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 Kimberlite

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AutoreMessaggio
Tila
Iniziato Sciamano
Iniziato Sciamano


Femminile Serpente
Numero di messaggi : 1826
Data d'iscrizione : 22.03.10
Età : 39
Località : Prov. CN

MessaggioOggetto: Kimberlite   Gio 1 Set 2011 - 5:26

Associata all'elementale fuoco, simboleggia la capacità del fuoco di purificare e trasformare altre sostanze ed inoltre l'evoluzione spirituale dell'anima.

Molto utile per chi sta cercando di crescere spiritualmente o accrescere la sua consapevolezza, poichè può essere in aiuto per creare un rapporto più stretto con il potere superiore e mantenere l'impegno di seguire il cammino spirituale.

FONTE: http://www.snazzdragon.com/main/az/stones/kim.php

Kimberlite

Kimberlite is an unusual volcanic rock which acts as a host for Diamonds. Diamonds need pressures in excess of 50,000 atmospheres to form, and pressures this intense only exist at a depth of more than 90 miles (150 km) below the surface of the Earth. Commercially mined Diamonds are taken from Kimberlite deposits that were brought close to the Earth's surface by ancient volcanic eruptions. No Kimberlite volcanoes have erupted for more than 60 million years, and this is a good thing. The speed needed to bring Diamonds to the surface of the Earth intact from such a depth is more than the speed of sound, so a Kimberlite volcanic eruption would be a truly devastating event.

Kimberlite is associated with the power of elemental Fire and is used magically to symbolize Fire's ability to purify and transform other substances. It also symbolizes the spiritual evolution of the human soul and is a very useful stone for anyone who is striving to grow spiritually or improve their awareness of the sacred nature of life. Kimberlite is said to help you 'walk your talk', create a closer relationship with your Higher Power and maintain a committment to the spiritual path you follow.


FONTE IMMAGINE: http://it.wikipedia.org/wiki/File:Kimberlite_picture.jpg


FONTE: http://it.wikipedia.org/wiki/Kimberlite

Kimberlite
Da Wikipedia, l'enciclopedia libera.

La kimberlite è una roccia di tipo peridoditico che costituisce il riempimento di particolari dicchi e camini vulcanici, solitamente si presenta con aspetto brecciato.
È una importante roccia platinifera e prende il suo nome dalla città di Kimberley in Sudafrica; in arte mineraria è detta roccia madre in quanto i diamanti che si rinvengono si sono formati direttamente al suo interno.

Origine e giacitura

I campioni di kimberlite sono delle intrusioni a forma di colonna con un diametro di qualche centinaio di metri formate da materiali provenienti dal mantello terrestre. Il platino i si trovano nella kimberlite fanno pensare che si origino nel mantello, esperimenti hanno dimostrato che il platino si può formare a delle profondità di 100-300 km[1]

L'origine della kimberlite, tuttavia, è fenomeno di discussione. La presenza di mellilite in alcuni ritrovamenti di questa pietra in America fa pensare a qualche basalto alcalino alterato, mentre altri ritrovamenti hanno portato campioni di roccia poco alterati ed in parte serpentinizzati che fanno pensare ad un tufo; in questo caso il diamante presente in questi pezzi di roccia ritrovati fa pensare che sia enallogeno, cioè non cristallizzato in quel luogo[1]

Note

^ a b c d Autori Vari, Scheda Kimberlite in: Il magico mondo di Minerali & gemme, De Agostini (1993-1996), Novara



FONTE IMMAGINE: http://commons.wikimedia.org/wiki/File:Kimberlite.jpg?uselang=it


FONTE: http://en.wikipedia.org/wiki/Kimberlite

Kimberlite
From Wikipedia, the free encyclopedia

Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush, eventually creating the Big Hole.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

Kimberlite has attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide information on the composition of the deep mantle, and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.

Morphology and volcanology

Kimberlites occur as carrot-shaped, vertical intrusions termed 'pipes'. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma which inherits a large proportion of both CO2 and H2O in the system, which produces a deep explosive boiling stage that causes a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982).

The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle-derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5–2 km (0.93–1.2 mi) of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved, but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer (up to 0.6 mile).

Two Jurassic kimberlite dikes exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners.[1]


Petrology

Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.

Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who re-named these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.

Group I kimberlites

Group-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm, 0.020–0.39 in) to megacrystic (10–200 mm, 0.39–7.9 in) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grained groundmass.

The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.

Group II kimberlites

Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite); Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite

Kimberlitic indicator minerals

Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.

These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence may indicate the presence of a kimberlite within the erosional watershed which produced the alluvium.

Geochemistry

The geochemistry of Kimberlites is defined by the following parameters:

Ultramafic; MgO >12% and generally >15%
Ultrapotassic; Molar K2O/Al2O3 >3
Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)
REE-enrichment[2]
Moderate to high LILE enrichment; ΣLILE = >1,000 ppm

LILE = large ion lithophile elements[3]

High H2O and CO2


Economic importance

Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. Only about 1 in 200 kimberlite pipes contain gem-quality diamonds.

The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite, and so was called yellow ground. Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground.

See also Udachnaya pipe.

The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 19th century accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time is that with flood of diamonds being found, the miners were undercutting each other's price of the diamonds and eventually decreased the diamonds' value down to cost in a short time.[4]

Notes

^ Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.
^ Nixon, P.H., 1995. The morphology and nature of primary diamondiferous occurrences. Journal of Geochemical Exoloration, 53: 41-71.
^ Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland
^ "South Africa: A New History of the Development of the Diamond Fields" (1902): New York Times Archives [PDF file], New York Times.



References

Bergman, S. C.; 1987: Lamproites and other potassium-rich igneous rocks: a review of their occurrences, mineralogy and geochemistry. In: Alkaline Igneous rocks, Fitton, J.G. and Upton, B.G.J (Eds.), Geological Society of London special publication No. 30. pp. 103–19
Clement, C. R., 1982: A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange free state. PhD Thesis, University of Cape Town.
Clement, C. R., and Skinner, E.M.W. 1985: A textural-genetic classification of kimberlites. Transactions of the Geological Society of South Africa. pp. 403–409.
Mitchell, R. H., 1995: Kimberlites, orangeites, and related rocks. Plenum Press, New York.
Mitchell, R. H.; Bergman, S. C. (1991). Petrology of Lamproites. New York: Plenum Press. ISBN 0-306-43556-X.
Smith, C. B., 1983: Lead, strontium, and neodymium isotopic evidence for sources of African Cretaceous kimberlite, Nature, 304, pp 51–54.
Edwards, C. B., Howkins, J.B., 1966. Kimberlites in Tanganyika with special reference to the Mwadui occurrence. Econ. Geol., 61:537-554.
Nixon, P.H., 1995. The morphology and nature of primary diamondiferous occurrences. Journal of Geochemical Exoloration, 53: 41-71
Wagner, P. A., 1914: The diamond fields of South Africa; Transvaal Leader, Johannesberg.
Woolley, A.R., Bergman, S.C., Edgar, AD, Le Bas, M.J., Mitchell, R.H., Rock, N.M.S. & Scott Smith, B.H., 1996. Classification of lamprophyres, lamproites, kimberlites, and the kalsilitic, melilitic, and leucitic rocks. The Canadian Mineralogist, Vol 34, Part 2. pp. 175–186.



FONTE IMMAGINE: http://commons.wikimedia.org/wiki/File:Chidliak_Kimberlite.jpg
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