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Occurrences of facetted re-entrants on rounded growth surfaces of natural diamonds
Journal of Crystal Growth 34(1976) 29—37 © North-holland Publishing Company
OCCURRENCES OF FACETTED RE-ENTRANTS ON ROUNDED GROWTH SURFACES OF NATURAL DIAMONDS S. SUZUKI and A.R. LANG H/I. Wills Phi’sics Laboratory, University of Bristol, Bristol BS8 I TL, England Received 24 October 1975; revised manuscript received 7 January 1976
Some unexpected growth-surface morphologies have been discovered as localised developments within the regions of non-facetted growth which are present (and can be dominant) in diamonds which have had epochs of mixed-habit growth during which crystallisation proceeded simultaneously both on normal octahedral faces and on non-crystallographic, hummocky surfaces having mean orientation {ioo}. These unexpected minor features are {ui }-facetted re-entrants, not involving twinning. They have been observed in two crystallographic settings. In one case pairs of {i ii facets develop to form re-entrant notches at the boundaries between adjacent sectors of non-facetted, mean- fi 00 }-orientation growth. In this case growth can propagate on the pair of re-entrant facets, forming a column of octahedral growth inserted within the surrounding matrix of non-facetted growth material. In the second case, {i ii }-sided pyramidal pits (size range from about 10 ~.smto 120 ~m) develop at points distributed over the area of non-facetted growth surface. There is evidence that they are initiated at particular growth horizons at which an episode of strong inhibition of growth on ~l 11 commences, and that they are preferentially located at dislocation outcrops. In this case no detectable propagation of octahedral growth upon the facets occurs: they are directly overlain by renewed non-facetted growth.
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1. Introduction
topography have also been fruitful. This report is concerned with local occurrences of {1 11 } facetted growth set within larger regions of crystal where the non-facetted mode of growth is dominant. Two varieties of this phenomenon will be described, differing in their geometrical settings. In each case planes of the form ~l 11 } combine to produce facetted re-entrants in the surface of the growing crystal. In one case the facetted re-entrant appears capable of advancing over distances of millimetre order (say 1/3 of the crystal radius) with little change in its state of development. Before the details are given, however, some leading characteristics of mixed-habit diamonds (as far as they are yet known) will be summarised.
Some strange crystal growth phenomena are being brought to light through studies of natural diamonds which have had epochs of mixed-habit growth. During these epochs the crystals were bounded in part by normal {i 11 } facets and in part by non-flat, generally hummocky surfaces whose mean orientation is {l00~. For the latter, non-crystallographic surfaces the descriptive name “cuboid” has been chosen, albeit locally the growth surfaces of cuboid growth sectors can be inclined up to about 300 off true {lOO} orientation [1,2]. It was from etch patterns produced on polished slices of diamonds cut parallel to cube planes [3,4] that Frank [5] deduced that facetted and non-facetted growth had proceeded contemporaneously. In subsequent studies of diamonds of this nature, X-ray topographic methods have been the investigative techniques of first choice of virtue of their non-destructiveness; but examinations of polished sections by birefringence, optical ultra-microscopy and cathodoluminescence
2. Some properties of mixed-habit diamonds
The most striking feature of these crystals is a population of microscopic or sub-microscopic particles distributed within their cuboid growth sectors. A few 29
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S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
cases have been encountered where the particles have been large enough for their individual birefringence
properties to be examined and a preferred orientation of the particles with respect to the diamond matrix to be detected [6]. The particles individually produce diffraction contrast images in X-ray topographs by straining the diamond matrix surrounding them: some cases have been found in which X-ray topographs revealed presence of particles whereas optical examination did not. Generally the particles make themselves evident by their scattering of light (and absorbing it, if present sufficiently profusely, and large individually). In cases where the crystal growth has been nicely symmetric (as in the specimen shown in fig. 5 of ref. [2]), a symmetrical “star” pattern can be seen inside the diamond provided its external surfaces are not too rounded or rough. There is very limited information on the chemical composition of the particles [7]. The relative development of normal {l 11] facets and of cuboid growth surfaces can vary widely from crystal to crystal, and within a given crystal during its growth. Whenever the fractional loss of crystal volume through post-growth dissolution has been sufficiently mild, it appears both from etch patterns on polished sections and from X-ray topographs that the final stages of growth were entirely ~111 }-facetted. On the other hand, accumulating X-ray topographic evidence is leading to the conclusion that when mixed-habit growth does occur, then the cuboid form is usually dominant in the early stages of growth. Dominant cuboid growth implies relatively large areas of growth sector boundaries between adjacent cuboid sectors. These boundaries sometimes follow {ll0} quite closely, and very often contain dense sheets of particles which are visible both by light-scattering under ultra-microscopic examination [I] as well as by X-ray diffraction contrast. Thereby they are readily traceable in the crystal. Octahedral growth sectors can be distinguished from cuboid growth sectors optically as well as by X-ray topography when the latter sectors contain light-scattering particles. However, even when such particles are not detectable (and they generally are absent in the outermost zones of cuboid growth), the growth sectors can be differentiated X-ray topographically because the integrated X-ray reflection from the diamond matrix in the octahedral growth sectors is generally stronger (often quite markedly so) than that from the particle-free regions of cuboid growth [1].
3. Facetted re-entrants at cuboid-cuboid growth sector boundaries Fig. 1 a is a surface reflection X-ray topograph of one face of a slice cut and polished parallel to (110), 0.59 mm thick, prepared from a diamond of mixed-habit growth which displayed a visible internal “star”. The slice contains the crystal centre, and this is nearer to the remote face than to the face which is imaged in fig. Ia. The specimen is notable for much stronger integrated X-ray reflection from its {l 11 } growth seetors than from its cuboid growth sectors: the former consequently appear considerably darker on the topograph prints. The (001) cuboid sector occupies the topmost part of the image, and the (DOT) cuboid sector lies below the centre. The images of the cuboid sectors show speckling due to their population of small partides which produce diffraction contrast, but the speck-
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(a)
S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces 0/diamonds
31
ling falls off quite abruptly when a distance of about 2 mm from the crystal centre is reached. The {l 11 } growth sectors which are intersected as expected are listed in the caption of fig. la To the left of the crystal centre, along the horizontal, the slice contains parts of the (100) and (010) cuboid growth sectors, the (100), (010) growth sector boundary lying largely (but not entirely) within the slice. Correspondingly, to the right
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of the crystal centre the (RIO) and (010) cuboid sectors are present, with the (100), (010) growth sector boundary contained within the specimen. Unexpected are the wedges of {l 11 } growth which lie centrally within both the left-hand and right-hand regions of cuboid growth. They are identified as being composed of {l 11 } facetted growth by their patent similarity in X-
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ray diffracting texture to the main ~l 11 } growth seetors, and by their geometry. On the left of the centre, extending outwards to the edge of the specimen along a radial distance of about 1 .25 mm, is a gently expanding column of octahedral growth on facets with indices (Ill) and (111), which facets together formed a Vee-shaped re-entrant in the growth surface. Close to the right-hand edge of the crystal slice, and also on the horizontal axis through the crystal centre, can be seen another wedge of {l 11 } growth. This involved the (Ill) and (Ill) facets, which together constituted a re-entrant the inverse of that which propagated itself in the left half of the slice. Fig. I b is an X-ray section topograph cutting through the slice not far from its left-hand edge. An important fact revealed by the sec-
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tion topogmaph is that the top and bottom apices of the parallelogram-shaped cross-section of material grown on the facetted re-entrant connect with the sheets of diffraction-contrast-producing particles which define the loci of growth sector boundaries between (100) and (010) cuboid growth sectors. Thus the ía(b) Fig.1. (a) Surface reflection X-ray topograph of diamond slice, 6.8 mm high, 4.7 mm wide, polished parallel to (110). Direction 10011 is vertical. A few residual polishing scratches parallel to 10011 produce dark vertical lines on the image. Darker areas are {i ii growth sectors: (111) upper right, (111) lower right, (111) upper left, (111) lower left. Close to centre of image, small outcrops of (111) facetted growth (above) and (111) facetted growth (below) appear. Near bottom apex, part of an outer shell of octahedral growth is intersected. Reflection 131, CuKo 1 radiation. (b) X-ray section topograph cutting vertically through the specimen shown in (a). The left-hand margin of the section topograph represents the inter-
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section with the face shown in (a), line of intersection mdicated by the solid marker lines in (a). The right-hand margin of the section topograph intersects the remote face of the specimen, line of intersection shown by interrupted marker lines. The section cuts through the main (111) growth sector (top), the main (111) growth sector (bottom), and through material grown on the re-entrant facet pair (ill) and (111) (left middle) - (010) cuboid growth is represented in the righthand margin of the Section at middle level; a smaller volume of (100) cuboid growth in the IefL half of the section, also at middle level. The reflection is 220, in symmetrical transmission, CuKcs1 radiation; the incident beam does not cut the specimen perpendicularly, but at 37.6°to its normal, 11101.
S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces u/diamonds
32
cetted re-entrant exists as an inset between these growth sectors. In fig. lb the section of (100), (010) growth sector boundary below the re-entrant runs vertically down the centre-line of the topograph, and it cannot lie far off(l 10) orientation. The short section visible running from the upper apex of the re-entrant crosssection upwards towards a surface outcrop makes about 300 with (110). The present external surface of the specimen is a rounded rhombic dodecahedron, the form to which octahedral diamonds progress as a consequence of dissolution after growth [8]. The specimen is somewhat elongated in the [001] direction. The epoch of mixed-habit growth was terminated (apparently abruptly) by one of entirely {l 11 }-facetted growth. Much of the enclosing octahedral shell has been lost by dissolution, however. In the slice shown in fig. la the only part of this enclosing facetted growth still retamed appears at and to the right of the lowest point of the image. Figs. la and Ib, together with information derived from other X-ray topographs, lead to an idealised recon-
Fig. 2. Idealized perspective drawing of dominant cuboid growth truncated by {i.ii facets and girdled by some {iii }-facetted re-entrants. The direction 10011 is vertical, [1001 points towards the observer and to the left, 10101 points to the right. A slice parallel to (110), containing the front left vertical edge and the rear right vertical edge, would represent the specimen of fig. 1~For simplicity, the internal edges of only one of the eight corner {i ii growth pyramids radiating from the crystal centre is drawn, i.e. that terminated by the (111) facet at the near top left corner. Similarly, the internal bounding edges of only one of the three facetted re-entrants are shown, i.e. that facetted by (111) and (111), corresponding to the facetted reentrant which develops on the left of the crystal centre in fig. 1.
struction (fig. 2) representing the stage in mixed-habit growth of the crystal when its diameter was about the same as the width of the slice in fig. Ia, and subsequent to its development of facetted re-entrants forming girdling notches. On the assumption that the specimen’s present elongation along [001] is an inheritance from the epoch of mixed-habit growth, fig. 2 is drawn simiharhy slightly elongated. The edges at which cuboid surfaces meet are shown as interrupted straight lines. In actuality they were not straight, but in what manner they were curved is largely unknown. Each of the eight octahedral facets which truncate the corners terininated a growth pyramid expanding from the crystal centre. To avoid confusing the drawing, only the internah edges of the (111) growth pyramid are sketched in, as faint interrupted straight lines: the X-ray topographs and the birefringence topographs (fig. 3) indicate how
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S. Suzuki, A.R. Lang / Facetted re-entants on rounded growth surfaces of diamonds
(b)
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(c)
Fig. 3. Birefningence of specimen shown in fig. 1 when viewed between crossed Polaroids. (a) Plane containing electric vector incident on crystal is vertical. (b) Plane of electric vector inclined at 45°to the vertical. (c) Key to (a) and (b) showing (in simplified fashion) traces of the principal fault surfaces detected in time coboid growth sectors, as derived from birefringence observations using the complete range of orientations of the electric vector. (The octahedral growths sectors are identified by vertical shading.)
great an ideahisation this straightness is. In fig. 2 only three out of the possible four facetted re-entrants lying in the median (001) plane are drawn (and none is shown in the corresponding (100) and (010) median planes). This is because only those actually drawn have been proven by X-ray topographic examination. There do exist however, some other quite small developments of {l 11 } facets at cuboid-cuboid growth sector boundaries; these will be referred to later. The birefringence topographs figs. 3a and 3b provide information both confirmatory and complementary to
that derived from the X-ray topographs. They show faint growth banding in the main {l 11 } growth seetons that is barely perceptible in the X-ray topographs.
In the cuboid sectors they show banding quite plainly in some regions where the X-ray topographs show it only faintly, and vice versa. The great sensitivity of the birefringence topographs to long range warping of the specimen results in the wide variations of extinction over the area of the slice. The actual extreme range of crystal misorientations was probably well under 1’ of arc. Birefringence observations reveal quite distinctly the curving fault surfaces which sub-divide the cuboid growth sectors into irregularly-shaped cells. However, it is difficult to photograph the whole pattern of fault surfaces in individual birefringence topographs because of great variation in extinction over the specimen area. The simplified sketch in fig. 3c, which is intended to
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S. Suzuki, AR. Lang / Eacetted re-entrants on rounded growth cur/aces of diamonds
serve as a key to figs. 3a and 3b. incot porates infoi nsalion accumulated From observations using all possible directions of the electric vector in the plane of the specimen - In order to simplify the drawing, all fault surfaces are shown by a single trace in fig. 3c. whether they be normal to the specimen plate or not. Only the most strongly-developed fault surfaces show up clearly in all of the figs. I a, 3a and 3b. Examples of such are seen about 1 mm above and 0.5 mum to time right of the centre of growth of the crystal, in the (001) cuboid growth sector, and also, in the (001 ) cuboid growth seebr, about 1 mm below the centre ot gi owtls and 0.5 inns to the right thereof. The major long-range strains in the specimen arise from coherency stresses at the boundaries between octahedral and cuboid growth, since these two matrices have a small (as yet undetermined) mis-match in mean hitflee parameter. When an oc tah edral—cuboid boundary changes direction sharply (i.e. ifit is stepped) the resultant localised deformations cause strong birefringence. Thus the saw-tooth nature of the boundary between the re-entrant facet composed of( 111) and (lii) growth and its enclosing cuboid matrix is well displayed in fig. 3
4. Facetted re-entrants within cuboid growth sectors The crystal shown in fig. 4, though clearly also a member of that tribe of diamonds which have had epochs of mixed-habit growth, exhibits many differences from the individual shown in fIgs. I and 3. Firstly to be remarked is that its outer shell of octahedral growth has been fairly well preserved: the existing crystal habit is an octahedron not very strongly modified by post-growth dissolution. Inside the octahedral shell, however, cuboid growth has been more strongly doniinant than ins the specimen previously discussed. (These geometrical circumstances, inter alia, determined the choice of (001) rather than (110) as the plane of sectioning, and the presentation of fig. 4 oriented with a cube direction diagonal rather than vertical as in fig. I a.) The innermost regions are remarkable for their complexly asymmetric disposition of growth sector boundaries. The particles which so intensely produce diffraction contrast include the largest of all those encountered to date in “star” diamonds (large enough for their individual birefringence properties to be studied [6]). However, it is certain features in the zone
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Ii. 4. Surface reflection X—nis topuL’raph 01 part oi one lace of a diamond slice cut parallel 10(001). The direction 11101 points horizontally to the right, and 11001 points towards the upper left corner. The field reproduced contains slightly more than one quadrant of the lace and is mainly occupied by (100) cuboid growth identified by the curvilinear growth bands runfling from lower left to upper right. Traces of octahedral planes run vertically (parallel to [110]) and horizontally (parallel to [110]). lield width 2 mm. Reflection 113, CuKoi radiation.
of cuboid growth outside the particle-containing zone that are of present interest. Noteworthy are the small Vee-shaped enhancements of diffraction contrast which. in the field of fig. 4, have their apices at (or very close to) points where dislocation images (running outwards from the crystal centre in the general direction [100]) intersect certain growth horizons well marked by diffraction contrast. From the geometry of the larger Vee-shaped figures in this and other fields it is clear that they are images of sections through small ~l 11 }facetted re-entrants. Upon these facets no volume of octahedral growth large enough for measurement by X-ray topographic means has crystallised: recommenced non-facetted growth directly overlies them. They are hard to detect unless they intersect the surface of the slice, for only then can stress relaxation produce the lattice curvature that generates easily visible diftrac-
S. Suzuki, A.R. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
tion contrast. In the facetted re-entrants contained in
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(Il 1), (TlT), (ill) and (iii) would be expected to have equal development in an average taken over all re-entrants, the general form of the latter being foursided pyramidal pits. [On a (001) section, as in fig. 4, it is not possible to distinguish between small outcrops of (Ill) and (Ill), both of which have horizontal traces, nor between small outcrops of (111) and (111) which have vertical traces.] In the case of such relatively slight distortions as accompany the re-entrants, only a surface layer of crystal not deeper than about 10pm contributes to the image of a re-entrant on a surface reflection topograph. Thus with facetted reentrants having facet edge lengths in the range from about 10 pm to 120 pm (as observed) only a minority will have their apices imaged in the topographs. However, it is these images which are most significant, for with them it is generally observed that the apex coincides with a dislocation (or small bundle of dislocations), and that re-entrants start developing at particular growth horizons which are also those at which an expansion of {1 11 } facets commences in the sector boundaries between cuboid and octahedral growth.
grooves belonging to adjacent cuboid sectors (thereby forming a col at the sector boundary), favours mitiation of a facetted re-entrant at the boundary, and its perpetuation. Direct evidence of association of grooves with a facetted re-entrant was obtained in the case of the re-entrant comprising (111) and (111) facets. This re-entrant was smaller than those bounded by the pairs of planes (111), (111), and (111), (111), which are seen in the slice shown in figs - 1 a and 3, but it had a more symmetrically rhombic section than that cut in fig. lb. The top and bottom apices of the rhomb connected with the sheet of particles defining the (100), (010) sector boundary, as expected. Birefringence showed that both of the other pair of apices (the diagonal joining which defines the re-entrant junction between (111) and (111)) connected with fault surfaces like those marking inter-cobble grooves. Fig. 5a is a birefringence topograph of part of a polished slice cut parallel to (110) about 0.75 mm distant from the slice shown in figs. la and 3. (The side edges of the slice are not natural surfaces: they have been sawn parallel to [001].) As in the case of the birefringence topographs figs. 3a and 3b, it is not feasible to show all fault surfaces with comparable contrast on one photographic print. However, the observations are presented diagram-
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matically ments of the in fig. (100), Sb which (010) shows growththe sector loci boundary of the seg-
the (100) cuboid sector shown in fig. 4, the planes
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During growth, the topography of cuboid surfaces of natural diamonds must have closely resembled the ‘cobble’-structure of impurity cells found on (0001) growth surfaces of synthetic quartz [9, 10]. In the specimen shown in fig. 4, the hummocky profile is revealed directly by the growth banding, but no fault surfaces trailing from the grooves between cobbles are evident. In this respect the specimen resembles the mixed-habit diamonds previously discussed [1]. On the other hand, in the specimen illustrated in figs. la and 3, traces of growth surfaces are only faintly visible, and the configuration of the fewer and larger cobbles present is shown most clearly by the fault surfaces which mark the trails of grooves between cobbles. This latter diamond thus resembles the more perfect specimens of synthetic quartz, prepared with minimum fluctuation in growth conditions, With regard to the facetted re-entrants described in section 3, it is considered likely that the junction of a groove with the cuboid—cuboid sector boundary, or possibly the merging together at the boundary of
joining the top and bottom apices of the central rhomb with the major (111) growth sector (above) and major (ill) growth sector (below), respectively, and also the traces of the fault surfaces which extend from the other two apices of the rhomb into the regions of cuboid growth on either side. It should be mentioned that birefringence topographs and X-ray transmission topographs reveal that both the left-hand and righthand columns of growth on the re-entrant facets contamed in the slice shown in figs. Ia and 3 made at least one ‘false start’ nearer to the crystal centre: re-entrant facets having no measurable volume of octahedral growth crystallised upon them can be detected buried in the cuboid matrix. Understanding of the process of initiation of re-entrant facets at cuboid---cuboid sector boundaries is impeded by lack of knowledge of the microstructure of the boundary, in particular of the reason for the sheets of particles within them. There appears to be a predisposition for small {1 11 1facets (not necessarily forming re-entrants) to develop at points along cuboid—cuboid sector boundaries. Such
S. Suzuki, AR. Lang / Far etted re-e,mtra,mts on rounded growth surfaces ~t diamonds
36
w:~inferred from the earlier studies II I~ and X-ray topographs of other regions of the specimen discussed in section 3 rein force this niotion. Regarding the sawtooth profile of the octahedral-cuboid boundary enveloping the column of octahedral growth on the (Iii), (111) re-en fran t, it is tempting to attribute this to repeated attainment of instability in the angle between the facetted and non-facetted growth surfaces along their junction. However, close examination of the X-ray topographs shows that the epochs of abrupt reduction in facet cross-section match ~vithigrowth banding extending across the whole width of the column of octahedral growth, and thus with fluctuations of the impurity con tent therein. Consequently, it appears more likely that the sawtooth profile is attributable to externally originating fluctuations in growth conditions. Turning to consider the re-entrants described in section 4, one question dominates: what interpretations can be put upon the often-seen coincidence of dislocations with the apices of re-entrants? (Dislocations received no mention in connection with the phenomena described in section 3, since the specimen concerned was dislocation-free in the regions involved.) For the initiation of facetted re-entrants, and for the coincidence of dislocations with the apices thereof, the following simple geometrical model comes first to mind. Facetting might commence at points along inter-cobble grooves (or at groove junctions) where the groove slope exceeded sonse critical value during a fluctuation in environmental conditions which favoured facet expansion. Grooves (and facetted re-entrants when once formed) would be expected to trap grown-in dislocations because the dislocation outcrop point should migrate towards the groove bottom (or re-entrant apex) in order
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(b) 11g. 5. (a) Birefringence topograph of central area of a slice parallel to (110) intersecting a small column of octahedral growth located in the bounthry between the (100) and (010) cuboid growth sectors. Width of slice 2 mm, thickness 0.5 mm. The vertical sides of the slice have been cut parallel to 10011. (b) Key diagram to (a). Octahedral growth represented by vertical shading. The main (111) growth sector is at the top of the field, the main (111) sector at the bottom. Heavy lines indicate the (100), (010) growth sector boundary. Loci of fault surfaces in the cuboid growth sectors are shown, with some simplification, by the light lines,
minimise the dislocation length grown in to the crystal per unit incremental thickness of crystal growth. There seems to be no reason for doubting that such dislocations trapping occurs in the upper left corner of the section szen in fig. 4, where the filling-in of the volume between the cuboid surface and an enclosing octahedral forum is achieved by growth on a limited number of coarsescale, alternating -[1 11 } facets. However, in the zone of present concern, where cuboid growth continues albeit with diminishing development relative to octahedral growth, a survey of locations of facetted re-entrants in relation to those of the inter-cobble grooves does not support the above-described model. The scale of the cobble
S. Suzuki, A. R. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
structure on the cuboid growth surfaces of this crystal can be estimated from the spacing of inward pointing cusps along the growth horizons, since these cusps mark inter-cobble grooves. On average, about three grooves are cut in each cuboid quadrant (estimated from a total of 16 quadrant sections imaged in X-ray topographs): and this leads to an average cobble diameter of about 0.5 mm. On the crystal section surfaces examined X-ray topographically, the total number of observed facetted re-entrants in cmmboid growth surfaces was counted, including only those cut not far from their apices, those clearly separable from their neighbours, and counting as “one” a sequence of related re-entrants occurring one above the other on succeeding growth horizons. (The count in fig. 4 is thus seven.) Coincidences of re-entrant apices with dislocation lines were noted: this count could be an underestimate since dislocations would be missed if they had lain entirely in material polished away from the crystal surface. Of the 41 re-entrants included in the census, in all but two cases was it possible to state with reasonable assurance whether or not coincidences with dislocations and/or grooves existed (within the resolution limits of a few micrometres). In seven cases re-entrant apices, dislocation outcrops and inter-cobble grooves coincided. In 32 cases the apex lay on on a dislocation line and was situated on convex growth surface, remote from a groove. In no cases were apices located on grooves without dislocations being there also. (A certain proportion of dislocations, say a quarter very roughly, have no detectable re-entrants developed at any point along them.) This manifest association of reentrant apices with dislocations prompts the question whether the re-entrants are etch pits formed during episodes of dissolution interrupting crystal growth. There then has to be explained the observed concordance of apices of re-entrants with special growth horizons rather than that the latter should define horizons of intersections of pit sides with cuboid growth surfaces. On evidence at present available, the etch hypothesis is re-
37
jected, weight being given to the observation mentioned at the end of section 4, viz., that episodes of apparent complete inhibition of growth on the main {l 11 } faces commenced at the same growth horizons at which reentrant apices are concentrated. The mechanism whereby facetting on a cuboid surface of diamond can be triggered-off at certain dislocation outcrops is mmknown. A conjecture is that adsorption of an impurity body (non-solid, perhaps) at the dislocation outcrop enables a nucleus of {1 11) facetting to be maintained there, available to expand when a change in the environment so dictates.
Acknowledgments The authors thank Mr. W.F. Cotty, Industrial Distributors Ltd., for specimens, and Professor F.C. Frank, F.R.S. for advice. One auther (S.S.) gratefully acknowledges support from the Japan Society for the Promotion of Science and from Industrial Distributors Ltd.
References Ill AR. Lang, Proc. Roy. Soc. (London) A340 (1974) 233. 121 A.R. Lang, J. Crystal Growth 24/25 (1974) 108. [31 E.R. Harrison and S. Tolansky, Proc. Roy. Soc. (London) A279 (1964) 490.
[41 M. Seal, Am. Mineralogist 50 (1965) 105. 151 F.C. Frank, in: Proc. Intern. Industrial Diamond Conf., Oxford, 1966, Vol. 1, Science, Ed. J. Burls (Industrial Diamond Information Bureau, London, 1967) pp. 119— 135.
161 S. Suzuki and A.R. Lang, Phil. Mag. 32 (1975) 1083. 171 M. Seal, Nature (London) 212 (1966) 1528. [8] M. 133.Moore and A.R. Lang, J. Crystal Growth 26 (1974) 19] CS. Brown and L.A. Thomas, J. Phys. Chem. Solids 13 (1960) 337. [10] A.R. Lang and V.F. Miuscov, J. AppI. Phys. 38 (1967) 2477.
OCCURRENCES OF FACETTED RE-ENTRANTS ON ROUNDED GROWTH SURFACES OF NATURAL DIAMONDS S. SUZUKI and A.R. LANG H/I. Wills Phi’sics Laboratory, University of Bristol, Bristol BS8 I TL, England Received 24 October 1975; revised manuscript received 7 January 1976
Some unexpected growth-surface morphologies have been discovered as localised developments within the regions of non-facetted growth which are present (and can be dominant) in diamonds which have had epochs of mixed-habit growth during which crystallisation proceeded simultaneously both on normal octahedral faces and on non-crystallographic, hummocky surfaces having mean orientation {ioo}. These unexpected minor features are {ui }-facetted re-entrants, not involving twinning. They have been observed in two crystallographic settings. In one case pairs of {i ii facets develop to form re-entrant notches at the boundaries between adjacent sectors of non-facetted, mean- fi 00 }-orientation growth. In this case growth can propagate on the pair of re-entrant facets, forming a column of octahedral growth inserted within the surrounding matrix of non-facetted growth material. In the second case, {i ii }-sided pyramidal pits (size range from about 10 ~.smto 120 ~m) develop at points distributed over the area of non-facetted growth surface. There is evidence that they are initiated at particular growth horizons at which an episode of strong inhibition of growth on ~l 11 commences, and that they are preferentially located at dislocation outcrops. In this case no detectable propagation of octahedral growth upon the facets occurs: they are directly overlain by renewed non-facetted growth.
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1. Introduction
topography have also been fruitful. This report is concerned with local occurrences of {1 11 } facetted growth set within larger regions of crystal where the non-facetted mode of growth is dominant. Two varieties of this phenomenon will be described, differing in their geometrical settings. In each case planes of the form ~l 11 } combine to produce facetted re-entrants in the surface of the growing crystal. In one case the facetted re-entrant appears capable of advancing over distances of millimetre order (say 1/3 of the crystal radius) with little change in its state of development. Before the details are given, however, some leading characteristics of mixed-habit diamonds (as far as they are yet known) will be summarised.
Some strange crystal growth phenomena are being brought to light through studies of natural diamonds which have had epochs of mixed-habit growth. During these epochs the crystals were bounded in part by normal {i 11 } facets and in part by non-flat, generally hummocky surfaces whose mean orientation is {l00~. For the latter, non-crystallographic surfaces the descriptive name “cuboid” has been chosen, albeit locally the growth surfaces of cuboid growth sectors can be inclined up to about 300 off true {lOO} orientation [1,2]. It was from etch patterns produced on polished slices of diamonds cut parallel to cube planes [3,4] that Frank [5] deduced that facetted and non-facetted growth had proceeded contemporaneously. In subsequent studies of diamonds of this nature, X-ray topographic methods have been the investigative techniques of first choice of virtue of their non-destructiveness; but examinations of polished sections by birefringence, optical ultra-microscopy and cathodoluminescence
2. Some properties of mixed-habit diamonds
The most striking feature of these crystals is a population of microscopic or sub-microscopic particles distributed within their cuboid growth sectors. A few 29
30
S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
cases have been encountered where the particles have been large enough for their individual birefringence
properties to be examined and a preferred orientation of the particles with respect to the diamond matrix to be detected [6]. The particles individually produce diffraction contrast images in X-ray topographs by straining the diamond matrix surrounding them: some cases have been found in which X-ray topographs revealed presence of particles whereas optical examination did not. Generally the particles make themselves evident by their scattering of light (and absorbing it, if present sufficiently profusely, and large individually). In cases where the crystal growth has been nicely symmetric (as in the specimen shown in fig. 5 of ref. [2]), a symmetrical “star” pattern can be seen inside the diamond provided its external surfaces are not too rounded or rough. There is very limited information on the chemical composition of the particles [7]. The relative development of normal {l 11] facets and of cuboid growth surfaces can vary widely from crystal to crystal, and within a given crystal during its growth. Whenever the fractional loss of crystal volume through post-growth dissolution has been sufficiently mild, it appears both from etch patterns on polished sections and from X-ray topographs that the final stages of growth were entirely ~111 }-facetted. On the other hand, accumulating X-ray topographic evidence is leading to the conclusion that when mixed-habit growth does occur, then the cuboid form is usually dominant in the early stages of growth. Dominant cuboid growth implies relatively large areas of growth sector boundaries between adjacent cuboid sectors. These boundaries sometimes follow {ll0} quite closely, and very often contain dense sheets of particles which are visible both by light-scattering under ultra-microscopic examination [I] as well as by X-ray diffraction contrast. Thereby they are readily traceable in the crystal. Octahedral growth sectors can be distinguished from cuboid growth sectors optically as well as by X-ray topography when the latter sectors contain light-scattering particles. However, even when such particles are not detectable (and they generally are absent in the outermost zones of cuboid growth), the growth sectors can be differentiated X-ray topographically because the integrated X-ray reflection from the diamond matrix in the octahedral growth sectors is generally stronger (often quite markedly so) than that from the particle-free regions of cuboid growth [1].
3. Facetted re-entrants at cuboid-cuboid growth sector boundaries Fig. 1 a is a surface reflection X-ray topograph of one face of a slice cut and polished parallel to (110), 0.59 mm thick, prepared from a diamond of mixed-habit growth which displayed a visible internal “star”. The slice contains the crystal centre, and this is nearer to the remote face than to the face which is imaged in fig. Ia. The specimen is notable for much stronger integrated X-ray reflection from its {l 11 } growth seetors than from its cuboid growth sectors: the former consequently appear considerably darker on the topograph prints. The (001) cuboid sector occupies the topmost part of the image, and the (DOT) cuboid sector lies below the centre. The images of the cuboid sectors show speckling due to their population of small partides which produce diffraction contrast, but the speck-
I . .
I
-~
c
I
I
(a)
S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces 0/diamonds
31
ling falls off quite abruptly when a distance of about 2 mm from the crystal centre is reached. The {l 11 } growth sectors which are intersected as expected are listed in the caption of fig. la To the left of the crystal centre, along the horizontal, the slice contains parts of the (100) and (010) cuboid growth sectors, the (100), (010) growth sector boundary lying largely (but not entirely) within the slice. Correspondingly, to the right
~
of the crystal centre the (RIO) and (010) cuboid sectors are present, with the (100), (010) growth sector boundary contained within the specimen. Unexpected are the wedges of {l 11 } growth which lie centrally within both the left-hand and right-hand regions of cuboid growth. They are identified as being composed of {l 11 } facetted growth by their patent similarity in X-
~ - .
-
p ~
ray diffracting texture to the main ~l 11 } growth seetors, and by their geometry. On the left of the centre, extending outwards to the edge of the specimen along a radial distance of about 1 .25 mm, is a gently expanding column of octahedral growth on facets with indices (Ill) and (111), which facets together formed a Vee-shaped re-entrant in the growth surface. Close to the right-hand edge of the crystal slice, and also on the horizontal axis through the crystal centre, can be seen another wedge of {l 11 } growth. This involved the (Ill) and (Ill) facets, which together constituted a re-entrant the inverse of that which propagated itself in the left half of the slice. Fig. I b is an X-ray section topograph cutting through the slice not far from its left-hand edge. An important fact revealed by the sec-
/
- ‘ .
-
tion topogmaph is that the top and bottom apices of the parallelogram-shaped cross-section of material grown on the facetted re-entrant connect with the sheets of diffraction-contrast-producing particles which define the loci of growth sector boundaries between (100) and (010) cuboid growth sectors. Thus the ía(b) Fig.1. (a) Surface reflection X-ray topograph of diamond slice, 6.8 mm high, 4.7 mm wide, polished parallel to (110). Direction 10011 is vertical. A few residual polishing scratches parallel to 10011 produce dark vertical lines on the image. Darker areas are {i ii growth sectors: (111) upper right, (111) lower right, (111) upper left, (111) lower left. Close to centre of image, small outcrops of (111) facetted growth (above) and (111) facetted growth (below) appear. Near bottom apex, part of an outer shell of octahedral growth is intersected. Reflection 131, CuKo 1 radiation. (b) X-ray section topograph cutting vertically through the specimen shown in (a). The left-hand margin of the section topograph represents the inter-
}
section with the face shown in (a), line of intersection mdicated by the solid marker lines in (a). The right-hand margin of the section topograph intersects the remote face of the specimen, line of intersection shown by interrupted marker lines. The section cuts through the main (111) growth sector (top), the main (111) growth sector (bottom), and through material grown on the re-entrant facet pair (ill) and (111) (left middle) - (010) cuboid growth is represented in the righthand margin of the Section at middle level; a smaller volume of (100) cuboid growth in the IefL half of the section, also at middle level. The reflection is 220, in symmetrical transmission, CuKcs1 radiation; the incident beam does not cut the specimen perpendicularly, but at 37.6°to its normal, 11101.
S. Suzuki, AR. Lang / Facetted re-entrants on rounded growth surfaces u/diamonds
32
cetted re-entrant exists as an inset between these growth sectors. In fig. lb the section of (100), (010) growth sector boundary below the re-entrant runs vertically down the centre-line of the topograph, and it cannot lie far off(l 10) orientation. The short section visible running from the upper apex of the re-entrant crosssection upwards towards a surface outcrop makes about 300 with (110). The present external surface of the specimen is a rounded rhombic dodecahedron, the form to which octahedral diamonds progress as a consequence of dissolution after growth [8]. The specimen is somewhat elongated in the [001] direction. The epoch of mixed-habit growth was terminated (apparently abruptly) by one of entirely {l 11 }-facetted growth. Much of the enclosing octahedral shell has been lost by dissolution, however. In the slice shown in fig. la the only part of this enclosing facetted growth still retamed appears at and to the right of the lowest point of the image. Figs. la and Ib, together with information derived from other X-ray topographs, lead to an idealised recon-
Fig. 2. Idealized perspective drawing of dominant cuboid growth truncated by {i.ii facets and girdled by some {iii }-facetted re-entrants. The direction 10011 is vertical, [1001 points towards the observer and to the left, 10101 points to the right. A slice parallel to (110), containing the front left vertical edge and the rear right vertical edge, would represent the specimen of fig. 1~For simplicity, the internal edges of only one of the eight corner {i ii growth pyramids radiating from the crystal centre is drawn, i.e. that terminated by the (111) facet at the near top left corner. Similarly, the internal bounding edges of only one of the three facetted re-entrants are shown, i.e. that facetted by (111) and (111), corresponding to the facetted reentrant which develops on the left of the crystal centre in fig. 1.
struction (fig. 2) representing the stage in mixed-habit growth of the crystal when its diameter was about the same as the width of the slice in fig. Ia, and subsequent to its development of facetted re-entrants forming girdling notches. On the assumption that the specimen’s present elongation along [001] is an inheritance from the epoch of mixed-habit growth, fig. 2 is drawn simiharhy slightly elongated. The edges at which cuboid surfaces meet are shown as interrupted straight lines. In actuality they were not straight, but in what manner they were curved is largely unknown. Each of the eight octahedral facets which truncate the corners terininated a growth pyramid expanding from the crystal centre. To avoid confusing the drawing, only the internah edges of the (111) growth pyramid are sketched in, as faint interrupted straight lines: the X-ray topographs and the birefringence topographs (fig. 3) indicate how
}
}
(a)
S. Suzuki, A.R. Lang / Facetted re-entants on rounded growth surfaces of diamonds
(b)
33
(c)
Fig. 3. Birefningence of specimen shown in fig. 1 when viewed between crossed Polaroids. (a) Plane containing electric vector incident on crystal is vertical. (b) Plane of electric vector inclined at 45°to the vertical. (c) Key to (a) and (b) showing (in simplified fashion) traces of the principal fault surfaces detected in time coboid growth sectors, as derived from birefringence observations using the complete range of orientations of the electric vector. (The octahedral growths sectors are identified by vertical shading.)
great an ideahisation this straightness is. In fig. 2 only three out of the possible four facetted re-entrants lying in the median (001) plane are drawn (and none is shown in the corresponding (100) and (010) median planes). This is because only those actually drawn have been proven by X-ray topographic examination. There do exist however, some other quite small developments of {l 11 } facets at cuboid-cuboid growth sector boundaries; these will be referred to later. The birefringence topographs figs. 3a and 3b provide information both confirmatory and complementary to
that derived from the X-ray topographs. They show faint growth banding in the main {l 11 } growth seetons that is barely perceptible in the X-ray topographs.
In the cuboid sectors they show banding quite plainly in some regions where the X-ray topographs show it only faintly, and vice versa. The great sensitivity of the birefringence topographs to long range warping of the specimen results in the wide variations of extinction over the area of the slice. The actual extreme range of crystal misorientations was probably well under 1’ of arc. Birefringence observations reveal quite distinctly the curving fault surfaces which sub-divide the cuboid growth sectors into irregularly-shaped cells. However, it is difficult to photograph the whole pattern of fault surfaces in individual birefringence topographs because of great variation in extinction over the specimen area. The simplified sketch in fig. 3c, which is intended to
34
S. Suzuki, AR. Lang / Eacetted re-entrants on rounded growth cur/aces of diamonds
serve as a key to figs. 3a and 3b. incot porates infoi nsalion accumulated From observations using all possible directions of the electric vector in the plane of the specimen - In order to simplify the drawing, all fault surfaces are shown by a single trace in fig. 3c. whether they be normal to the specimen plate or not. Only the most strongly-developed fault surfaces show up clearly in all of the figs. I a, 3a and 3b. Examples of such are seen about 1 mm above and 0.5 mum to time right of the centre of growth of the crystal, in the (001) cuboid growth sector, and also, in the (001 ) cuboid growth seebr, about 1 mm below the centre ot gi owtls and 0.5 inns to the right thereof. The major long-range strains in the specimen arise from coherency stresses at the boundaries between octahedral and cuboid growth, since these two matrices have a small (as yet undetermined) mis-match in mean hitflee parameter. When an oc tah edral—cuboid boundary changes direction sharply (i.e. ifit is stepped) the resultant localised deformations cause strong birefringence. Thus the saw-tooth nature of the boundary between the re-entrant facet composed of( 111) and (lii) growth and its enclosing cuboid matrix is well displayed in fig. 3
4. Facetted re-entrants within cuboid growth sectors The crystal shown in fig. 4, though clearly also a member of that tribe of diamonds which have had epochs of mixed-habit growth, exhibits many differences from the individual shown in fIgs. I and 3. Firstly to be remarked is that its outer shell of octahedral growth has been fairly well preserved: the existing crystal habit is an octahedron not very strongly modified by post-growth dissolution. Inside the octahedral shell, however, cuboid growth has been more strongly doniinant than ins the specimen previously discussed. (These geometrical circumstances, inter alia, determined the choice of (001) rather than (110) as the plane of sectioning, and the presentation of fig. 4 oriented with a cube direction diagonal rather than vertical as in fig. I a.) The innermost regions are remarkable for their complexly asymmetric disposition of growth sector boundaries. The particles which so intensely produce diffraction contrast include the largest of all those encountered to date in “star” diamonds (large enough for their individual birefringence properties to be studied [6]). However, it is certain features in the zone
k~
Ii. 4. Surface reflection X—nis topuL’raph 01 part oi one lace of a diamond slice cut parallel 10(001). The direction 11101 points horizontally to the right, and 11001 points towards the upper left corner. The field reproduced contains slightly more than one quadrant of the lace and is mainly occupied by (100) cuboid growth identified by the curvilinear growth bands runfling from lower left to upper right. Traces of octahedral planes run vertically (parallel to [110]) and horizontally (parallel to [110]). lield width 2 mm. Reflection 113, CuKoi radiation.
of cuboid growth outside the particle-containing zone that are of present interest. Noteworthy are the small Vee-shaped enhancements of diffraction contrast which. in the field of fig. 4, have their apices at (or very close to) points where dislocation images (running outwards from the crystal centre in the general direction [100]) intersect certain growth horizons well marked by diffraction contrast. From the geometry of the larger Vee-shaped figures in this and other fields it is clear that they are images of sections through small ~l 11 }facetted re-entrants. Upon these facets no volume of octahedral growth large enough for measurement by X-ray topographic means has crystallised: recommenced non-facetted growth directly overlies them. They are hard to detect unless they intersect the surface of the slice, for only then can stress relaxation produce the lattice curvature that generates easily visible diftrac-
S. Suzuki, A.R. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
tion contrast. In the facetted re-entrants contained in
35
(Il 1), (TlT), (ill) and (iii) would be expected to have equal development in an average taken over all re-entrants, the general form of the latter being foursided pyramidal pits. [On a (001) section, as in fig. 4, it is not possible to distinguish between small outcrops of (Ill) and (Ill), both of which have horizontal traces, nor between small outcrops of (111) and (111) which have vertical traces.] In the case of such relatively slight distortions as accompany the re-entrants, only a surface layer of crystal not deeper than about 10pm contributes to the image of a re-entrant on a surface reflection topograph. Thus with facetted reentrants having facet edge lengths in the range from about 10 pm to 120 pm (as observed) only a minority will have their apices imaged in the topographs. However, it is these images which are most significant, for with them it is generally observed that the apex coincides with a dislocation (or small bundle of dislocations), and that re-entrants start developing at particular growth horizons which are also those at which an expansion of {1 11 } facets commences in the sector boundaries between cuboid and octahedral growth.
grooves belonging to adjacent cuboid sectors (thereby forming a col at the sector boundary), favours mitiation of a facetted re-entrant at the boundary, and its perpetuation. Direct evidence of association of grooves with a facetted re-entrant was obtained in the case of the re-entrant comprising (111) and (111) facets. This re-entrant was smaller than those bounded by the pairs of planes (111), (111), and (111), (111), which are seen in the slice shown in figs - 1 a and 3, but it had a more symmetrically rhombic section than that cut in fig. lb. The top and bottom apices of the rhomb connected with the sheet of particles defining the (100), (010) sector boundary, as expected. Birefringence showed that both of the other pair of apices (the diagonal joining which defines the re-entrant junction between (111) and (111)) connected with fault surfaces like those marking inter-cobble grooves. Fig. 5a is a birefringence topograph of part of a polished slice cut parallel to (110) about 0.75 mm distant from the slice shown in figs. la and 3. (The side edges of the slice are not natural surfaces: they have been sawn parallel to [001].) As in the case of the birefringence topographs figs. 3a and 3b, it is not feasible to show all fault surfaces with comparable contrast on one photographic print. However, the observations are presented diagram-
5
matically ments of the in fig. (100), Sb which (010) shows growththe sector loci boundary of the seg-
the (100) cuboid sector shown in fig. 4, the planes
~
During growth, the topography of cuboid surfaces of natural diamonds must have closely resembled the ‘cobble’-structure of impurity cells found on (0001) growth surfaces of synthetic quartz [9, 10]. In the specimen shown in fig. 4, the hummocky profile is revealed directly by the growth banding, but no fault surfaces trailing from the grooves between cobbles are evident. In this respect the specimen resembles the mixed-habit diamonds previously discussed [1]. On the other hand, in the specimen illustrated in figs. la and 3, traces of growth surfaces are only faintly visible, and the configuration of the fewer and larger cobbles present is shown most clearly by the fault surfaces which mark the trails of grooves between cobbles. This latter diamond thus resembles the more perfect specimens of synthetic quartz, prepared with minimum fluctuation in growth conditions, With regard to the facetted re-entrants described in section 3, it is considered likely that the junction of a groove with the cuboid—cuboid sector boundary, or possibly the merging together at the boundary of
joining the top and bottom apices of the central rhomb with the major (111) growth sector (above) and major (ill) growth sector (below), respectively, and also the traces of the fault surfaces which extend from the other two apices of the rhomb into the regions of cuboid growth on either side. It should be mentioned that birefringence topographs and X-ray transmission topographs reveal that both the left-hand and righthand columns of growth on the re-entrant facets contamed in the slice shown in figs. Ia and 3 made at least one ‘false start’ nearer to the crystal centre: re-entrant facets having no measurable volume of octahedral growth crystallised upon them can be detected buried in the cuboid matrix. Understanding of the process of initiation of re-entrant facets at cuboid---cuboid sector boundaries is impeded by lack of knowledge of the microstructure of the boundary, in particular of the reason for the sheets of particles within them. There appears to be a predisposition for small {1 11 1facets (not necessarily forming re-entrants) to develop at points along cuboid—cuboid sector boundaries. Such
S. Suzuki, AR. Lang / Far etted re-e,mtra,mts on rounded growth surfaces ~t diamonds
36
w:~inferred from the earlier studies II I~ and X-ray topographs of other regions of the specimen discussed in section 3 rein force this niotion. Regarding the sawtooth profile of the octahedral-cuboid boundary enveloping the column of octahedral growth on the (Iii), (111) re-en fran t, it is tempting to attribute this to repeated attainment of instability in the angle between the facetted and non-facetted growth surfaces along their junction. However, close examination of the X-ray topographs shows that the epochs of abrupt reduction in facet cross-section match ~vithigrowth banding extending across the whole width of the column of octahedral growth, and thus with fluctuations of the impurity con tent therein. Consequently, it appears more likely that the sawtooth profile is attributable to externally originating fluctuations in growth conditions. Turning to consider the re-entrants described in section 4, one question dominates: what interpretations can be put upon the often-seen coincidence of dislocations with the apices of re-entrants? (Dislocations received no mention in connection with the phenomena described in section 3, since the specimen concerned was dislocation-free in the regions involved.) For the initiation of facetted re-entrants, and for the coincidence of dislocations with the apices thereof, the following simple geometrical model comes first to mind. Facetting might commence at points along inter-cobble grooves (or at groove junctions) where the groove slope exceeded sonse critical value during a fluctuation in environmental conditions which favoured facet expansion. Grooves (and facetted re-entrants when once formed) would be expected to trap grown-in dislocations because the dislocation outcrop point should migrate towards the groove bottom (or re-entrant apex) in order
(a)
~
______________
(b) 11g. 5. (a) Birefringence topograph of central area of a slice parallel to (110) intersecting a small column of octahedral growth located in the bounthry between the (100) and (010) cuboid growth sectors. Width of slice 2 mm, thickness 0.5 mm. The vertical sides of the slice have been cut parallel to 10011. (b) Key diagram to (a). Octahedral growth represented by vertical shading. The main (111) growth sector is at the top of the field, the main (111) sector at the bottom. Heavy lines indicate the (100), (010) growth sector boundary. Loci of fault surfaces in the cuboid growth sectors are shown, with some simplification, by the light lines,
minimise the dislocation length grown in to the crystal per unit incremental thickness of crystal growth. There seems to be no reason for doubting that such dislocations trapping occurs in the upper left corner of the section szen in fig. 4, where the filling-in of the volume between the cuboid surface and an enclosing octahedral forum is achieved by growth on a limited number of coarsescale, alternating -[1 11 } facets. However, in the zone of present concern, where cuboid growth continues albeit with diminishing development relative to octahedral growth, a survey of locations of facetted re-entrants in relation to those of the inter-cobble grooves does not support the above-described model. The scale of the cobble
S. Suzuki, A. R. Lang / Facetted re-entrants on rounded growth surfaces of diamonds
structure on the cuboid growth surfaces of this crystal can be estimated from the spacing of inward pointing cusps along the growth horizons, since these cusps mark inter-cobble grooves. On average, about three grooves are cut in each cuboid quadrant (estimated from a total of 16 quadrant sections imaged in X-ray topographs): and this leads to an average cobble diameter of about 0.5 mm. On the crystal section surfaces examined X-ray topographically, the total number of observed facetted re-entrants in cmmboid growth surfaces was counted, including only those cut not far from their apices, those clearly separable from their neighbours, and counting as “one” a sequence of related re-entrants occurring one above the other on succeeding growth horizons. (The count in fig. 4 is thus seven.) Coincidences of re-entrant apices with dislocation lines were noted: this count could be an underestimate since dislocations would be missed if they had lain entirely in material polished away from the crystal surface. Of the 41 re-entrants included in the census, in all but two cases was it possible to state with reasonable assurance whether or not coincidences with dislocations and/or grooves existed (within the resolution limits of a few micrometres). In seven cases re-entrant apices, dislocation outcrops and inter-cobble grooves coincided. In 32 cases the apex lay on on a dislocation line and was situated on convex growth surface, remote from a groove. In no cases were apices located on grooves without dislocations being there also. (A certain proportion of dislocations, say a quarter very roughly, have no detectable re-entrants developed at any point along them.) This manifest association of reentrant apices with dislocations prompts the question whether the re-entrants are etch pits formed during episodes of dissolution interrupting crystal growth. There then has to be explained the observed concordance of apices of re-entrants with special growth horizons rather than that the latter should define horizons of intersections of pit sides with cuboid growth surfaces. On evidence at present available, the etch hypothesis is re-
37
jected, weight being given to the observation mentioned at the end of section 4, viz., that episodes of apparent complete inhibition of growth on the main {l 11 } faces commenced at the same growth horizons at which reentrant apices are concentrated. The mechanism whereby facetting on a cuboid surface of diamond can be triggered-off at certain dislocation outcrops is mmknown. A conjecture is that adsorption of an impurity body (non-solid, perhaps) at the dislocation outcrop enables a nucleus of {1 11) facetting to be maintained there, available to expand when a change in the environment so dictates.
Acknowledgments The authors thank Mr. W.F. Cotty, Industrial Distributors Ltd., for specimens, and Professor F.C. Frank, F.R.S. for advice. One auther (S.S.) gratefully acknowledges support from the Japan Society for the Promotion of Science and from Industrial Distributors Ltd.
References Ill AR. Lang, Proc. Roy. Soc. (London) A340 (1974) 233. 121 A.R. Lang, J. Crystal Growth 24/25 (1974) 108. [31 E.R. Harrison and S. Tolansky, Proc. Roy. Soc. (London) A279 (1964) 490.
[41 M. Seal, Am. Mineralogist 50 (1965) 105. 151 F.C. Frank, in: Proc. Intern. Industrial Diamond Conf., Oxford, 1966, Vol. 1, Science, Ed. J. Burls (Industrial Diamond Information Bureau, London, 1967) pp. 119— 135.
161 S. Suzuki and A.R. Lang, Phil. Mag. 32 (1975) 1083. 171 M. Seal, Nature (London) 212 (1966) 1528. [8] M. 133.Moore and A.R. Lang, J. Crystal Growth 26 (1974) 19] CS. Brown and L.A. Thomas, J. Phys. Chem. Solids 13 (1960) 337. [10] A.R. Lang and V.F. Miuscov, J. AppI. Phys. 38 (1967) 2477.