Platelet development in cuboid diamonds: insights from micro-FTIR mapping

Abstract

Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixedhabit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentrationadjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.

Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

“…It is well known that the formation of the twinned micro-lamellae is an important mechanism for deformation accommodation in diamond [37]. Previous high-pressure and high-temperature (HP-HT) experiments carried out on synthetic diamonds suggested that the {111} micro-twins begin to form under a range of P and T conditions typical for the upper mantle [26,38,39]. Howell et al (2012) [26] showed that further differential stress brings a continuous crystal bending that produces the formation of subgrains.…”

“…Previous high-pressure and high-temperature (HP-HT) experiments carried out on synthetic diamonds suggested that the {111} micro-twins begin to form under a range of P and T conditions typical for the upper mantle [26,38,39]. Howell et al (2012) [26] showed that further differential stress brings a continuous crystal bending that produces the formation of subgrains. In particular, when multiple slip systems {111} <110> occur simultaneously, a rotation of subgrains arises around <112> axes.…”

Nitrogen isotope systematics and origins of mixed-habit diamonds

“…However, two recent studies (Howell et al, 2012;2013a) have shown compelling evidence that the rate of nitrogen aggregation does not differ between sectors, but that the subsequent rate of platelet formation can be retarded in cuboid sectors due to the presence of "disc-crack-like" defects (Walmsley et al, 1987). Hydrogen-related absorption features are significantly stronger in the cuboid sectors (Rondeau et al, 2004;Howell et al, 2012;2013a), most likely due to C-H bonding occurring on the internal surfaces of the disc-crack-like defects. However, due to the possible abundance of IR-inactive hydrogen within a diamond (Sellschop, 1992;Sweeney et al, 1999), there remains no quantitative evidence regarding hydrogen partitioning between growth sectors.…”

“…Carbon-isotope analysis via both gas-sourced mass spectrometry (Cartigny et al, 2003) and secondary-ion mass spectrometry (SIMS; Bulanova et al, 2002;Zedgenizov & Harte, 2004;Howell et al, 2012;2013a) has revealed no evidence of isotopic fractionation between the growth sectors.…”

“…Nitrogen concentrations in mixed habit diamonds are often very high (900 ->2000 ppm) and N is preferentially partitioned into the octahedral sectors; with enrichment ranging from 107 -157 % relative to the cuboid sectors (Welbourn et al, 1989;Rondeau et al, 2004;Lang et al, 2004;Zedgenizov & Harte, 2004;Howell et al, 2012;2013a).…”