Morphology and FTIR Characteristics of the Alluvial Diamond from the Yangtze Craton, China

A total of 48 natural alluvial diamonds from the Yangtze Craton, China, also called Hunan diamonds, were studied using morphology and IR spectroscopy. These diamond samples, collected downstream of the Yuan River, Hunan Province, with unknown host-rock source(s), were observed by scanning electron microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR). Most Hunan diamonds are monocrystal forms of octahedra, tetrahexahedra (THH) and dodecahedra; octahedral–rhom-dodecahedral transitional behaviors and irregular forms are also visible. Trigons and tetragons, terraces and shield-shaped laminae are surface features that frequently indicate dissolution and reabsorption; green and brown spots, network patterns, and other mechanical abrasion marks are typical evidence of long-time deposition and transportation of Hunan diamonds. The main types of Hunan diamonds are type IaAB and type Ⅱa. Diamond samples have a wide range of total nitrogen content (N_tot) from 196–1094 ppm. Two populations are distinguished by two-peak distribution models of N_A (A-center concentrations) and %B (proportion of aggregated nitrogen). Hunan diamonds are low in structure hydrogen (0.03–4.67 cm⁻¹, mostly below 1 cm⁻¹) and platelets (0.23–17 cm⁻¹, mostly below 2 cm⁻¹). Moreover, there is a significant positive correlation between the hydrogen correlation peak and N_tot, which is similar to Argyle diamonds. The temperature conditions of the diamond formation have been estimated at 1075–1180 °C, mainly conforming to the kimberlite diamond range. Besides, some samples with slightly higher temperatures are close to the ultramafic-related Juina diamonds. Therefore, the FTIR characteristics analysis and comparison indicate the multiple sources of Hunan diamonds.