THE AMER TCAN MINERALOGIST, VOL. 56, JULY_AUGUST, 197 1

CARBON FROM UBANGI-A MICROSTRUCTURAL STUDY

LucrBu F. Tnunn, Metallurgy and Materials Science Diais,ionDenver Research Institute, Uniaersity of Denver,

Denaer, C olorad.o 802 10

AND

E. Cnnrs'rraAN DE Wus, Dept. of Geosciences, Texas Tech Llniaersity,Lubbock, Teras 79406

Ans:rn,q.ct

Carbon originating from Ubangi (Central African Itepublic) is a dark-colored variety ofpolycrystalline diamond resembling Brazilian carbonado. It consists of a porous matrix ofrandomly oriented, mainly anhedral crystallites ranging from 0.5 to 20 pm which oftenencloses well-formed octahedral and cubic diamonds rvith an edge length of up to 500 pm.The pores contain inclusions of florencite and small amounts of other minerals which areusually associated with diamond in alluvial deposits.

IN'rnooucuoN

Until the beginning of this century, the state of Bahia, Brazil wasthe sole commercial source of black diamond, the so-called carbonado.Because of its polycrystall ine habit, this material is extremely toughand is sti l l much in demand for use in dri l l bits designed to pierce thehardest types of rock. As the Brazil ian deposits were being played out atthe beginning of this century, the price of carbonado rose dramatically,but this development was completely reversed around 1925 when therich Central African deposits were put into production. Today, only afew thousand carats of dri l l ing grade carbonado are mined in Brazil eachyear, most of the demand for this type of_diamond being covered bycarbon from the Ubangi area in the Central African Republic.

Diamond dealers usually reserve the term carbonado for stones ofBrazil ian origin, while the African variety is designated as carbon.Because it appears to be well established, this somewhat unfortunatenomenclature wil l be used throughout this paper. It should thus be keptin mind that in the present instance the term carbon refers specificallyto the black polycrystall ine diamond from central Africa.

Diamond is found throughout the territory of the Central AfricianRepublic, but mining is concentrated in two areas: Berbezati, Carnot,Nola in the West (West Ubangi), and Ouadda, N'Dele in the North-East(East Ubangi), the two centers being some 600 km apart. In the East,the carbons are usually small and form only 6 to 7 percent of the totaldiamond production, while in the West they are quite large and theirproportion reaches up to 30 percent. In this latter area, large, white,monocrl 'stall ine diamonds are relatively rarel this negative correlation

1252

CARBON TIROM UBANGI 1253

has been explained by the assumption that carbon was formed in higher

areas of its sti l l unknown matrix where rapid decompression and cooling

may occur, resulting in a high nucleation rate and the growth of many

small,. imperfect crystall i tes. Since the Ubangi diamond fields are strictly

sedimentary, carbon would be most abundant in the stratigraphicallylowest deposits formed during the early stages of erosion. Conversely,large moncrystalline diamond the growth of which needs a long time

and steady pressure-temperature conditions would be found in the

lower parts of the matrix which are the last to erode and thus deposited

in the youngest layers of the sediment. (Bardet, 1959, private communi-

cation). It must be noted, however, that the central African diamond

fields consist of cretacious alluvial sandstones and conglomerates; de-

spite a great amount of prospecting and geological surveying' no kim-

berlite pipes nor any kimerlitic mineral have ever been found in Central

Africa.Aside from the newly-discovered fields in the USSR (Central Ural

and Yakutiya), black diamond is found exclusively in Brazil and CentralAfrica, which has led to the speculation that these latter deposits may

have a common origin. In fact, the two locations would have beenseparated by less than 1300 km before the assumed break-up of Pangea.

However, despite some microstructural similarit ies between carbonado

and carbon to be described below, this theory cannot be unequivocally

corroborated by the present investigation.Carbon is found in irregularly-shaped lumps with an average diameter

of 8 to 12 mm (approximately 20 carats), although much larger stones

up to several hundred carat are by no means rare. The largest known

Central African stone weighs 740.25 carats and is exhibited at the

Smithsonian Institution in Washington D.C.To our knowledge, most published microstructural work concerning

black diamond was performed on Brazil ian samples; reviews of thisliterature have been recently published (Trueb and Butterman, 1969;

Trueb and de Wys, 1969). Aside from a report in book-form which deals

mainly with the geography and geology of Ubangi diamond fields(Bruet, 1952), carbon from Ubangi has received very l itt le attention sofar, despite its easy availability and the mystery surrounding its origin.These reasons prompted us to undertake the present investigation.

ExpBnruuNrAL RESULTS axn DrscussroN

All the results reported here were obtained on a series of ten carbons

from Ubangi weighing between 4.0 and 5.4 carat; they were chosenrandomly from a 20,000 carat batch. It could not be ascertained whetherthe samples originated from.East or West Ubangi.

1254 LUCIEN F. TRUEB AND ]', CIIRISTIAAN DE WYS

Frc. 1. Two samples of carbon from Ubangi, as received.Background subdivisions: 1 mm.

The samples were uniformly dark-gray to black and were either dullor lustrous, depending on the eimount of polishing they received aftererosion and subsequent transport by the drainage svstem. Also, someof the samples were obviously fragments of larger stones, while otherswere in the form of bean-shaped, well-rounded pebbles. The surfaceswere usually pitted and contained variable numbers of irregularly-shaped pores with a diameter of 100 to 300 pm, often fi l led with reddish-brown to yellowish-white impurit ies. Figure 1 shows the surface struc-ture of two representative specimens.

The densities as determined by a precision pycnometric methodvaried from 3.176 to 3.301, the average value being 3.292 g-cm-3. Sincethe density or pure white diamond is 3.511 g. cD-3, the average porositvof the samples was 6.2 percent.

Inclusions. The distribution of the inclusions was determined by contactX-ray radiography. For this purpose, the carbons were tied to a pieceof KODAK high resolution plate by means of "Saran Wrap" and ex-posed for a few seconds to nickel-fi l tered copper radiation. A modifiedDebye-Scherrer camera operated without coll imators and providedwith a pinhole aperture at the incident beam porthole was used. Photo-graphic enlargements were prepared directly from the resulting expo-sures and two examples are given in Figure 2. Sample A in Fig. 2 con-tained a reiatively large amount of inclusions or aggregates thereof, thesize of which varied from 40 to 500 prm. Sample B contained considerably

CARBON FROM UBANGI

Frc. 2. X-ray contact radiographs of the two carbons shown in Fig. 2.(\,Iagnification 15le higher). The black spots are inclusions.

fewer highly absorbing inclusions and their average diameter was ap-proximately 60 pm. In the latter instance several areas with a preferredorientation of the inclusions could be observed, which may be due to amore or less parallel alignment of the pores; this effect has already beennoted in carbonado, where it occurs to a considerable extent (Trueb andBrrtterman, 1969).

Fracture SurJaces. In order to investigate the microstructural features ofthe bulk of the Specimens, fresh fracture surfaces were produced bycarefully crushing the carbons in a large vise. Considerable force wasrequired to that effect, and the jaws of the vise were usually dentedextensively before fracture occurred with a loud report. The light-grayfractured interfaces were then examined by light-optical, scanning andtransmission electron microscopy.

Figure 3 shows the appearance of a fracture interface of one of themore porous specimens (p:3.267 B.cm-r; in the l ight microscope at lowmagnification. The individual crystallites cannot be resolved very clearlyin this instance since the irregularly fractured interfaces emit a greatmany specular reflections. Ifowever, the presence of a considerablenumber of usually elongated pores is clearly evident. Furthermore, closeexamination of the pores showed that they contained inclusions whichoften filled them out completely, which is difficult to recognize in blackand white photographs.

B

1256 LUCIEN F, TRUEB AND E. CHRISTIAAN DE WYS

Figure 4 is a scanning electron micrograph showing the fracture inter-face of a relativelv pore-free specimen (p: 3.335 g. cm-3). At thismagnification, the diamond crystallites appear to be mainly anhedral,and their size ranges from 2 to 40 pm, the main fraction having a diam-eter of 5 to 10 pm.

Figure 5 shows rather large monocrystals distributed within thematrix of 10 pcm crystall i tes. This feature, while not being unusual, was

Irrc. 3. Fracture interface of Ubangi carbon showing eiongatedpores and inlergranular inclusions.

observed only in part of the carbon samples. In this instance the edgeIength was of the order of 250 pm. This crystal was fractured duringcrushing and only its cross-section can be seen in Figure 5; it exhibitsthe characteristic pattern of steps which appears whenever diamondbreaks under the effect of stresses that are not parallel to a cleavageplane. At the lower right in Figure 5, a smaller, well-preserved octa-hedron with an edge length of 100 pm (arrow) can be seen with two

{111} planes emerging from the polycrystall ine matrix. These relativelylarge crystals usually appeared in clusters and their surfaces were oftenpeeled out of the surrounding material during crushing, which indicatesrelatively poor adhesion to the matrix. Carbons with inclusions of largeoctahedral and sometimes cubic diamond crystals were thus presumably

roo-_. F

CARBON FROM UBANGI

Frc.4. Scanning electron micrograph of fracture intBrface.

Frc. 5. Scanning electron micrograph showing cross-section of large diamond crystal

(center) and smaller octahedron (lower right, arrow) embedded in the polycrystalline

matrix.

1258 LUCIL,N F. TRL:IIB AND It. CIIRISTIAAN DIt WYS

{ormed in two stages: a slow nucleation and growth process leading tosmall but near-perfect crystals, followed by the formation of a matrixof mainly anhedral, small and disordered crystall i tes which appearedunder conditions favoring a high nucleation rate, most l ikelv a rapidrelease of pressure due to a seismic or volcanic process.

In order to gain more detailed information about the size, shape andfracture mode of the diamond crystall i tes in carbon, freshly fracturedinterfaces were replicated by means of a two-stage cellulose acetate,/carbon rrethod with platinum palladium pre-shadowing. These replicaswere examined by electron microscopl-;Figures 6 and 7 are representativemicrographs depicting the substructure of the polycrvstall ine matrix.Figure 6 shows densely packed and interlocked crvstall i tes the size oIwhich varies from less than I to 20 pm. While the larger crvstall i tesappeared to be mainiv anhedral, octahedra and deformed octahedrawere often observed within the 1 to 4 trrm range. Both the intergranularand transgranular fracture mode were apparent, the latter usuallf in-volving {111} cleavage which leads either to verl 'smooth surfaces or

Frc. 6. Electron micrograph of fracture interface of Ubangi carbon, showing bothintergranular and transgranular fracture. (Replica)

CARBON FROM UBANGI

Frc. 7. Electron micrograph of fracture interface; the individual crystai)ites

are outlined by intergranular cracks or voids. (Replica)

patterns of steps with alternating directions, depending on the orienta-

tion of the crystallites with respect to the stress applied when breaking

the specimen.Intergranular fracture exposes the true crystallite boundaries which

invariably exhibit a mottled pattern (Fig.6, top right). Close examina-

tion revealed that this mottling consisted of large numbers of minute

octahedra ranging from 200 to 1500 A and arranged in preferred orienta-

tion on each crystal facet. Analysis of the shadow direction on the

replica showed that all these octahedra were depressions on the outer

surface of the crystallites. They may thus be negative crystals formed

under the effect of gases or fluids (e.g., carbon dioxide which is often

found as an inclusion in monocrystalline diamond) entrapped under high

pressure between two adjacent boundaries.Similar features can also be observed in Figure 7 in which a majority

of the crystall i tes are outl ined by relatively broad black ribbons. This is

an artifact due to the intrusion of replicating plastic between crystallites,

but indicates the presence of interstices or cracks at the grain boundaries'

1259

L(ICIliN F TRU]'B AND L. CIIRISTIAAN DE WVS

l-tc. 8. liracture interface of 250 pm diamond crystal embeddedin the polycrystalline matrix. (Replica)

Most of the cleaved surfaces were practically featureless, the main im-perfections being series of lens-shaped voids with a length of 0.2 to 0.3pm and a width of 200 to 600 A appearing in parallel ur*y. and runningin one or two definite directions. (Fig. 7, upper lef t).

A somewhat unusual fracture pattern of one of the large diamondcrystals embedded in the polycrystall ine matrix is shown in Figure 8. Inthis instance the stress applied to fracture the stone must have beenoriented rather exactly in the [100] direction, which resulted in localized.symmetrical cleavage on {111} planes forming a pattern of similarlyoriented octahedra extending over the entire fracture surface.

Although the crystall i tes themselves apparently contain small num-bers of more or Iess spheroidal inclusions, the bulk of the inclusions werefound either within pores or along grain boundaries where they cause apartial coating of the diamond crystall i tes. Figure 9 shows that theinclusions apparently seeped into the sample through intergranulargaps, forming irregular patterns of more or less aggregated 0.1 to 0.5prm diameter hexagonal platelets. No satisfactory extraction replicaswere obtained of such surfaces, so that the identif ication of the platelets

CARBON FROM UBANGI

Fig. 9. Fracture interface of Ubangi carbon showing small pore fiiled with foreign phase(arrow) and platelet-shaped inclusions which seeped into the intergranular voids. (Replica)

by selected area electron diffraction was not possible. However, theirgeometry suggests that they consist of f lorencite, the most abundantinclusion in the carbons as reported below.

Defects Wi,thin Diamond, Crystallites. In order to investigate the defectsand possible microinclusions within the diamond crystall i tes themselves,the smallest fragments obtained by crushing carbons were dispersed onspecimen support grids coated with a fi.lm of amorphous carbon andexamined by transmission electron microscopy. Figure 10 shows severalconcentric dislocation loops in what appears to be a Frank-Read source:a dislocation helix and two isolated dislocations bowing out from one ofthe edges of the crystall i te are also observed. The dislocation densitydetermined from a series of micrographs similar to Figure 10 by meansof a grid-intercept method (KEH, 1960) was 108 to 1010 cm-2, the samplesbeing ti l ted in the electron microscope for maximum dislocation con-trast. Although at least part of these defects were presumably producedduring crushing, the relatively high density of dislocations tends to indi-

t262 LUCII'N F TR(IEB AND ]'. CHRISTIAAN DD WYS

Flc. 10. Transmission electron micrograph of carbon fragment showingdislocations and dislocation loops (center).

cate a significant level of residual stress. This is not unexpected sincethe crystall i tes are believed to have grown under conditions well aboveequil ibrium, resulting in rapid nucleation. These observations werefurther corroborated by electron spin reasonance measurements ofpowdered samples which had been boiled for at least t hr in hydrofluoricacid to remove all inclusions. A singlet signal was obtained in the posi-tion characteristic for free electrons, and the concentration of free elec-trons was found to be approximately 2.6X1017, which compares with apublished value of 6.6X10tt for Brazil ian carbonado (Mears and Bow-man, 1966). Because of differences in the particle size distribution andthe resulting packing density in the specimen holder, these values areprone to significant errors; however, a direct qualitative comparisonconfi.rmed that the free electron density of Brazilian material is signifi-cantly higher than that of the Ubangi variety.

Other types of lattice defects which were occasionally observed infragments of carbon were dislocation loops resulting presumably fromthe collapse of vacancv discs, and elongated dislocation dipoles (Figures11 and 12) which are commonly found in monocrystall ine diamonds and

CARBON FROM ABANGI

Frc. 11. Transmission electron micrograph showingdislocation loops and helices.

are assumed to be caused by high-temperature motion of screw disloca-tion containing long jogs (Evans and Phaal, 1962). Although a numberof fragments in (100) orientation were carefully examined, the character-istic defect patterns indicating the presence of nitrogen platelets as de-scribed by Evans and Phaal (1962) could not be observed. From thispoint of view, the crystall i tes forming the carbons can be classified asType II, although no spectroscopic data are available to corroborate thispoint.

Figure 12 and many other fragments of carbon that were examined,showed evidence that they contained inclusions, mainly in the form ofhexagonal or rectangular particles. However, such evidence alone is notsufficient to decide whether a given suspected inclusion is truly situatedwithin a crystallite or simply happens to be a fragment adhering to oneof its surfaces. For this reason, the samples were tilted in the electronmicroscope unti l a set of extinction fringes appeared. Only in the in-stance of true inclusions wil l a break in the fringe pattern occur; particlesadhering to a surface have no effect on the fringes. By means of thismethod, it was determined that the great majority of suspected inclu-

1264 LUCIEN F. TRUIl,B AND I'. CITRISTIAAN DE WYS

r l

, '

Fro. 12. Transmission electron micrograph showing dislocaticndipoles and suspected inclusions.

sions did not pass the test, and only very few true incltrsions were ob-served. This scarcity confirms the result of the study by replicationreported above. The diameter of the inclusions observed bv transmissionelectron microscopy was usually below 500 A, which precluded theiridentif ication by selected area diffraction.

Amount and Composition of Impuriti,es. The above findings further con-firm previous evidence that the majority of the impurit ies found in car-bon is confined to pores, cracks and intergranular interstices. In order todetermine the total amount of these impurit ies, a series of fragments ofcarbon weighing 0.5 to 1 carat were heated in air at 1100'C for 2 to 3 hr.This caused total combusion of the diamond, the residue consisting ofsponge-like skeletal polycrystall ine structures ranging from yellowishto brown-gray, sti l l assuming the original shape of the carbon frag-ments, and small yellowish-white single crvstals with an edge lengthvarying between 10 and 30 pm. The weight of this ash was used to deter-mine the percentage value of inclusions which scattered between 1.7 and3.6 percent, the average being 2.5 percent.

CARBON FROM UBANGI

T A B L E 1

E l e n e n t a l C o m p o s i t i o n o f U b a n g i C a r b o n A s h a f t e r C o m b u s t i o n

a t 1 1 0 0 0 c

1265

E l e m e n t a l C o n s l i E u e n t s

( M a j o r E l e m e n t s U n d e r l i n e d , T r a c e s i nP a r e n t h e s e s )

P , S , C a , F e , L a , C e ( Z n , Y ,I ' h i t e s k e L e t o n

Y e 1 1 o w s k e l e t o n

L a r k - b . r o - t r r t s k e l e t o n

Y e l l o w i s h - w h i t ec r y s t a l l i t e s

e o f A s h ? a r t i c l e

A I , S i ,P r , N d )

a l q j p

B a ( 5 1 , S ,

A 1 , T i , F e ,

T i , C a , N i , L a , C e , N d ,

N i , C e , N d ( S i , C a , C r , L a

S i , P , C a , F e , L a , C e , P r ( A 1 , T i )

The elemental constituents of the ash particles were determined byelectron probe microanalysis. The results are l isted in Table 1 and areidentical to the elemental compositions obtained for inclusions that weredirectly extracted from the carbon specimens as described in the nextparagraph. Because of possible modifications of the minerals duringcombustion and microscopic evidence that the ash particles consistedof complex mixtures, only a cursory identif ication by powder X-raydiffraction was conducted in this instance. It showed that the mainconstituents of the skeletal fraction was florencite with variable amountsof magnetit ite, ruti le, i lmenite, akermanite, and gehlenite. The smallsingle crystals consisted of monazite and quartz.

In order to obtain a more detailed, localized and nondestructiveanalysis of the impurities in carbon, inclusions were extracted indi-vidually under a binocular microscope by means of a fine needle, bothfrom the outside of the stones and from freshly fractured interfaces. Inthis instance, the identification proceeded by combining electron probemicroanalysis with X-ray diffraction and in some instances electron dif-fraction and polarization microscopy. The results are l isted in Table 2and show that the predominating inclusion was the rare-earth phosphateflorencite which, besides cerium, contained easily detectable amounts ofIanthanum, praseodymium, neodymium, and samarium.

The presence of rare earths in carbon is of special interest since theyhave been detected in diamond-bearins alluvial sediments in Brazii

F l o r e n c i t e

H i s i n g e r i t e

f l n e n i t e

K a o l i n i t e

P y r o p h y l l i r e

Q u a r t z

R u t i l e

1266 LUCIEN F. TRUDB AND IJ. CHRISTIAAN DE WYS

T A B L E 2

E x t e r n a l I n c l u s i o n s i n U b a n g i C a r b o n s( r e m o v e d f r o n o u t s i d e P o r e s )

M i n e r a l C o n p o s i t i o n

A k e r m a n i t e - g e h l e o i t e C a r A l . r S i O r - C a Z M g S i Z O . .

a l u n i n i a n s e r p e n t i n e 5 M g O ' A 1 2 O 3 ' 3 S ] . O 2 ' 4 H 2 0

( C e , L a , P r , N d , S m ) A 1 , ( P o 4 ) 2 ( O H )

6

( r J i r r * r o . + a " " : * . r ) ( s i r . o r r . J + z A l o . z ) o r oF e O ' T i O

2

A 1 2 S i 2 O 5 ( O H ) 2

A 1 2 O 3 ' 4 S i O 2 . H 2 O

s i o 2

T i o 2

W e i n s c h e n k i t e ( c h u r c h i E e ) ( Y , E r , L a , N d , C e ) r O a ' P

Z O 5 ' 4 1 1

Z O

I n t e r n a l I n c l u s i o n s i n U b a D g i C a r b o n s( r e n o v e d f r o m f r e s h l y f r a c t u r e d i n t e r f a c e s )

I

M i n e r a l C o m p o s i t i o n

A l u n i n i a n s e r p e n t i n e 5 M e O ' A l r O r ' 3 S i O Z ' 4 H Z O

H i g h - A 1 c h l o r i t e ( a n e s i t e ? ) 4 M g O ' 2 A L 2 0 3 ' 2 S i O Z ' 4 H Z O

F e 0 ' C r r O ,

( C e , L a , P r , N d , S n ) A 1 3

( P o 4 ) 2

( o E ) 6

F e 0 ' T i 0 2

F e - 0 ,

C a o ' T i o 2

s i o 2

( F e r M g ) r S i 0 O

T i o 2

(the so-called favas), the origin of which has not yet been determined.It is worth noticing in this connection that rare earths also occur inkimberlites; more particularly, it has been found that the kimberlitesin Yakutil'a are typified by an unusually high cerium content by com-

c h r o m i t e

F l o r e n c i t e

I l m e n i t e

M a g n e t i t e

P e r o v s k i t e

Q u a r t z

0 l i v i n e

R u t i l e

CARBON FROM UBANCI

parison with the l ithosphere in general and many magmatic rocks(Bardet 1969). This point is thus confirmed by the present investigation.AIso, the yellowish gravel (so-called feoes) cited by Bruet (1952) astypically associated with central African diamond and assumed by theabove author to be a phosphate is very l ikely to be florencite and thusclosely related to the Brazilian favas. The presence of florencite but theabsence of allanite found in Brazil ian carbonado (Trueb and de Wys,1969) deserves some attention. Allanite is a typical accessory mineral indeep-seated igneous rocks where the temperature conditions have beensuch as to allow a stable epidote type structure. The occurrence offlorencite which is closely related to the alunite group, tends to indicatean association with an acid type volcanic rock subjected to sulfuricacid solution or vapors. Such conditions are typically those of the highpressures and high temperatures which are characteristic of fumaroles.The occurrence of Weinschenkite which is often associated with gypsumin volcanic regions, is indicative of the replacement of calcium by rareearths in a magmatic residual.

Perovskite has not been previously detected as an inclusion in mono-crystall ine diamond. According to Bardet (1970) cit ing the theories ofMilashev, it is considered as an inhibitor for diamond formation inkimberlites. Whether this point is also related to the scarcity of goocmonocrystall ine diamonds in deposits rich in carbon remains an openquestion.

The occurrence of olivine, i lmenite, ruti le, magnetite, chromite, andthe melli l i tes indicates a basic hypo to mesothermal type of intrusive.Pyrophil l i te is typical of schistose rocks and is also found in the Minas

Geraes area of Brazil in foliated masses of great extent. Finally, theoccurrence of kaolinite and hisingerite is probably indicative of secon-dary alteration. It is worth noticing that ruti le, magnetite, quartz, andilmenite are cited by Bruet (1952) as being among the typical satell i tesof diamond, i.e. these minerals are invariably associated with diamondin sedimentary deposits formed by multiple-stage erosion and concen-tration of heavy components.

The presence of graphite as an impurity in carbon in amounts closeto the detectability limit of X-ray diffraction is suspected, but couldnot be confirmed unambiguously due to the overlap of all strong graphiteIines by reflections caused by some of the minerals listed in Table II.

The particular combination of minerals found in carbons tends to indi-cate a basic high temperature, high pressure intrusive into a more acidicmetamorphosed host. There is I itt le doubt that the diamonds were asso-ciated for at least part of their history with some type of volcanic activ-ity. The fact that no volcanic pipes have been found so far in Ubangi

t267

1268 LUCIIiN F. TRUI'B AND I', CIIRISTIAAN DE WYS

could readily be due to the presence of deep layers of sediments of thelarge lake which covered the entire Ubangi area during the tertiarl-.Furthermore, the laterit ic coverage in central Africa is a considerableobstacle to geological prospection of this area.

Acxnowr,nocnurltts

This investigation was suggested by M. G. Bardet, BRGM, Orleans, Irrance, and wouldnot have been possible without the kind collaboration of Diamond Distributors, Inc. (NewYork, N.Y.) who donated the specimens. Particular thanks are due B. Jolis and L. R.Kagan for helpful discussions. We also wish to gratefully acknowledge the assistance ofM. D. Coutts (RCA, Princeton, N.J.) for scanning electron microscopl', of G. S. Reddy(E. I. du Pont, Wilmington, Del.) for ESR measurements, and of A. L. Mangelsen (ClimaxMolybdenum Corp., Golden, Colo.) for part of the X ray diffraction work.

RBrBnrnpcns

Banntr, M. G. (1970) Chronique Mines Rech. Mini.ere (in press)Bnurr, E. (1952) Le Diamanl. Bibliotheque Scientifique Payot (Paris)Eva.Ns, T., exn R. Pn.ler, (1962) Imperfections in 1'ype I and 'Iype

II diamonds Prac.Roy. Soc. A, 27O, 538-552.

Krrr, A. S. (1960) Direct Obserztalion oJ Lattice DeJects in Crystals. Interscience Publishers,Inc., New York, 213.

Mn,rns, W. H., aNo R. F. Bowue,r, (1966) Indust. Di,omond. Assoc. Amer., Proc. Tech.Sess. March7,1966, Boco Raton, Florid.a.

Tnunr, L. F., .lxo W. C. Burrnnuen, (1969) Carbonado, a microstructural sttrdy. Am.er.MineraL. 54, 412-425.

aNo E. C. or Wvs (1969) Carbonado: Natural polycrystalline diamond. Science165,799-802.

Manuscripl, receit:eil, January 2 l, 197 1; acceptel .for publication February 1, 197 1.