banded-iron formation. The former are represented by the deposits at Matsiloje.

Swaziland

The largest deposit of hematite in Swaziland was mined at Ngwenya, 25 km north-west of Mbabane. Hematite was the main iron ore mineral although magnetite, siderite and goethite were also present. Very attractive pseudomorphs of hematite after pyrite come from the Devil's Reef gold mine area. Cubes up to several centimetres on edge have been replaced by minute, steel-grey, sparkling hematite crystals, and specularite was abundant at the mine. Iron Hill, 2 km south of the Havelock mine in the Pigg's Peak district, contains significant iron ore. Hematite also exists in deposits at Gege and Maloma.

Early mining of Hematite in the Northern Cape

The earliest mining activities at Doornfontein in the Northern Cape have been dated at 2000 BC or even earlier and Khoisan hunters and gatherers, who periodically mined the deposit, are thought to have extracted specularite before AD 80. Hematite at Doornfontein was not the only source of pigment in the region. Old workings were found at Sishen and a famous old mine is located near the Groenwater River close to Postmasburg. The old mine was known as Catkoppies, and was visited by Europeans as early as 1801 and is described by a number of early travellers.

In 1804, the German physician Hinrich Lichtenstein visited Gatkoppies to gain information about the Koranna and Tswana. So too did the famous explorer and naturalist William Burchell, who journeyed through the region in 1812 and reported that the Tswana knew specularite as 'sibilo'. Mining methods at Gatkoppies were haphazard, and both Lichtenstein and Burchell recorded deaths from cave-ins. Gouges and scrape marks left by the tools of the Tswana and Hottentot miners are still clearly visible on the walls. The site was an important economic centre, yielding specularite that was distributed all over southern Africa.

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

Book title: Field Guide to Rocks and Minerals of Southern Africa
Author: Bruce Cairncross
Photos: Bruce Cairncross
Struik Publishers
Cape Town, South Africa 2004
ISBN: 1868729850
Softcover, 15x21 cm, 288 pages, throughout colour photos by Bruce Cairncross

Cairncross, Bruce im Namibiana-Buchangebot

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