An unusual paragenesis of diamond, graphite and calcite: a Raman spectroscopic study
Thomas, R and Rericha, A and Davidson, P and Beurlen, H, An unusual paragenesis of diamond, graphite and calcite: a Raman spectroscopic study, Estudos Geologicos, 31, (2) pp. 3-15. ISSN 1980-8208 (2021) [Refereed Article]
This work presents a study of an unusual synthetic diamond sample, where calcite is the main phase in addition to diamond. Using Raman spectroscopy it can be shown that in addition to diamond carbon forms four different generations: highly ordered graphite-II, lower ordered graphite-I as spandrel between different mineral grains, weakly-ordered carbonaceous material as graphite-III and the hexagonal diamond lonsdaleite (here graphite-IV) as last formation in calcite. Graphite-III can be found mainly in the calcite body. According to the fine dispersion of this carbonaceous material and the arrangement on grain boundaries, we assume that carbon was dissolved in the calcite melt, and that by the activated state of carbon in the calcite melt the formation of diamond is favored near 1760°C, and 6.8 GPa. Lonsdaleite as minor phase may have been formed as an longlived intermediate state under standard conditions between graphite and diamond. Evidence shows that there are two different lonsdaleite phases (possibly hexagonal and monocline) present. The prevailing diamond is characterized by the first-order Raman line at 1333 cm-1. However, there are also present diamonds with the first-order Raman line down to 1310.6 cm-1, corresponding to 13C = 0.511. Significantly there is the strong decrease of the optical damage threshold with increase of the 13C content.