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Diamond in kimberlite (Mir Kimberlite Pipe, Malo-Botuoba Kimberlite Field, ~354-360 Ma; Mir Diamond Mine, Siberia, Russia) | by James St. John
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Diamond in kimberlite (Mir Kimberlite Pipe, Malo-Botuoba Kimberlite Field, ~354-360 Ma; Mir Diamond Mine, Siberia, Russia)

Diamond crystal in kimberlite from the mid-Paleozoic of Siberia, Russia. (Stan & Pris Woollams collection)

 

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

 

Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.

 

To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state.

 

The element carbon occurs principally in its native state as graphite (C) and diamond (C). Graphite is the common & far less valuable polymorph of carbon. A scarce polymorph of carbon is diamond. The physical properties of diamond and graphite couldn’t be more different, considering they have the same chemistry. Diamond has a nonmetallic, adamantine luster, typically occurs in cubic or octahedral (double-pyramid) crystals, or subspherical to irregularly-shaped masses, and is extremely hard (H≡10). Diamonds can be almost any color, but are typically clearish, grayish, or yellowish. Many diamonds are noticeably fluorescent under black light (ultraviolet light), but the color and intensity of fluorescence varies. Some diamonds are phosphorescent - under certain conditions, they glow for a short interval on their own.

 

Very rarely, diamond is a rock-forming mineral (see diamondite - www.flickr.com/photos/jsjgeology/14618393527).

 

The above specimen is a very rare, ultramafic igneous rock called kimberlite, which is the ultimate host rock for most diamonds. Kimberlites are unusual igneous bodies having overall pipe-shaped geometries. Their mode of formation is only moderately understood because they have not been observed forming. Kimberlites are known from scattered localities throughout the world - only some are significantly diamondiferous. Classic localities for diamonds are India and Brazil. Africa was also discovered to have many kimberlites and is world-famous for producing large numbers of diamonds. Other notable diamondiferous kimberlite occurrences include Russia, China, and northwestern Canada.

 

Kimberlites are named for the town of Kimberley, South Africa. Several kimberlite pipes occur in the Kimberley area. Kimberlites have a gently tapering-downward, pipe-shaped cross-section, somewhat like a carrot.

 

This sample of diamondiferous kimberlite is from Siberia’s Mir Diamond Mine. Identifiable large fragments in the matrix of this rock include serpentinized olivine (greenish) and garnet (very dark red). The Mir Kimberlite Pipe was the first diamondiferous kimberlite found in Siberia and one of the most important diamond mines on Earth (see: lumq.com/wp-content/uploads/2008/06/mir_mine_5.jpg). The Mir Kimberlite is one of several ultramafic pipes in the Malo-Botuoba Kimberlite Field of the Siberian Platform. Additional diamondiferous kimberlite fields have been found elsewhere in Siberia.

 

Geochronologic studies on rocks and minerals from the Mir Kimberlite Pipe have given dates ranging from the Archean to the Phanerozoic. Ages from small inclusions in diamond crystals date well into the Precambrian, although Mir diamonds appear to have crystallized not long before eruption of the pipe in the Devonian (there is mounting evidence indicating diamond inclusion dates don’t necessary reliably indicate diamond crystallization dates).

 

Available evidence indicates that the Mir Pipe erupted some time in the Devonian or Mississippian (Early Carboniferous). Devonian to Mississippian ages have also been obtained from other pipes in the Malo-Botuoba Kimberlite Field. Published eruption dates for the Mir Pipe range from 403 to 324 million years. Many studies report dates clustering around 354 to 360 Ma (~latest Devonian to Early Mississippian). These dates roughly correspond to a time of intraplate basalt volcanism in Siberia.

 

The Mir Kimberlite’s geometry is that of a gently tapering-downward pipe, as are most other kimberlite bodies. The pipe intrudes Cambrian and Ordovician carbonate rocks, which occur throughout the Siberian Platform.

 

Reported xenoliths in the Mir Pipe include Paleozoic sedimentary rocks, Paleozoic igneous rocks, Precambrian basement rocks, and mantle rocks (eclogites & several types of peridotites).

 

Location: Mirnyy, Yakutia/Sakha, Siberia, Russia (62˚ 31’ 43.93” North, 113˚ 59’ 38.60” East)

 

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Taken on May 1, 2010