Formation of unusual yellow Orapa diamonds

Abstract

Twenty eclogitic diamonds from Orapa Mine (Botswana) with an unusual yellow colour are characterised for their growth structure, N systematics, and C isotope composition, and the major element composition of their silicate inclusions. The diamonds show complex luminescence with green, blue and non-luminescent zones and occasional sector zonation. All parts of the diamonds have low total N concentrations (<50 at.ppm, with one exception of <125 at.ppm) and a limited range in C isotope composition (−5.7 to −10.6‰). Fourier Transform Infrared spectra show bands at 1334, 1332, 1282, and 1240 cm −1 typical for Ib-IaA diamonds. Relict unaggregated N defects (N s o and N s +) are present and the preservation is likely caused by the low N concentrations and possible low mantle residence temperatures rather than young diamond formation (inclusion ages of 140, 1096, 1699 Ma; Timmerman et al. Earth Planet Sc Lett 463:178-188, 2017). Garnet and clinopyroxene inclusions extracted from 14 diamonds have an eclogitic composition with relatively low Ca contents and based on all characteristics, these diamonds form a distinct population from Orapa.

Is the Imposition of Diamond Morphology on Mineral Inclusions a Syngenetic or Post-Genetic Process with Respect to Diamond Formation?

“…In this light, it is evident that a correct analysis of the two possible processes requires determination of the position of the inclusions inside the diamond: are they near the barycenter, i.e., in an intermediate position, or in the periphery of the diamond? Nevertheless, these data can be misleading when they are not associated to specific growth zonations observed by cathodoluminescence (CL). ,− Indeed, only when diamond grows homothetically (i.e., equivalent faces advance with the same rate), the barycenter of the crystal (blue circle in Figure a) coincides with the central growth zone of the diamond (red square in Figure a). When the growth is not homothetic, then the central growth zone of the diamond no longer coincides with the gravity-center of the crystal (Figure b).…”

Morphology of Monocrystalline Diamond and its Inclusions

“…In recent years, there have been several studies that document the internal features of rare natural Ib diamonds from French Guiana, eastern Sierra Leone, NE Siberia, and Botswana (Smith et al 2016a;Zedgenizov et al 2016;Smit et al 2018b;Timmerman et al 2018). These diamonds are mostly yelloworange due to the presence of isolated nitrogen.…”

“…In some diamonds, growth zoning correlates with heterogeneous yellow color distribution. In Botswana, the Orapa mine has produced some irregular cubo-dodecahedral diamonds, with some color differences between the different growth sectors (Timmerman et al 2018).…”

Effect of HPHT Treatment on Spectroscopic Features of Natural Type Ib-IaA Diamonds Containing Y Centers

“…Recently, considerable attention has been paid to natural type Ib diamonds [6][7][8][9][10][11][12][13]. These diamonds are rare, accounting for less than 0.1% of natural diamonds [14].…”