Dianite, rare Siberian gemstone, - mineralogy, origin, and terminology issues

Abstract

Dianite, rock named after the Princess Diana, was first found and described by Konev et al. (1988). It occurs only in one deposit on the Earth -the Kedrov stock within the alkaline Murun complex in Yakutia (Eastern Siberia). Both deep-blue colour and nephrite-resembling fabrics determine the gemquality properties of this rock (Fig. 1). Dianite has formed in a complex magmatic-hydrothermal system overprinted by a later metasomatic event, as shown by i.e. the presence of combined W-and M-type tetrad effect (Dumańska-Słowik et al., 2022).It is mainly composed of amphibole-supergroup species represented by: early-magmatic magnesio-hastingsite -pargasite, late-magmatic potassic-magnesio-arfvedsonite, and minor replacive potassic-richterite (Dumańska-Słowik et al., 2022). Among these species, the main component is potassic-magnesioarfvedsonite, which appears as fine-fibrous crystals that tightly interlock with anhedral K-feldspars. Subordinately, a peculiar mica containing Na in octahedral sites has also been found in this rock. It forms nest-like aggregates, whilst its composition is close to shirokshinite that has been described only in Khibiny massif (Kola Peninsula, Russia) -see Pekov et al. (2003).In the marketplace, dianite is offered as ,,blue jadeite" or ,,Siberian blue nephrite". However, the term jade strictly corresponds to two tough, fine-grained gem materials of strictly metamorphic affinity and mostly muted green colouration: jadeite and nephrite (Hobbs, 1982). Thus, dianite does not follow the basic criterion of jade, even though it shows fibrous-felted micro-texture related to the presence of amphibole-supergroup species.