Sectorial Structure of the Ural Demantoids and “Horsetail” Inclusions

The presence of “horsetail” inclusions—hair-like inclusions diverging in bundles from the central part of the grain towards its periphery—is considered a sign of the Ural origin of demantoid, although such inclusions are not unique to the Urals. Demantoid grains are usually rounded and have a spherulitic or sectorial structure (spherocrystals). Well-formed crystals are rare, but they contain the same inclusions, which emerge at the surface as hollow channels with a polygonal cross-section. The question arises: are these growth features or the result of dissolution? Demantoid grains and crystals were studied by optical methods, scanning electron microscopy (SEM) and EBSD mapping. It was established that when a cluster of demantoid crystal nuclei arose, their growth proceeded under conditions of geometrical selection and was blocked by adjacent crystals. Grains located at the periphery of the aggregate continued to grow in the direction of their sector. In nodules from serpentine–carbonate veins, the grains are separated by serpentine, whereas in oriented intergrowths of crystal nuclei with euhedral crystals, serpentine interlayers are absent. In demantoid crystals, the cross-sections of the hollow channels have the shape of a parallelogram or rhombus, a square and an equilateral triangle, which possibly allows them to be assigned to negative crystals. Demantoid crystals have a sectorial structure (spherocrystals) with numerous newly nucleated spherocrystals expressed by hyperboloids of revolution with induction surfaces and by hollow-channel inclusions. The cause of the sectorial growth of demantoid may be decompression conditions.