Diamond Genisis and Chemical composition

 

Diamond Genesis

Even today it is not completely established how diamond is formed though it is known to be the product of the deep-seated crystallization of ultrabasic igneous magmas which have intruded as dikes or pipes of kimberlite or lamproite (Dana’s New Mineralogy, 1997). These primary deposits on erosion have concentrated diamond crystals in placers of different geo- logical ages. Conglomerates and different stream and wave deposits are prospected. Readers are recommended, in view of the dated nature of most monographic literature on diamond genesis, to check the latest journals in a national or university library: gem textbooks cannot accommodate this type of information in addition to all the other topics they need to cover.
For any study of diamond genesis the study of inclusions is essential. While Koivula and Gübelin, Photoatlas of Inclusions in Gemstones (cited pas- sim) is invaluable, Koivula has produced a study of inclusions in diamond alone, The Micro World of Diamonds (2000; ISBN 0964173352). Here all the solid inclusions are shown in colour where appropriate (there are no liquid inclusions in diamond) and there is a useful bibliography. Readers are advised to obtain this book.

 

Diamond Composition

Diamond, crystalline carbon, is polymorphous with graphite, chaoite and lonsdaleite. The hardness is 10 and the SG 3.515. The RI is 2.42 and the dis- persion 0.044. Crystals are commonly octahedral, also cubic, dodecahedral or tetrahedral. The unique hardness arises from the three-dimensional covalent linkage of each carbon atom to the four neighbouring ones. The

32/

, possibly bar 43

point group is 4/m–      m                             – m (for an explanation of point
 
groups and related topics see International Tables for Crystallography: Brief Teaching Edition of Space Group Symmetry, second, revised edition, 1988, published at Dordrecht for the International Union of Crystallography by D Reidel Publishing Company, ISBN 9027725780). A more compre- hensive survey for the International Union is Lima de Faria et al., 1990 (ISBN 079230649X). Here the history of the terms used in crystal descrip- tions is given and there is a unique and excellent bibliography.
Diamond crystals present special problems in fashioning. Diamond crys- tals unless very small are not available to collectors; some are well illus- trated from the standpoint of the polisher in two books written by Nizam Peters and published by the American Institute for Diamond Cutting, Rough Diamonds: internal and external features, 1998 (ISBN 0966585402). There are many studies of diamond cutting; one fairly recent one is Basil Watermeyer’s The Art of Diamond Cutting, 1994 (ISBN 0412984113).
Diamond crystals have an adamantine to greasy lustre and sometimes show curved and striated faces; some crystals are spherical with an internal radial structure. Contact twinning with {111} as the twin plane is common and crystals are often flattened on {111}. For explanations of these symbols, which are given in this text only for diamond, see any text of mineralogy, for example Dana’s New Mineralogy (1997) pages xviii–xxxii. The ISBN of this essential text for anyone needing to know about validated mineral species is 0471193100. Crystal faces are commonly marked with trigons whose outline does not echo the edges of the face but is reversed. Etch pits do follow the crystal edges.
Penetration twins, sometimes repeated, are also found. There is a perfect cleavage on {111} and a conchoidal fracture. Diamond shows fluorescence and phosphorescence and is triboelectric. It possesses the highest ther- mal conductivity of any known substance. Diamond is transparent to translucent and may be colourless, pale to deep yellow, brown, white and blue-white; less common colours reported are orange, pink, green, blue, red and black. Those diamonds with strong colours (not merely off-white rather than canary yellow) are known as fancies. Cell data: space group Fd3m. a 3.5595 Z 8. The X-ray powder patterns (given in this text only for diamond) are 2.06 (100), 1.261 (25), 1.0754 (16), 0.8182 (16), 0.8916 (8).
Diamond has an SG of 3.52 and varies little; the single RI is 2.42 and the dispersion (fire) 0.044. The ideal polished diamond is cut as a colourless round brilliant – if the ideal combination of brilliance (from the unique hardness) and dispersion is to be achieved. The flashes of colour from the small upper facets surrounding the table are irresistible and are best seen by distant single white spotlights – as in many cathedrals and muse- ums. Direct sunlight is less effective (less subtle) in producing the best dispersion. Whatever the desired effect from a diamond, the polisher has to pay attention to the facet angles – in a round brilliant, light must not leak from the base or the effects of dispersion will be lost. The flat (spread) brilliants with their large tables may impress (and give the impression of greater weight) but in comparison with the deeper old-cut diamonds with their smaller tables and steeper facet angles their disper- sion is low. Old-cut stones show their fire however small they are.
There are many books describing diamond polishing but I recom- mend, in particular, Herbert Tillander, Diamond Cuts in Historic Jewellery, 1381–1910, London, 1995; ISBN 1874044074. For the grading of polished colourless or near-colourless diamonds GemA have issued a useful text, Diamond Grading Manual (2000; no ISBN) and there are several editions of the commercially produced grading handbook by Verena Pagel-Thiesen (Diamond Grading, Antwerp, 1980). Gary A. Roskin, Photo Masters for Diamond Grading, 1994; ISBN 0964173301 will also be found useful.
A study of diamond crystals should begin with the early consultation of the English translation of Yuri L. Orlov, Mineralogiia almaza (The Mineralogy of the Diamond), New York, 1977; ISBN 0471018694. The original Russian edition was published in 1973 and one of its strengths is the presence of profuse illustrations of crystals and a very strong list of references. By the time of the English translation, Orlov had been able to add to his material.
Details of individual modes of occurrence are given below as are different responses to UV and X-rays.
The history of diamond mining is on the whole exhaustively written up, occurrences in Africa being especially well covered. South African dia- mond deposits and mining are described by Gardner F. Williams in The Diamond Mines of South Africa, second edition, New York 1905 [Sinkankas #7234]. Also on South African diamond is Alpheus Fuller Williams, The Genesis of the Diamond, 2 vols, London, 1932 [Sinkankas #7224]. Gardner Williams’ book has been criticized for shortcomings in some details of the mineralogy and for omitting descriptions of mines not at the time under De Beers control. Much of the text discusses personalities (the second edition had undergone some revision). Nonetheless the text gives details of the discovery of the South African diamond deposits upon which many subsequent writers have drawn.
On the other hand, Alpheus Williams’ book gives detailed studies of the petrology and mineralogy of the materials filling kimberlite pipes and should be consulted in the early stages of investigation into this topic.