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FIFTH INTERNATIONAL. KIMBERLITE. CONFERENCE. Araxa, Brazil 1991. Volume 2. Diamonds: Characterization, Genesis and. Exploration. Henry O.A.Meyer.
Proceedings of the FIFTH INTERNATIONAL KIMBERLITE CONFERENCE

Araxa, Brazil 1991

Volume 2 Diamonds: Characterization, Genesis and Exploration Editors

Henry O. A. Meyer and Othon H. Leonardos

Published by Companhia de Pesquisa de Reeursos Minerais - CPRM Av. Pasteur, 404 Urea - CEP 22290-240 - Rio de Janeiro, RJ - Brasil CPRM - Special Publication lIB Jan/94 Brasilia

I?\\ CNPq @{ Conselho

Nacional de Desenvolvimento Cientifico e Tecno16 gico

136 Proceedings of the Fifth International Kimberlite Conference

CARBONADO

AND YAKUTITE: PROPERTIES

AND POSSIBLE

GENESIS

By Felix V. Kaminsky

ABSTRACT Two types of carbonado are currently known: common carbonado of Brazilian type, and yakutite. Both have a nonkirnberlitic origin; they differ in morphology, structure and composition. Yakutite was first encountered in the 1960's in placers of Northern Yakutia. It is represented by grains of flattened hexagonal form, not more than 2-3 carat in mass. Its main peculiarity is the presence of the hexagonal carbon modification - lonsdaleite. Yakutite is characterised by heavier carbon isotopic composition compared with carbonado (-9 to -23 %0 against -21 to -31 %0 013CPDB) and very low paramagnetic nitrogen content (less than 1015 crrr"). Primary occurrences of yakutite are recorded in impact metamorphosed rocks, and also in stone and iron meteorites. It was formed as a psuedomorph after graphite during extremely high momentary (explosive) loading. To explain the genesis of carbonado a hypothesis of diamond formation without high pressure is proposed.

INTRODUCTION Carbonado is one of the most interesting, but rare, diamond varieties, representing a cryptocrystalline aggregate, which, in the opinion of some scientists, is worth classifying as an independent subspecies (Orlov, 1984). In spite of its modest appearance, it has excellent abrasive properties. That is why, in synthetic diamond production a great deal of attention is paid to production of synthetic carbonado (although it is not truly analogous with the natural species). During the last decades, scientists in different countries have studied properties of carbonado, such as texture, impurities, isotopic composition, and mineral associations. The main objective of this work is to consider the classification of carbonado and to understand its genesis. Two types of carbonado are currently known, which are essentially different in composition, properties, the mode

of formation: common carbonado of Brazilian type, and yakutite, a new variety. Carbonado was first diagnosed as diamond in the 1840's in the Sincor6 county in Brazil, while in the 185060's production started from placers in the States of Bahia, Parana, and Minas Gerais, sometimes accouting for 60-70% of all the diamonds in Brazil (Stutzer, 1931). At present, carbonado is known not only in Brazil, but also in Venezuela (Gran Sabana region), in East Australia, in the Ubangi region of the Central African Republic, and in Zaire. In Russia grains of typical carbonado were first encountered in the gold placers of the Far East (Karninsky et al., 1978)_ Carbonado has been found only in placers; never in primary deposits of kimberlite, nor lamproite. Yakutites were earlier known as "hexagonal diamonds" (Bundy and Kaspar, 1967; Hanneman et aI., 1967), "carbonado-like diamond" (Bartoshinsky et al., 1980), "lonsdaleite-bearing polycrystalline diamond" (Rumyantsev et al., 1980), "carbonado with lonsdaleite" (Orlov and Kaminsky, 1981). In 1966, in alluvial deposits of the Northern yakutia, shapeless, grains frequently of dark-brown to steel-gray colour, resembling slag, were discovered. In most cases an alteration of fine zones of various shades was observed. After X-ray diagnosis of these grains they were identified as diamonds of the carbonado type. These were called "yakutite", taking into account their specific nature (Chumak and Bartoshinsky, 1968). A detailed mineralogical study of these diamonds established, that they incorporate the hexagonal carbon modification - lonsdaleite (Krajnyuk and Bartoshinsky, 1971; Karninsky et al., 1978, 1985a; Klyuev et al., 1978). Similar polycrystalline diamond aggregates have been encountered in alluvial deposits in the Ukraine (polkanov et al., 1973, 1978). Primary sources of these grains have been suggested to be meteorites (Hanneman et al., 1967) and impact metamorphosed terrestrial rocks resulting from meteorite impact. For physical and chemical studies see Galimov et al. (1980), Rumyantsev et

Central Research Institute of Geological Prospecting for Base and Precious Metals (TsNIGRI), 129-b Warshavskoye Shosse, Moscow 113545, Russia

Diamonds: Characterization, Genesis and Exploration 137

al. (1980). The diamonds are psuedomorphs after graphite formed as a result of extremely high momentary explosive loads (Shafranovsky, 1985). The term "yakutite" as first used (in a newspaper article) by Chuman and Bartoshinsky (1968) is considered entirely appropriate for diamond hayiJilg.well defined properties first described from meteorites 'and placers of Yakutia,

PROPERTIES OF CARBONADO AND YAKUTITE Carbonado and yakutite differ markedly in texture and properties. Their main characteristics are shown :inTable 1. Carbonado has an irregular grain shape but most frequently isometric. Samples are both porous and consolidated, flint-like. In contrast, yakutites are of flattened hexagonal shape with rounded edgess. Irregular fragments

Table 1. Comparative characteristics of carbonado and

yakutite grains. Grain properties

Carbonado

Yakutite

Shape

irregular, isometric

flattened-hexagonal

Mass, carat

1-40 (up to 3167)

0,OHl.2 (up to 2.2)

Crystallite size,

0,5-80

0,1-1

(usually 10-40)

microns

Structure

absent

present

Non-diamond

absent

lonsdaleite

(up to

50%), chaoite also

carbon phases

could be present Photoluminescence spectra

Paramagnetic

N3, H3, H4, Tl

wide textureless band,

systems

580-610nm

4x1018 - 3x1019

absent (less than 1015)

nitrogen impurity

Other paramagne-

55x1017

_ 4,5x1018

2xl018 - L2xlO19

tic centers, cm-I

Carbon isotope

-23,2 to -30.6

-9,9 to -20,1

T) = -

(V di - Vgr)dP;

(2)

o where:

P - pressure, Pe - equilibrium pressure, V - atomic volume. As chemical potentials are a function of temperature and pressure, then assuming:

a P Vgr(P,T 0) = Vgr(O,To) x [1 - _1_ 1+a2P

];

(3)

where: Vgr(O,~~ at T.=298 K equals 5.31 cm3Jmole; al = 1.5xlO MPa, a2 = 11.3xlO-4 Mpa; diamond compressibility 1.6xlO-6 MPa; we obtain the dependence of equilibrium between graphite and diamond in the temperature-pressure coordinates: Po = 600 + 2.7 T, K (in MPa). . The most complete phase diagram (Bundy, 1964), shows the graphite-diamond equilibrium curve within the interval of 4-4.5 GPa. These data are widely used when producing synthetic diamond crystals. During the last decade, research on fine (up to tens of nm) diamond films has established that the surface energy of the smallest newly formed crystallites is an independent thermodynamic potential, which under certain conditions has an opposite direction in relation to the chemical potential. Due to this fact, the parameters of phase

Diamonds: Characterization, Genesis and Exploration 141

transmons for small crystalline particles depend on their size. It was called "phase size effect" (Komnick, 1979). Taking this effect into account, Tauson and Abramovich (1986) concluded that crystallization of micro-octahedric diamonds from elongated graphite prisms may be possible under pressure 3.0-3.5 gPa. If crystallographic parameters of graphite and diamond are also considered, the value of this dislocation could be more considerable (Chaikovsky and Rosenberg, 1984). According to calculations, the limit of the diamond stability area should be described by a certain surface in the space with "pressure-temperature-crystallite size (r.F)" coordinates. This diagram constructed with consideration of reference data concerning graphite and diamond properties within the temperature range 0 to 3000 K, is represented in Fig. 4.

DIAMOND

r, nm

~~~~:q=.

p. kb /

100 f-

50

o

1000

2000

3000

T. OK

Figure 4. Graphite-diamond phase diagram in the "pressure(p)-temperature(T)-crystallite size (r)" coordinates with the eccentricity r=2.5. After Chaikovsky and Rosenberg (1984).

When crystallite size is small (r ;? 100 nm), the graphite-diamond equilibrium diagram does not differ from the known one. However, as crystallite size is decreased (r ::; 10 nm), the surface of phase equilibrium notably declines to the domain of the low pressure, while at r=1 nm diamond turns out to be stable even in the absence of external pressure within the temperature range up to 2000 K.

Geometric parameters of crystallites - "r" and 'T" should satisfy the condition: ~d(r/a+b) I"> 2r[cl - ~f.!v(r/a+b)],

(4)

where: c - energy of o-link and 1t-link in graphite and diamond as calculated for one atom; cr cr cl=£gr - £di; cr 1t Ez=acdi - 2eg~ d - distance between (001) planes in graphite; a - diameter of circle, inscribed into an elementary hexagon, equal to 2.455A for graphite and 2.518A for diamond; r - effective radius of cylinder in the graphite lattice; r - eccentricity, equal to nd/2r From the expression (4) the crystallite size range, where the presence of crystallites with diamond structure could be expected at P=O, is f=0.3-1.5 nm. Some examples are known of the occurrence of diamond matter in extraordinary conditions. In our opinion, the above mentioned studies could explain these odd occurrences. Thus, when studying Mesozoic uranium-bearing carbonaceous rocks of the Colorado Plateau, Breger (1964) found diamond-like clusters therein. Breger explained their formation by the influence of radioactive radiation. Dubinchuck et al. (1976) carried out similar studies in the USSR. They found newly formed diamond particles up to lu km in size in a number of samples from uranium-bearing carbonaceous rocks (kerogen, kerite, brown coal, etc.). We don't exclude the fact that the carbon ado formation could occur under low pressure, while the required energy sources for graphite (or other carbon-bearing matter) transformation into diamond could be different and not related to pressure. In particular, it does not seem odd that diamond particles were found in carbonaceous rocks near radioactive sources. Acknowledgements I gratefully acknowledge the kind, yet critical reviews of my manuscript by Dr. Melissa Kirkley and two anonymous reviewers. For their help and guidance and that of the Editors, I am most appreciative.

142 Proceedings of the Fifth International Kimberlite Conference

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Diamonds: Characterization, Genesis and Exploration 143

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