Carbonado-like diamond from the Avacha active volcano in Kamchatka, Russia

Abstract

In addition to a series of finds of diamond in mafic volcanic and ultramafic massive rocks in Kamchatka, Russia, a carbonado-like diamond aggregate was identified in recent lavas of the active Avacha volcano. This aggregate differs from ‘classic carbonado’ by its location within an active volcanic arc, well-formed diamond crystallites, and cementing by Si-containing aggregates rather than sintering. The carbonado-like aggregate contains inclusions of Mn–Ni–Si–Fe alloys, native β-Mn, tungsten and boron carbides, which are uncommon for both carbonado and monocrystalline diamonds. Mn–Ni–Si–Fe alloys, trigonal W2C and trigonal B4C are new mineral species that were not previously found in the natural environment. The formation of the carbonado-like diamond aggregate started with formation at ~850–1000°C of tungsten and boron carbides, Mn–Ni–Si–Fe alloys and native β-Mn, which were used as seeds for the subsequent crystallization of micro-sized diamond aggregate. In the final stage, the diamond aggregate was cemented by amorphous silica, tridymite, β-SiC, and native silicon. The carbonado-like aggregate was most likely formed at near-atmospheric pressure conditions via the CVD mechanism during the course or shortly after one of the volcanic eruption pulses of the Avacha volcano. Volcanic gases played a great role in the formation of the carbonado-like aggregate.