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DIAMOND in SOUTH AFRICA

Dry Diggings (suite)

The garnet is of constant occurrence, and always in relatively considerable amounts. It is found in the form of rounded or angular grains, crystals, or even indications of crystalline form, being never observed. The grains are usually in a fresh and unaltered state, and therefore appear bright and transparent, some, however, are cloudy and opaque, and of a reddish-brown colour, in consequence of a process of decomposition having commenced. The colour of the unaltered garnets is variable, a deep wine- or hyacinth-red is most frequent, and red tinged with violet is less common, while light or dark brownish-yellow, and a beautiful ruby-red are colours which are seen but rarely. Garnets of this ruby-red colour are cut for gems and enter the market under the name of "cape rubies". In size the garnet grains vary from mere dust up to the size of a walnut. All specimens yet examined contain a little chromium, and have the chemical composition of pyrope, itself well known and much used in jewellery under the name Bohemian garnet.

The members of the pyroxene group most frequently met with are enstatite (and bronzite) and chrome-diopside. The enstatite has the usual composition, but not the usual appearance of this mineral. It occurs generally in fragments about the size of a hazelnut. It is transparent, with the colour of green bottle-glass, and has a distinct cleavage and a conchoidal fracture. It closely resembles olivine in appearance, and is frequently confused with this mineral. It is often found intergrown with garnet in such a way that a shell of enstatite encloses single grains of garnet. This variety of enstatite is of more common occurrence than is the garnet. Another variety of enstatite (bronzite) also occurs, but more rarely. It is brown in colour and less unlike the ordinary mineral in appearance; moreover, it has a distinct plane of separation in one direction.

Chrome-diopside, sometimes referred to as chromiferous diallage, though less common than garnet, is yet very frequently met with. It occurs in irregular polyhedral grains of about the same size as the garnet grains and with no trace of crystal-faces. It is emerald-green in colour, translucent in mass, but transparent in thin splinters, and is usually distinctly cleavable in one direction. Wollastonite, another mineral of the pyroxene group, is also said to occur in the "blue ground".

The amphibole group of minerals is represented by the green smaragdite, which, however, is of rare occurrence. It has possibly been derived by the alteration of chrome-diopside. The occurrence of tremolite and asbestos has also been reported.

Altered magnesium mica occurs everywhere in small shining scales of a greenish or brownish colour or completely bleached. These thin plates or prisms frequently have a regular six-sided outline, and show the characteristic cleavage of mica; optically they are almost uniaxial. This altered mica, which is distinguished as vaalite, sometimes occurs aggregated into brown balls the size of a hen's egg, and in some places forms the chief constituent of the "blue ground". The glittering scales of mica embedded in the "blue ground" are sometimes mistaken at first sight by the unpractised eye for diamonds.

Ilmenite (titanium iron ore) is another mineral frequently associated here with the diamond. It occurs in shining black rounded grains with no indication of crystal-faces. It contains some magnesia and is not magnetic. Formerly the diamond miners imagined this mineral to be the black variety of diamond known as carbonado, and at present found almost exclusively in Brazil. They were not easily convinced of their error, and the name they gave it, carbonado, still remains. True carbonado occurs only very sparingly at the Cape. Magnetite (magnetic iron ore) in grains and of the usual character is said to be of frequent occurrence. Chromite (chromic iron ore), found in brilliant shining black grains up to the size of a pea, and with a conchoidal fracture, is also fairly common. Zircon, known to the Kimberley miners as "dutch bort" occurs very rarely in transparent to translucent grains of a very pale flesh colour, and about the size of a lentil or pea. Other minerals associated with diamond in the "blue ground" are pyrite, sapphire, kyanite, topaz, and on very rare occasions colourless olivine. Apatite has been detected by chemical tests, and gold was once found enclosed in eclogite at the Jagersfontein mine. Under the microscope, graphite, tourmaline, rutile, and perofskite, among other minerals, have been detected. The common mineral quartz, on the contrary, has never yet been observed.

The majority of the minerals mentioned above occur in all the mines, but some are confined to particular pipes. Thus gold has been met with only in the Jagersfontein mine, and up to the present the occurrence of sapphire also is confined to the same mine.

The parti-coloured residue left by washing the "blue ground" after sorting out the large rock fragments consists largely of grains of red garnet and zircon, the green minerals of the pyroxene and amphibole groups, and black ilmenite and magnetite mixed with small fragments of diabase. The absence from this residue of the other minerals mentioned above is explained either by their rarity or by their having been lost in the washing process. Diamonds are of course present, and are picked out by hand.

All the minerals mentioned above are original constituents of the "blue ground," and were already formed at the time it first filled the pipes. There are others, however, which are of secondary formation, owing their origin to the weathering processes undergone by the upper layers of the "blue ground". Such a secondary mineral is calcite, a not unimportant constituent of the rock-mass, and occurring also in veins and crevices and as crystals encrusting the walls of cavities in the rock. Other secondary minerals are zeolites, especially mesolite and natrolite, sometimes found in beautiful groups of acicular crystals; also in places rough fragments of a bluish hornstone. Barytes, which is of rare occurrence, is also probably a later-formed mineral. All these minerals of secondary origin, but particularly the zeolites, are found most abundantly in the upper part of the pipes, which is more exposed to the action of atmospheric agencies. At successively lower levels they diminish in amount and finally disappear.

Stanislas Meunier has described a total of eighty different species from the "blue ground," but the existence of some of these as distinct mineral species probably requires confirmation.

One other rock found in the "blue ground" of De Beer's mine remains still to be mentioned, but is of no great importance. It penetrates the "blue ground" as a dyke five to seven feet thick, and on account of its tortuous path is known locally as "the snake". It is a compact greenish-black rock of much the same composition and consisting of essentially the same minerals as the "blue ground", but it contains no diamonds.

The manner in which the pipes have been filled with the material we have been considering has been explained in many and various ways. The first investigator to formulate a theory in accordance with all the observed facts was Emil Cohen, and the views he propounded in 1873 have never been seriously challenged by any one of the numbers of observers who have followed him in this field of inquiry.

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