Raman investigations and ab initio calculations of natural diamond-lonsdaleite originating from New Caledonia

Here, we report the first occurrence so far of cubic diamond and lonsdaleite in a siliceous breccia hosted in nickel laterite from the Tie & PRIME;baghi mine, New Caledonia. Diamond and lonsdaleite polytype occurred as minerals in surficial formed siliceous rocks. The vibrational properties and the calculated elastic properties and electronic band structure of 3C-diamond and lonsdaleite were studied by micro-Raman spectroscopy and ab initio calculation. Our measured Raman spectra demonstrate the coexistence of cubic diamond and lonsdaleite based on the correlation between ab initio calculation and experimental results. We showed that the position of the single Raman vibration mode for cubic diamond corresponds to the first order scattering of F-2g symmetry at 1332 cm(-1). For lonsdaleite, the ab initio calculations predict three fundamental vibrational modes: 1207 cm-1 (E-2g), 1307 cm(-1) (A(1g)), and 1330 cm(-1) (E-1g). The calculated indirect bandgaps of 3C-diamond and lonsdaleite at room temperature are 5.7 eV and 5.2 eV, respectively. The elastic anisotropy calculations show that lonsdaleite has the greatest shear modulus, bulk modulus and young's modulus compared to cubic diamond, which indicates a high degree of hardness of this hexagonal structure of diamond.