Kimberlite
The kimberlite , formerly also called "blue ground", is a blue-green to black ultramafic rock containing olivine and phlogopite of igneous origin , which belongs to the group of peridotites and can lead to diamonds as an accessory . Other ingredients include orthopyroxene , clinopyroxene , rutile , perovskite , pyropreicher garnet and rare titanium containing Andradite and Schorlomit . The composition of kimberlite corresponds to that of its area of origin, the upper mantle . The density is between 3.3 and 5.7 g / cm³.
etymology
The kimberlite was named after the South African city of Kimberley . The first scientific description and name of the rock came from Henry Carvill Lewis and was published in 1887. The sample material for this came from the Dutoit's Pan mine in Kimberley.
Formation and dismantling
The rock is carried to the surface through the so-called pipes . These are very deep, vertical chimneys of volcanic origin. For this reason, kimberlites occur almost entirely on the earth's surface as volcanic debris ( breccias ). Only in the root zones of the pipes is kimberlite found as solidified gangue rock . Depending on the specific formation, a distinction is made between massive kimberlites (mainly storage tunnels and tunnels), intrusive kimberlite breccias and kimberlite tuffs .
In addition to diamonds, kimberlite also contains fragments of other foreign rock ( xenolites ) that were torn up by the eruptions from the deep crust of the earth, especially eclogite . There are no diamonds where the kimberlites have penetrated crevices and passages under rather calm conditions . The initially bluish-green rock ( blue ground ) takes on a yellowish-brown color ( yellow ground ) under the influence of weathering on the earth's surface .
Most of today's kimberlites were formed between 70 and 150 million years ago, but the oldest kimberlite was formed around 1.2 billion years ago. They are found in Africa , Australia , North America , India , Brazil and Siberia . In general, the occurrence of kimberlites is linked to cratonic continental blocks.
If the content is sufficient, kimberlite vents can represent primary deposits for diamonds . If the diamond-bearing kimberlite weathers, the diamonds relocated by water can form secondary deposits (e.g. soap deposits in streams, rivers and on coasts).
history
The first kimberlite was found in South Africa . Diamond-bearing kimberlites were later found in Siberia , followed by Sierra Leone in 1961.
See also
literature
- Wolfhard Wimmenauer : Petrography of igneous and metamorphic rocks. Enke, Stuttgart 1985, ISBN 3-432-94671-6 .
Web links
- Pete Rowley: Diamond Geyser - anatomy of a kimberlite eruption , blog entry by the geologist ( About Lithics ) from June 28, 2012 (English)
Individual evidence
- ↑ Ashish N. Dongre, KS Viljoen, NV Chalapathi Rao, A. Gucsik: Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy . In: Mineralogy and Petrology . tape 110 (2-3) , 2016, pp. 295-307 , doi : 10.1007 / s00710-016-0428-4 .
- ↑ Ludwig Pfeiffer, Manfred short, Gerhard Mathé: Introduction to Petrology. Akademie-Verlag, Berlin 1981, p. 144.
- ↑ Walter Ehrenreich Tröger : Special petrography of igneous rocks. A nomenclature compendium. Publishing house of the German Mineralogical Society, Berlin 1935, p. 297.