Diamond formation in melts of model carbonatites (experiment at 7.5-8 GPa)

Solopova N.A., Spivak A.V., Litvin Yu.A.

Institute of Experimental Mineralogy RAS, Institutskaya 4, Chernogolovka. 142432, Russia

Currently a new method of diamond crystallization under high pressure has been developed. A distinctive feature of this method is carbonate melt which acts as a solvent for starting carbon (graphite or diamond). The diamond-forming growth medium is presented by K-Na-Mg-Ca-carbonate-carbon melt-solution. It is significant that this multi component carbonate melt reproduces most important carbonate part of the natural carbonate-silicate growth medium for dominant mass of the mantle-derived diamonds. Hence, experimental results of this study are also applicable to physicochemical characterization of natural process of diamond genesis.

Starting material was homogenized carbonate mixture of the composition (wt. %): K₂CO₃ 35.0, Na₂CO₃ 10.0, MgCO₃ 25.0, CaCO₃ 30.0 which is a Fe-free version of composition studied earlier. Graphite powder mixed with the carbonate material was the carbon source. Experiments were carried out at HPHT apparatus of the toroidal anvil-with-hole type. The experimental assemblage was used within the temperature 1610-1650^oC and pressure 7.75 - 8.50 GPa intervals for time duration of 5-30 min. Experimental samples were studied with the use of electron microscope Tescan Vega II XMU in the IEM (Chernogolovka, Moscow Region).

It was found that density of diamond nucleation and size of spontaneous single crystals are changeable within the 7.75 - 8.50 GPa interval and 5 – 30 min. time duration (the temperature is kept fixed at 1630 ± 20^oC). The density of diamond nucleation in the carbonate-carbon melt-solution decreases from around 1.3∙10⁵ to 7.3∙10³ crystals/mm³ with pressure lowering. For a given value of time duration, linear size of diamond single crystals increases in regular way with pressure decrease and peaks at about 0.25 mm. The linear size of single crystalline diamonds was found to demonstrate an exponential increase with time duration. This nonlinear kinetic behavior is local for each diamond grain and is caused by re-crystallization of less dense graphite into more dense diamond (‘volume loss” effect) under conditions of fixed general pressure in the high pressure apparatus with non hydrostatic compressive medium.

It should be pointed out that the kinetic peculiarities of diamond crystallization reflect the changeable behavior of carbon oversaturation in respect to diamond in the K-Na-Mg-Ca-carbonate-carbon melt-solution in dependence of PT parameters and time duration. Carbon oversaturation is the main moving force of the process of diamond crystallization. The FTIR-spectroscopy and color cathodoluminescence methods were used for studies of admixture composition and real physical structure of the carbonate-synthetic diamonds.

By earlier study of IR-spectra of carbonate-synthetic (CS) diamonds by Shiryaev et al, 2005, it was determined that nitrogen at the diamond crystals is presented by two forms: (1) single substitutional atoms (C-defects) and (2) nitrogen pairs (A-defects, which are the most widespread in natural diamonds of type Ia).

The objects of research were the CS – diamonds with linear size of 30 -60 mkm generated in experiment at pressure of 8,5 GPa, temperature from 1600 to 1800 ºС and time duration of 5-30 min.

The luminescence spectra of CS – diamonds crystals were observed on the Labram - spectrometer at Shubnikov Institute of Crystallography of the Russian Academy of Sciences by Shiryaev A.A. Photoluminescence spectra contain the peak at 504 nm concerned to Н3 - defect, which is additional to the basic A - defect (characteristic defect for natural diamonds). The peak at 575 nm describes NV centre (a complex nitrogen-vacancy). At the same time, the peak at 638 nm, concerning NV^- defect (negatively charged complex nitrogen-vacancy) is presented at luminescence spectra. This defect testifies of considerable quantity (first hundreds of ppm) of single atoms of nitrogen (defect C). All this data allow to quality the CS-crystals under study as a mixed type Ib+Ia.

The cathodoluminescence spectra of diamonds crystals were obtained in the “Borok” Geophysical Observatory of the Russian Academy of Sciences by Tselmovich V.A. The CL-spectra contain a strip A – luminescence with maximum positions close to 420 nm, 465 nm and 520 nm. The cathodoluminescence spectra demonstrate a heterogeneous distribution of the admixtured nitrogen within experimental samples. The high carbon oversaturation in melt and high growth rates of diamond crystals could give explanation to this phenomenon. The obtained data does not contradict to photoluminescence information and allow to quality a carbonate-synthetic diamonds to the type Ib + Ia.

Support: the RF President grants MK-4754.2009.5, MK-4735.2009.5 and RFBR grant 08-05-00110, 10-05-00654.