The results of a microscopy and FTIR and PL spectra study of the natural polycrystalline diamonds from the Mengyin kimberlite pipes show that they can be classified as the euhedral faceted polycrystalline diamonds (EFPCDs) and anhedral rounded polycrystalline diamonds (ARPCDs). Different diamond grains or their points were formed in different conditions or processes. They were not formed in diamond nucleation stage, but in the diamond growth period. They probably originated from the relatively deeper mantle and were formed in the environment like the peridotitic (P) type diamond single crystals. The EFPCDs did not undergo a remarkable dissolution process during their formation and were possibly fast formed shortly before the kimberlite eruption. The ARPCDs not only were formed at a higher temperature than the EFPCDs but also underwent a notable dissolution process and had been stored relatively longer in the mantle. Fluids or melts probably participated in the formation of the ARPCDs or modified them during the period of their storage in the mantle. 1. Introduction Polycrystalline diamonds are multigranular aggregates consisting almost entirely of diamonds. Based on the qualitative external characteristics, there are many and diverse polycrystalline varieties of diamond. Among them, balas displaying radial fibrous structure is spherulite, bort is a distinctly granular aggregate, and carbonado consisting of submicroscopic diamond particles is a cryptocrystalline aggregate [1]. But based on the scientific origin of the natural polycrystalline diamonds, previous studies [2] classified them as carbonado and framesite. Carbonado is multigranular and porous sensu stricto refers specifically to the diamond aggregates from the Central African Republic and Brazil, and it has been found that most of them occur in placers [3]. Framesite is used more broadly to describe clusters of randomly oriented microcrystalline diamonds found in association with the kimberlites all over the world. In addition, framesite is also called diamondite, which is a monomineral rock consisting almost entirely of diamonds and minor amount of other minerals such as silicate and sulfide [4, 5]. Study on the polycrystalline diamonds has not only important scientific significance but also notable practical value. Firstly, in the polycrystalline diamonds, there are randomly oriented microcrystalline diamond grains and high densities of grain boundaries which effectively impede the fracture propagation of the ( 1 1 1 ) cleavage in diamonds, so their hardness, toughness, and wear resistance
References
[1]
Y. L. Orlov, The Mineralogy of the Diamond, John Wiley & Sons, New York, NY, USA, 1977.
[2]
P. J. Heaney, E. P. Vicenzi, and S. De, “Strange diamonds: the mysterious origins of carbonado and framesite,” Elements, vol. 1, no. 2, pp. 85–89, 2005.
[3]
G. J. H. McCall, “The carbonado diamond conundrum,” Earth-Science Reviews, vol. 93, no. 3-4, pp. 85–91, 2009.
[4]
G. Dobosi and G. Kurat, “Trace element abundances in garnets and clinopyroxenes from diamondites—a signature of carbonatitic fluids,” Mineralogy and Petrology, vol. 76, no. 1-2, pp. 21–38, 2002.
[5]
G. Dobosi and G. Kurat, “On the origin of silicate-bearing diamondites,” Mineralogy and Petrology, vol. 99, no. 1, pp. 29–42, 2010.
[6]
T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, and K. I. Funakoshi, “Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature,” Physics of the Earth and Planetary Interiors, vol. 143-144, pp. 593–600, 2004.
[7]
D. E. Jacob, K. S. Viljoen, N. Grassineau, and E. Jagoutz, “Remobilization in the cratonic lithosphere recorded in polycrystalline diamond,” Science, vol. 289, no. 5482, pp. 1182–1185, 2000.
[8]
D. E. Jacob, R. Wirth, F. Enzmann, J. O. Schwarz, and A. Kronz, “Constraints on presses of diamond formation from inclusions in polycrystalline diamond (Framesite),” in Proceedings of the 9th International Kimberlite Conference Extended Abstract (IKC '08), 2008, Abstract no. 9IKC-A-00159.
[9]
T. Maruoka, G. Kurat, G. Dobosi, and C. Koeberl, “Isotopic composition of carbon in diamonds of diamondites: record of mass fractionation in the upper mantle,” Geochimica et Cosmochimica Acta, vol. 68, no. 7, pp. 1635–1644, 2004.
[10]
R. Burgess, L. H. Johnson, D. P. Mattey, J. W. Harris, and G. Turner, “He, Ar and C isotopes in coated and polycrystalline diamonds,” Chemical Geology, vol. 146, no. 3-4, pp. 205–217, 1998.
[11]
M. Honda, D. Phillips, J. W. Harris, and I. Yatsevich, “Unusual noble gas compositions in polycrystalline diamonds: preliminary results from the Jwaneng kimberlite, Botswana,” Chemical Geology, vol. 203, no. 3-4, pp. 347–358, 2004.
[12]
W. Wang, E. Takahashi, and S. Sueno, “Geochemical properties of lithospheric mantle beneath the Sino-Korea craton; evidence from garnet xenocrysts and diamond inclusions,” Physics of the Earth and Planetary Interiors, vol. 107, no. 1–3, pp. 249–260, 1998.
[13]
T. Jiang and K. Xu, “FTIR study of ultradispersed diamond powder synthesized by explosive detonation,” Carbon, vol. 33, no. 12, pp. 1663–1671, 1995.
[14]
J. E. Field, The Properties of Natural and Nynthetic Diamond, Academic Press, London, UK, 1992.
[15]
Y. F. Meng, C. S. Yan, J. Lai et al., “Enhanced optical properties of chemical vapor deposited single crystal diamond by low-pressure/high-temperature annealing,” Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 46, pp. 17620–17625, 2008.
[16]
A. M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer, Berlin, Germany, 2001.
[17]
R. M. Chrenko, R. E. Tuft, and H. M. Strong, “Transformation of the state of nitrogen in diamond,” Nature, vol. 270, no. 5633, pp. 141–144, 1977.
[18]
Z. Yang, H. Li, M. Peng, J. Chen, F. Lin, and Y. Su, “Study on the HPHT synthetic diamond crystal from Fe–C(H) system and its significance,” Chinese Science Bulletin, vol. 53, no. 1, pp. 137–144, 2008.
[19]
P. Cartigny, J. W. Harris, and M. Javoy, “Diamond genesis, mantle fractionations and mantle nitrogen content: a study of δ13C–N concentrations in diamonds,” Earth and Planetary Science Letters, vol. 185, no. 1-2, pp. 85–98, 2001.
[20]
O. Navon, I. D. Hutcheon, G. R. Rossman, and G. J. Wasserburg, “Mantle-derived fluids in diamond micro-inclusions,” Nature, vol. 335, no. 6193, pp. 784–789, 1988.
[21]
S. R. Boyd, F. Pineau, and M. Javoy, “Modelling the growth of natural diamonds,” Chemical Geology, vol. 116, no. 1-2, pp. 29–42, 1994.
[22]
B. Harte and J. W. Harris, “Lower mantle mineral associations preserved in diamonds,” Mineralogical Magazine A, vol. 58, pp. 384–386, 1994.
[23]
P. Deines, J. W. Harris, and J. J. Gurney, “Carbon isotope ratios, nitrogen content and aggregation state, and inclusion chemistry of diamonds from Jwaneng, Botswana,” Geochimica et Cosmochimica Acta, vol. 61, no. 18, pp. 3993–4005, 1997.
