Elsevier

Lithos

Volume 77, Issues 1–4, September 2004, Pages 83-97
Lithos

Features of coated diamonds from the Snap Lake/King Lake kimberlite dyke, Slave craton, Canada, as revealed by optical topography

https://doi.org/10.1016/j.lithos.2004.04.028Get rights and content

Abstract

Confocal photoluminescence (PL) and local absorption spectroscopy were used to study the types and spatial distribution of point defects in coated diamonds, the input of which is about 30% in the Snap Lake deposit, Canada. Nitrogen concentration is on the level of several hundreds of ppm in the core, with a nitrogen-poor layer in its outer part, whereas in the coat it is usually several times higher as a result of fast growth. Nitrogen defects in the core are strongly aggregated with N3, B and B′-forms dominating, whereas A-defects are typical of the coat. The rounded shape of the coated diamonds is a result of the combined effect of partial dissolution of the octahedral core and the “abnormal” growth of the coat, which produces a fibrous structure. Analysis of PL and PL excitation spectra showed that structureless yellow-green PL of the coat is likely to be due to nickel-nitrogen complexes with their fine structure broadened in the strain fields. The presence of irradiation/annealing products such as vacancies V0 and nitrogen-vacancy complexes NV, N2V2 shows that the diamonds studied have undergone post-growth ionizing irradiation with further low-temperature annealing in natural conditions.

Introduction

Coated diamonds are specific diamonds, which consist of a clear colorless core, often a well-formed octahedron of good quality, enveloped by a yellow, green or grey coat. The coat material is also diamond, but filled with a particulate matter, the individual particles of which are generally of submicron size and are not clearly resolved with an optical microscope. Most of the recorded coated diamonds have been from the Democratic Republic of the Congo (DRC), where they form about 90% of the population (Kamiya and Lang, 1965), and Sierra Leone (up to 50% in some deposits). They are very characteristic of Mbuji Mayi in the DRC, which is the world's largest producer of this type of diamond (Boyd et al., 1987). The systematic study of coated diamonds began in 1950 with a general description by Custers (1950). Using X-ray topography Kamiya and Lang (1965) have shown that the coat has a fibrous structure interrupted by layers of fine particles. Later Lang (1974) concluded that the structural difference resulted from different growth mechanisms for the core and coat. The core grew continuously in the octahedral habit, by concentric, complete {111} growth layers, while the coat demonstrated a much rarer, “abnormal” mode whereby the growing crystal is broken up into a bundle of columns, growing outwards independently but with equal velocities. Faulkner et al. (1965) found in an Electron Spin Resonance (ESR) study that the concentration of single nitrogen atoms (P1 centers in ESR or C-defects in infrared (IR) spectroscopy) is low in the coated diamonds: only about 30 ppm in the samples cut from the coat. Angress and Smith (1965) discovered, using IR spectroscopy, that a considerable part of the nitrogen in the coat (up to 1700 ppm) is aggregated into pairs (A-defects) with a dominant 1282 cm−1 peak. In the core the nitrogen was found in A and/or B-form, the latter is believed to be a nitrogen-vacancy complex of the N4V structure and is usually accompanied by platelets (B′ defects). There were some additional bands at 475, 1086 and 1105 cm−1 in the IR spectra for the coat. Numerous narrow lines, related to OH, HOH, CH, CO2, CO3 and to silicate and other compounds in the numerous inclusions, were found in the coat related IR spectra of diamonds with strongly colored gray coat Navon et al., 1988, Schrauder and Navon, 1994. The same authors also found that inclusions in fibrous/coated diamonds are rich in various oxides such as SiO2, K2O, CaO, FeO, MgO and P2O5. Logvinova et al. (2003) found S–Fe–Ni-rich inclusions in fibrous diamonds from Yubileinaya pipe, Yakutia. Inclusions are characterized by a wide S/(S+Ni+Fe) range and the high Ni content (up to 40±7 wt.%). Broad compositional variations of S–Ni–Fe inclusions, the constant Ni/(Ni+Fe), the presence of varying amounts of alkalis and Mg and the association with carbonatitic melts suggest that the melt-bearing micro-inclusions do not contain a mineral phase, but should be regarded as sulfide melts.

The geochemical features and isotopic characteristics of the Snap Lake/King Lake (SL/KL) kimberlites combined with the results of the study of crystalline inclusions in the diamonds suggest that their mantle sources are unusual for known kimberlites (Pokhilenko et al., 2001): The lithosphere beneath the Snap Lake area is abnormally thick (at least 300 km). This means that the pressure interval of the natural diamond formation in the lithosphere was from 37 to at least 110 kbar while the normal pressure interval is from ∼37 to ∼65 kbar. As a result differences in the internal structure and physical characteristics of the Snap Lake diamonds, formed at abnormally high pressures, would be expected.

In the present paper the similarity between the coated diamonds from SL/KL and other deposits (DRC, etc.) is demonstrated using a microscopic study, birefringence, photoluminescence (PL) and X-ray topography. Using different methods of optical spectroscopy we show for the first time that in diamonds with fibrous structure nickel is present not only in inclusions but is also incorporated into diamond lattice. Its concentration is particularly high in the coat, and nickel–nitrogen complexes are responsible for its yellow-green luminescence. An attempt to reconstruct the growth history of coated diamonds was undertaken.

Section snippets

Experimental

A group of several hundred samples from the SL/KL was studied using optical and electron scanning microscopy. Two typical coated diamonds (#SL-00/31 and #SL-00/14) were prepared as {110} plates, about 0.5 mm thick. For comprehensive studies we used all available techniques: transmission, birefringence, PL patterns, X-ray topography and optical spectroscopy.

In the case of (110) sections the octahedron faces in the core are perpendicular to the diamond plate and a good spatial resolution is

Samples description

Examination of the sample morphology using optical and ES microscopy showed that the coated diamonds are wide spread among diamonds from SL/KL: according to results of visual study their input is about 30%. Such crystals are somewhat rounded, but their unambiguous identification is possible only in the case when the coat is partly destroyed. Six samples of this type with sizes in the 1.2–3 mm range are shown in Fig. 1. The coat thickness varies considerably from very thin, which is hardly

Conclusions

  • 1.

    Coated diamonds are a result of a multistage growth process with the stages separated in time. Examination of a large batch of diamonds from the SL/KL deposit in Canada allows us to suppose that many of these stones have or initially had a fibrous coat: thus an abnormal growth on the last stage is likely to be typical of all SL/KL diamond association.

  • 2.

    Nickel in the form of point defects was found for the first time in coated diamonds: in the coat it is present in the form of nickel–nitrogen

Acknowledgements

This work was supported partly by the Grants nos 98-05-65283, 02-05-65075 and 03-05-64040 of the Russian Foundation for Basic Research. A.P.Y. thanks the Royal Society for the Visiting Research Fellowship. Authors are grateful to G. Rylov (IMP SB RAS) for X-ray topography and H.J. Milledge (University College, London) for the cathodoluminescence study of the examined stones. The authors are greatly thankful to I. Chinn from De Beers Group Exploration Macrodiamond Laboratory, reviewer, for kind

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