| Diamonds ascend to the Earth's surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from Earth's mantle, this magma rises and erupts in small but violent volcanoes. Just beneath such volcanoes is a carrot- shaped "pipe" filled with volcanic rock, mantle fragments, and some embedded diamonds. The rock is called kimberlite after the city of Kimberley, South Africa, where the pipes were first discovered in the 1870s. Another rock that provides diamonds is lamproite. | |
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The volcano that carries diamond to the surface emanates from deep cracks and fissures called dikes. It develops its carrot shape
near the surface, when gases separate from the magma, perhaps accompanied by the boiling of ground water, and a violent
supersonic eruption follows. The volcanic cone formed above the kimberlite pipe is very small in comparison with volcanoes like Mount Etna, but the magma originates at depths at least 3 times as great. These deep roots enable kimberlite to tap the source of diamonds. Magmas are the elevators that bring diamonds to Earth's surface. |
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In some circumstances little if any of the magma will reach the surface having been diverted into fissures and cracks in 'weak rock' during it's ascent - the path of least resistance. These will form into kimberlite dykes and sills which like the pipes may or may not be diamondiferous. |
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Certain minerals, present in the rocks from the upper mantle,
are useful indicators of the presence of kimberlite. These
make ideal kimberlite indicator minerals for several reasons:
they are far more abundant in kimberlite than diamonds, they
occur almost exclusively in kimberlite, they can withstand
extreme crushing and grinding and they are visually and
chemically distinct. Geologists look for these indicator
minerals in areas they feel may contain kimberlite. Some of the most commonly used kimberlite indicator minerals are: Cr-pyrope (purple colour, kelyphytic rims) Eclogitic garnet (orange-red) Cr-diopside (pale to emerald green) Mg-ilminite (black conchoidal fracture) Chromite (reddish-black, irregular to ochtaheral crystal shape) Olivine (yellowish-green) |
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The following are links to some interesting sites featuring kimberlite.
American Natural History Museum
BC Ministry of Energy and Mines
University of British Columbia