MICRO-DIAMONDS IN CHROMITITE AND PERIDOTITE

Diamonds can be formed naturally in many ways but most diamonds originated in eclogite and peridotite rocks in upper mantle. They are brought to earth’s surface by three types of igneous rocks: kimberlite, lamproite and lamprophyre. Recently, micro-diamonds from chromitites and peridotites in ophiolites were reported (e.g. Yang, 2014; Yang, 2015; Xu, 2015; Xiong, 2015; Robison, 2015). These small diamonds are not gem-quality and thus, they are not economically important. However, since they occurred in a new source, they are geologically very important in order to understand thoroughly about diamond genesis.

We studied thirty micro-diamonds of average 200 microns in size from three sources: (1) chromitite ore in Luobusa, Tibet, (2) peridotite in Luobusa, Tibet, and (3) chromitite ore in Polar Ural, Russia. These are translucent diamonds with yellow to yellowish green in color. Most of them are crystal fragments but some show well-formed cubo-octahedral form. Step-like etch features were observed on the surface of samples. We characterized spectral features of these diamonds using Thermo Nicolet iN10 FTIR microscope and Renishaw inVia Raman microscope.

These diamonds are mixed type IaB and Ib as revealed by mid-IR spectra. They also contain micro-inclusions, such as water and carbonates. The unusual IR peaks, which were never found in gem-quality diamonds, were also detected along with strong CH bands. Additional micro-inclusions of less than 20 microns, e.g. chromite, magnesiochromite, magnetite, feldspar, hematite, and moissanite, were identified by Raman spectral analysis. PL spectra measured at LNT suggested that these diamonds contain nitrogen-vacancy (NV⁰ and NV^-) and H2 (NVN^-) centers. Samples found in chromitite from Luobusa showed nickel-related defects at 882-884 nm doublet, contributing green color component.

Differences in IR, Raman and PL spectra of these diamonds from HPHT synthetic and CVD synthetic diamonds supported that they are of natural origin. All spectra suggested that these micro-diamonds were formed in different but unique geological environment compared to that of gem-quality diamonds.