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Summary Article: Kimberlite
from Rock and Gem

THE MAJOR SOURCE OF DIAMONDS, kimberlite is a form of peridotite. It is mica-rich, often with well-formed crystals of brown phlogopite mica. Other abundant minerals include chromium- and pyrope-rich garnet and chrome-bearing diopside; lesser amounts of ilmenite, serpentine, pyroxene, calcite, rutile, perovskite, magnetite, and diamond can also be present.

Kimberlite occurs in pipes – intrusive igneous bodies roughly circular in cross section with vertical sides, and usually less than 3/4 mile (1km) in diameter – and other igneous bodies that are also steep-sided and intrusive. These tend to be found in the uplifted centers of continental platforms, and appear to be roughly the same age, having formed during the Late Cretaceous Period (100 to 65 million years ago). It appears that the kimberlite was injected into zones of weakness in the crust as a relatively cool “mush” of crystals rather than as a liquid magma that crystallized within the pipes themselves, as there is usually little contact metamorphism of surrounding rocks. Xenoliths (fragments) of mantle rock are often brought to the surface in kimberlites, making them a valuable source of information about the mantle. Diamonds need pressures of over 50,000 atmospheres to form. This corresponds to a depth below the Earth’s surface of at least 90 miles (150km), well into the upper peridotite mantle. An examination of minute mineral grains trapped inside growing diamonds reveals that most diamonds formed one to three billion years ago. Once diamonds formed in the upper mantle, they remained there for millions of years before being blasted to the surface at supersonic speeds in devastatingly volatile eruptions. Fortunately, no kimberlite volcanoes are known to have erupted for over 60 million years.

Kimberlite is also called blue ground, a reference to its color. Weathered kimberlite is yellowish in color, and is called yellow ground. It is in areas of yellow ground that diamonds were first discovered in South Africa, and later in the USA in Arkansas, Colorado, and Wyoming.

  • Properties
  • Rock type Ultramafic, volcanic, igneous
  • Major minerals Olivine, pyroxene, mica
  • Minor minerals Garnet, ilmenite, diamond
  • Color Dark gray
  • Texture Fine to coarse/porphyritic

Kimberlite pipe

The mined-out diamond-bearing pipe at Kimberley, South Africa, has now filled with water. Kimberlite pipes do not have a great lateral extent, but they are very deep.

Weathered kimberlite

This heavily weathered specimen of kimberlite from Kimberley, South Africa, holds an octahedral diamond crystal.

Mining kimberlite

Diamonds were first discovered at De Kalk Farm on the banks of the Orange River, South Africa, in 1867, but the host rock was not discovered until two years later. At first, diamonds were extracted by panning from weathered kimberlite in open pits, but as the pits became deeper and the rock harder it was necessary to mine underground. After the rocks have been blasted, they are brought to the surface and the diamonds are separated out. Wet, crushed kimberlite is passed across grease-covered tables: water does not stick to diamond, so the dry stones stick to the grease, while the kimberlite washes away (see Ancient mining). The diamond crystals are scraped off. Now many of the mines are worked out and South Africa is no longer one of the top diamond-producing nations.

Drilling

Drill-holes packed with explosives are blasted to break up the rocks.

Rough diamond

Most gem-quality diamonds are found as octahedral crystals.

© 2008 Dorling Kindersley Limited

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