Investigation of Nitrogen Aggregation in Diamond Using Long Duration Multi-Anvil Experiments and Optical and Small Angle X-Ray Scattering Analyses

Andrei A. Shiryaev (a_shiryaev@mail.ru)¹, Mark T. Hutchison (mhutchis@lpl.arizona.edu)² & Yu A. Klyuev³

¹ Vernasdky Inst. of Geochemistry and Analytical Chemistry RAS, Kosygin St. 19, Moscow B-334, Russia

² Lunar and Planetary Laboratory, University of Arizona, Tucson AZ 85721-0092, U.S.A.

³ VNII Almaz, Gilyarovskii St., 65, Moscow, Russia

Nitrogen is the main elemental impurity in diamond and is believed to be initially incorporated into the diamond lattice as single substitutional atoms (C-defect, Kaiser and Bond, 1959). Diffusion during high temperature and pressure annealing forms more complex nitrogen and nitrogen-vacancy aggregates (A and B defects, Boyd et al., 1994 and 1995). Despite significant advances in constraining aggregation kinetics and the structure of the main defects, many important points, e.g. diffusion activation energy, are still controversial. In order to better characterise nitrogen aggregation, a series of experiments are being conducted using the Walker-style 1000 ton multi-anvil press of the University of Arizona. Runs on synthetic stones grown with Ni catalysts and characterised by photoluminescence, FTIR and small-angle X-ray scattering (SAXS) have been carried out at 7 GPa, 1800°C for durations of up to one month. Due to changes in vacancy concentration and speciation during quenching, and since nitrogen aggregation is believed to be vacancy-assisted, one can expect significant differences in diffusion kinetics between sequences of short runs (all previous studies) and long continuous runs (this study), likely to be more relevant to interpretation of the natural sample. Our results from relatively short (~ 10 hour) runs show that almost all nitrogen aggregated from single atoms to pairs (A-defect). Photoluminescence spectra show appearance of lines, which are tentatively ascribed to the creation of Ni-N-vacancy complexes. Importantly, initial SAXS results indicate formation of large diameter (~500Å) clusters with low contrast with the diamond matrix consistent with the view that nitrogen aggregation proceeds by the decay of supersaturated solutions of N in the diamond lattice. We expect that one month runs will yield well-structured nitrogen-rich clusters of ~90 Å diameter similar to those found in natural IaB diamonds (Shiryaev et al., 1999). Results from long duration annealing will be reported at the conference.

Boyd SR, Kiflawi I and Woods GS, Phil. Mag, B 69, 1149-1153, (1994).

Boyd SR, Kiflawi I and Woods GS, Phil. Mag, B 72, 351-361, (1995).

Kaiser W and Bond WL, Phys. Rev, 115, 857-863, (1959).

Shiryaev AA., Klyuev YuA, Naletov M and Dembo AT, Abs. of the 9th Ann. Goldschmidt Conf. (LPI, Houston), 275, (1999).

An Experimental Study of Chlorine Partitioning between Fluids and Hydrous Phonolitic Melts

Stefano Signorelli (s.signorelli@bristol.ac.uk)¹ & Michael Carroll (carroll@campus.unicam.it)²

¹ Dept. Earth Sciences, Bristol University, Bristol BS8 1RJ, UK

² Dipt. Sci. della Terra, Università di Camerino, 62032 Camerino, Italy

Chlorine solubility and partition coefficients between aqueous chloride solutions and silicate melts were investigated experimentally at 860-890°C, 25 to 250 MPa, using peraluminous (Vesuvius, Italy) and peralkaline (Teide, Spain) natural phonolites. Depending on the experimental conditions the peralkaline phonolite was in equilibrium with either a Cl-bearing supercritical aqueous fluid or a subcritical assemblage of aqueous vapour + Cl-rich brine, whereas the peraluminous phonolite was always equilibrated with subcritical fluids. In both cases the composition of fluid was calculated by mass balance. For experiments below 200 MPa and in the presence of Cl-bearing supercritical aqueous fluid, Cl concentrations in the glass increase linearly with increasing Cl molality in the fluid phase. This relationship was used to calculate fluid/melt partition coefficients which range from 0.8 at 50 MPa to 6 at 150 MPa and show an inverse correlation with pressure. At 200 to 250 MPa, the concentration of Cl in the silicate melt and that calculated in the bulk fluid was directly used to calculate partition coefficients. When vapour and Cl-rich brine coexists with the silicate melt, Cl contents in the melt achieve a typical plateau of solubility. Cl contents of brine-saturated melts vary approximately linearly from 0.89 wt.% at 25 MPa to 0.57 wt.% at 150 MPa in Vesuvius phonolite; results for the Teide phonolite are similar. These results provide useful constraints on the maximum amount of Cl expected in phonolitic melts and information on how Cl behaviour in silicate melts depends on melt composition. The comparison of experimental results with the amount of Cl in natural volcanic samples such as glass inclusions and matrix may help in the identification of eruptive products preserved in the geologic record which may have been associated with large halogen emissions.

Synthesis of Carbon Nanotubes from Hydrocarbon Gases at Low Temperatures and Pressures

Sergei Simakov (simakov@vap.usr.pu.ru)¹, Alexandr Graphchikov², Irina Drozdova³, Andrei Lapshin & Elena Grebenshchikova⁴

¹ S-Petersburg University, Universitetskaya emb. 7/9, S-Petersburg, 199034, Russia

² Institute of Experimental Mineralogy, Chernogolovka, Russia

³ Institute of Silicate Chemistry, S-Petersburg, Russia

⁴ A.F.Ioffer Physical Institute, S-Petersburg, Russia

In the nature rare cubic form - fullerene was discovered inner graphite in the Karelian shungites (Buseck et al., 1992). Experimentally fullerenes were synthesised from graphite at very high temperatures. But mineralogical data shows that in the metamorphic rocks there is another mechanism of the fullerene formation. Metamorphic graphite can be formed from fluids (Frost, 1979). We modelled the process of graphite formation at low temperatures and pressures (750°C and 5 kbar) from the methane and hydrocarbon gases (oxygen fugacity keeps lower CCO buffer). Products of the experiments were studied by the X-ray diffraction and electronic microscopy. The main phase in the products of the experiments is graphite. The second widespread phase is l-carbin. Except graphite and carbin rare nanotubes with inner channel were founded. It has length up to 1 µm, out diameter nearly 600 Å and inner diameter nearly 40 Å. The same tubes are associated with the fullerenes. From the morphological data and associative phases we can come to conclusion that it is fullerite pipes. As a result we can conclude that in the natural processes except high-temperature way another way of fullerite formation can take place. It's the formation from the hydrocarbon gases at relatively low temperatures and pressures. From it follows that the origin of the metamorphic fullerite is connected with the hydrocarbon-rich metamorphic fluids.

Buseck PR, Tsipursky SI & Hettich R, Science, 257, 215-217, (1992).

Frost BR, Am. J. Scie, 279, 1033-1059, (1979).