Carbonado

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Three carbonados (4.05 ct, 4.07 ct and 5.09 ct) from the Bangui region , Central Africa

Carbonado ( Spanish for charred ), often also called black diamond , is a very rare polycrystalline variety of the mineral diamond .

As a porous aggregate composed of many small diamond crystallites of predominantly irregular - rarely cubic or rounded cubic - shape, Carbonados contain many admixtures such as carbon or graphite, various metal oxides, as well as minerals containing potassium, aluminum or silicon. Iron, nickel, chromium, copper, magnesium, beryllium and sometimes calcium were also found to fill the pores. According to various researchers, however, they were only introduced after the carbonados were formed and therefore cannot be used to explain the history of the carbonados. These admixtures are also responsible for the dark gray or rust brown to black color.

properties

The size of the individual units generally varies between 0.001 and 0.2 millimeters. So far, only once has a carbonado of exceptional size been found, even surpassing the famous Cullinan diamond . The Carbonado do Sérgio was discovered in the area around Lençois (Bahia) in 1895 and weighed 3167 carats (Cullinan: ≈3106 carats), i.e. about 633 grams.

The density of Carbonado fluctuates between 3.1 and 3.4 g / cm³ (locality: Macaúbas basin). The carbonados of unknown origin examined by Stephen E. Haggerty in 1998 even had a density of 2.8 g / cm³. With diamonds, however, the density is usually around 3.515 g / cm³. Despite its low density compared to diamonds, the hardness of a carbonado is so high that it cannot be processed with normal grinding methods. In 2004, a research group led by K. De Corte was only able to shape and polish carbonados with the help of a laser . Its great hardness and, as a result, its correspondingly excellent abrasive properties would be of great benefit to the technical industry. However, its extremely rare occurrence stands in the way of large-scale industrial use as an abrasive. The primary goal of research is therefore to decipher the formation conditions of carbonados and, if possible, to produce them synthetically.

history

The earliest reports of carbonado finds and their assignment to diamonds come from "Chapada Diamantina " (Bahia) in 1840 , where they were washed from the local river soaps .

After the discovery of large diamond deposits in South Africa , the mining of Brazilian diamonds collapsed , but the demand for carbonados remained almost unchanged due to its valuable properties for the industry. The proportion of carbonados rose to 60 to 70% compared to total production (as of 2004). However, Bahia does not mine diamonds or carbonados today. When the entire area was declared a national reserve, any prospecting activity was also strictly prohibited.

In January 2011 the new Schwarzer Diamant extension of the German Mining Museum in Bochum received a carbonado weighing ≈3.4 carats. The jeweler Michael Mauer humorously justified his donation with the words "A black diamond belongs in the black diamond" and that he was on coal here, learned here to make " coal " and therefore could give some coal back to the city.

Education and Locations

Since a diamond only arises under terrestrial conditions at great depths at high pressure and high temperature, which results in a compact structure in the known form, it was assumed early on that carbonado did not arise on earth. Its pores contain hydrogen and are also filled with quartz , which has made it impossible to examine the infrared spectrum to clarify its origin. However, Jozsef Garai and Stephen Haggerty of Florida International University found a way: They ground samples of carbonado and removed the quartz with the help of hydrochloric and hydrofluoric acid . Subsequent exposure to intense synchrotron radiation showed that the spectrum of carbonados is very similar to that of diamonds from meteorites.

In fact, there are still only assumptions about the exact formation conditions of carbonados, more or less based on the composition of the samples examined so far or the geological characteristics of the respective sites.

It is particularly noticeable that the “black diamonds” are never isolated, but are always found together with normal diamonds. In contrast, they have so far not been found in kimberlites or related rocks , the typical parent rock of diamonds, but only secondarily in soaps . For example, the region of the Macaúbas River in Minas Gerais was formed by glacial (Ice Age) sedimentation around 950 million years ago (according to Pedrosa-Soares et al., 2000). Since, in addition to diamonds and carbonados, chrysoberyl , monzanite , staurolite and almandine were found in the Macaúbas Basin , which, according to their known formation conditions, could not have originated in the geological formations there, only transport through ice age glaciers from north-north-west direction (Bahia) with subsequent alluvial enrichment in question.

So far, only Diamantina and Lençois in Bahia, the region of the Macaúbas River (also Macaúbas Formation, formerly Macahúbas) in Minas Gerais , Ubangui in the Zaire lowlands , Western Australia , Venezuela , English Guyana , northern China , Borneo as well as have been identified as sites Sacha (Yakutia) known in Russia .

literature

  • Joachim Karfunkel, Vitaly Petrovsky, Maximiliano S. Martins, Mário LSC Chaves, Reinhard Wegner: Carbonado - Black Diamond - from Minas Gerais, Brazil: geological occurrence and considerations for its formation . In: The opening. Journal for the friends of mineralogy and geology . tape 5 . VFMG e. V., Heidelberg 2008, p. 258-266 .
  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th, revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 88, 282 .

Web links

Commons : Carbonado  - collection of images, videos and audio files

Individual evidence

  1. ↑ Ronny von Wangenheim: Present: The little black one. In: ruhrnachrichten.de. Ruhr Nachrichten , January 13, 2011, accessed on October 14, 2019 .
  2. ↑ Jozsef Garai, Ute Kehse: Black diamonds from eternity. In: Wissenschaft.de. Image of Science , January 11, 2007, accessed October 14, 2019 .
  3. ^ Rainer Kayser: Black diamond from space. In: astronews.com. Astronews, January 15, 2007, accessed October 14, 2019 .