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The largest specimen of carbonado known was found July 15, 1895, in Bahia, in the neighborhood of the town of Leñoes, between the Rio Rancardor and the stream known as the "Bicas." It is about the size of a man's fist, and when first found weighed 3167 carats; since it was taken from the ground it has gradually lost 19 carats in weight, so that its present weight is 3,148 carats, or about 650 grains (nearly 1 1/2 pounds avoirdupois). The heaviest specimen previously known weighed only 1,700 carats, and was of inferior quality. The essential constituent of carbonado, as of ordinary diamond, is carbon; the former, however, contains a much larger amount of impurity than does the latter. After combustion, this impurity remains behind as an incombustible residue, and sometimes forms a skeleton outline of the original fragment or nodule of carbonado. The amount of incombustible ash varies from 1/4 to over 4 per cent of the weight of the carbonado burnt. Three specimens examined by Rivot contained 96.84, 99.10, and 99.73 per cent of carbon, and 2.03, 0.27 and 0.24 per cent of ash respectively. This ash resembled in appearance yellow, ferruginous clay, and enclosed microscopic crystals of an undetermined substance. By treating finely powdered carbonado with aqua regia a portion of the mineral matter constituting the ash may be dissolved out, the solution being found to contain iron and a little calcium, but no aluminum. Dana gives as the composition of carbonado: carbon 97, hydrogen 0.5, and oxygen 1.5 per cent. The presence of the last two constituents however requires confirmation. The view has been expressed that carbonado is a mixture of crystallized and amorphous carbon, but it is not supported by a microscopic examination of the material. The hardness of carbonado not only equals that of diamond, but may even exceed it, and its hardness is supposed to be greater the less distinctly it is crystalline. Carbonado cannot therefore be cut by ordinary diamond powder, or at least only with extreme difficulty; it forms a valuable cutting material for ordinary diamonds, and large quantities are used as a grinding material for this and other purposes, which require exceptionally hard material. On account of its great hardness, combined with the absence of cleavage (in the mass), carbonado is specially suitable for the rock-drills of boring machinery; moreover, it possesses another advantage over the diamond in that it can be easily shaped into any required form and size, while with ordinary diamond either a natural crystal or a cleavage fragment must be used.

The specific gravity of carbonado is, on account of its porous nature, lower than that of diamond crystals. The values 3.012, 3.141, 3.255, 3.416 have been determined; the last three of these values were determined with the three specimens of which the chemical composition is given above, and in the same order. Carbonado, when reduced to powder, has of course the same specific gravity as ordinary diamond.

That the occurrence of carbonado is almost entirely confined to the district of the Serra da Cincora has already been mentioned. It was found for the first time in the year 1843 in the "gupiarras" of the river San Jose, and all the carbonado required for technical purposes is derived from this source. In Minas Geraes carbonado may be said to be completely absent; in South Africa it is present in very small amount; and in India and Australia no trace of it has been met with. In the diamond sands of Borneo it is less rare, and here are to be found nodules of carbonado enclosed by a shell of colorless diamond. In every locality in which it occurs this black, porous variety of diamond is associated with crystals of the usual kind, they are found in the same rocks, and have no doubt a common origin and mode of formation.

The production of carbonado in the Serra da Cincora, which in former times was considerable, has now appreciably diminished, being scarcely more than 350 grams per month. This, and its ever-increasing application for technical purposes, has caused a tremendous rise in price. When first found little use was made of it, and it could be bought for a low price; later, the price shows a tendency to rise higher. Diamonds in considerable numbers have also been found in the southern part of the State of Bahia, near the border of Minas Geraes. This district may be regarded as a continuation in a northeasterly direction through Grao Mogol of the diamond-fields of Diamantina. The stones are found near Salobro (signifying brackish) in the alluvial deposits of the Rio Pardo. This river and the diamantiferous river Jequetinhonha (Rio Belmonte) both empty themselves into the Atlantic Ocean at the foot of the Serra do Mar and near the small haven of Canavieiras. The mines are about two days' journey inland from this seaport town, and from it they derive their name of the Canavieiras mines.

The discovery of diamonds here was made in 1881 or 1882 by a forester who had previously searched for diamonds in other districts. Scarcely was the occurrence made known, when 3,000 or more diamond miners peopled the virgin forest, in spite of the unhealthy malarial climate. The treasure was obtained at a depth of two feet below white clay containing decomposing vegetable matter, so that the deposit was a very recent one. The diamantiferous stratum is much more clayey than any in Minas Geraes; it has throughout the character of a plateau-deposit. Diamonds are also found in the rivers Salobro and Salobrinho, tributaries of the Rio Pardo, especially in the "gupiarras" or valley-deposits lying above the present high-water level, just as in the valleys about Diamantina.

The minerals associated with the diamond in these clays are not only less in amount but also differ in kind to a certain extent from those found in Minas Geraes. Monazite in yellowish and reddish broken crystal fragments is present in abundance, also zircon, usually brownish to white in color, but sometimes violet, and in addition kyanite, staurolite, almandine, hematite, ilmenite, magnetite, iron-pyrites, and a somewhat considerable amount of corundum. The occurrence of corundum is remarkable, as hitherto it has been found in no other Brazilian deposit, while all the other minerals mentioned do occur in association with diamond in various parts of Brazil. We may contrast with the occurrence here of corundum the complete absence of certain minerals, which in other parts of Brazil are frequently found with diamond, namely, rutile, anatase, tourmaline, and the hydro-phosphates.

As regards the origin of the diamonds found here, it has been supposed that they are derived from the gneiss, granite, and other ancient crystalline rocks of the neighboring coast range, the Serra do Mar. In the diamantiferous deposit, however, there is no trace of feldspar or mica, two essential constituents of these rocks; moreover, the minerals chrysoberyl, andalusite, tourmaline, beryl, etc., which are frequently present in such rocks in Brazil, are also conspicuous by their absence from these deposits, so that this suggested origin for the diamond seems decidedly doubtful. For a satisfactory determination of the mother-rock of these diamonds, further investigation is required; in any case, it does not seem to be itacolumite, since this rock has not been observed in any part of the surrounding district.

Diamond Geology [ 1  India  3  4  5  6  7  8  Brazil  10  11  12  13  14  15  16  17  18  19  20  Borneo  22   South Africa  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  Venezuela, Guyana  42  Australia  44  Argyle  Congo  46  47  48  49  50  51  52  53  54  55  Angola  57  58  59  Guinea  ]

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011