The relationship between platelet size and the B′ infrared peak of natural diamonds revisited

Abstract

Platelets in diamond are extended planar defects that are thought to be generated during the nitrogen aggregation process in type Ia diamonds. They were subjected to intensive research during the 1980s and 1990s but the techniques used for observation of defects in diamond have improved since that time and new insights can be gained by further study. This study combines high resolution Fourier Transform Infrared (FTIR) analysis, with an emphasis on the main platelet peak, and transmission electron microscopic (TEM) imaging. By performing TEM and FTIR analyses on volumes of diamond that were closely spatially related it is shown that the average platelet diameter, D, follows the relationship D = a x−b where x is the position of the platelet peak in the infrared spectrum, a is a constant and b is the minimum position of the platelet peak. The best fit to the data is obtained if a value of b = 1360 cm-1 is used, giving a fitted value of a = 221. The observed variation in infrared (IR) peak width can also be explained in terms of this relationship. Additionally, platelet morphology was found to vary according to diameter with large platelets being more elongated. The tendency to become more elongated can be described by the empirical equation AR = 11.9 D+19.6 + 0.4 where AR is the aspect ratio. Using the relationships established here, it will be possible to study platelet abundance and size as a function of parameters such as nitrogen concentration, nitrogen aggregation and diamond residence time in the mantle. This work therefore will open up new methods for constraining the geological history of diamonds of different parageneses and from different localities.

The behaviour of platelets in natural diamonds and the development of a new mantle thermometer

“…The kinetics and exact mechanism of platelet degradation are not well understood. Goss et al (2003) predicts a stable platelet radius of only a few nano-meters, significantly smaller than commonly observed platelets (see Speich et al 2017, and references therein). In fact, 'giant platelets' with diameters of up to several micro-meters have been reported (e.g.…”

“…The relationship between these two parameters was re-investigated by Speich et al (2017), who also confirmed that the integrated intensity of the B ′ peak is controlled by population density as was previously determined by Sumida and Lang (1988). However, the variation in peak width and symmetry are most likely a consequence of the functional form of the relationship between average platelet radius and peak position (Speich et al 2017), not linked to variations in platelet morphology or size distribution as was proposed in the past (Sobolev et al 1969;Kiflawi et al 1998). These observations are the basis for the interpretation of the data discussed in the current paper.…”

Deep Carbon

“…The major recent developments have been: (1) improvements in the methods for acquiring and processing FTIR spectroscopy maps 34 ; (2) a better understanding of the temperature history that is available from zoned diamonds 36 ; and (3) unlocking the abundant information that is provided by the FTIR spectroscopy signal of plateletsplanar defects created with B centers during nitrogen aggregation. 37,38 Figure 5.1a shows a map of "model temperatures" made up by automated fitting of several thousand FTIR spectra in a map of a diamond from Murowa, Zimbabwe. The higher temperatures in the core and lower temperatures in the rim reflect a growth and annealing history with at least two stages.…”

“…36 spectroscopy measurements on platelets, we now have a much better understanding of the meaning of the FTIR spectroscopy platelet peak characteristics (position, area, width, and symmetry) and how platelets evolve with thermal history. 37,38 In addition to diamonds with regular platelet behavior and irregular, platelet-degraded behavior, we have identified a new class of sub-regular diamonds with anomalously small platelets that have experienced unusually low mantle temperatures (below about 1120 C).…”

Natural-Color Fancy White and Fancy Black Diamonds: Where Color and Clarity Converge

“…Dislocation loops in diamonds have been experimentally shown to form from platelet defects in type IaB diamonds when heated to very high temperatures (2400-2700°C; Evans et al, 1995). Platelets are extended defects thought to form when A aggregates transition into B aggregates; they involve carbon interstitials (i.e., clusters of carbon atoms not in normal positions in the lattice) and nitrogen in thin layers (Humble, 1982;Speich et al, 2017). In these diamonds, dislocation loops are impossible to discern directly with an optical microscope, unlike a foreign inclusion with distinct boundaries; however, the effects of these dislocation loops appear to scatter light.…”