Abstract
The term “demantoid”, first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. “Horsetail” inclusions are considered a sign of the Ural type demantoid. Although these inclusions are large (visible to the naked eye), their diagnostics remains debatable: some researchers attribute them to byssolite (amphibole-asbestos), others consider them chrysotile. We investigated the horsetail inclusions in the Ural demantoids through various methods: optical microscopy, scanning electron microscopy (SEM), Raman spectrometry, X-ray powder diffraction, and thermal analysis. In most cases, “horsetail” inclusions in the Ural demantoid were represented by hollow channels and only the outcrops, on the demantoid surface, were occasionally filled with serpentine (established by SEM); in one case, magnetite was observed. Hollow canals were usually collected not in bundles, such as a “horsetail”, but in fans, sometimes curved into cones. The structure of the grains was spheroidal, sectorial, and sometimes had induction surfaces, which, to the periphery of the grain, were replaced by tubular channels assembled in a fan. The specifics of the growth of the “horsetail” inclusions of the demantoid grains can be explained by the decompression conditions that arose when the ultrabasites (a crust-mantle mixture) were squeezed upwards during collision.
Highlights
Demantoid is a jewelry variety of andradite
“Horsetail” inclusions are represented by hair-like formations of various sizes assembled in fan or sheaf-like bundles tapering toward the central part of the grain
This can explain the problems associated with identifying these inclusions by optical microscopy, X-ray powder diffraction, thermal methods, and Raman spectroscopy
Summary
Demantoid is a jewelry variety of andradite. Its chemical formula is Ca3 Fe2 Si3 O12 , and it often contains admixtures of Cr, Ti, V, Al, and Mg. The demantoid was first discovered in gold-bearing alluvial placers of the Bobrovka river, 100 km NNW from Yekaterinburg (Figure 1). It was named in 1856 by the Finnish mineralogist Nils von Nordensheld for its strong dispersion (0.057), comparable with that of diamonds (0.044). In 1874, demantoids were discovered in gold-bearing placers near the Bobrovka river (a widespread name for small rivers in Russia) near the Poldnevaya village, 80 km south of Ekaterinburg (see Figure 1), by V.G. and A.V. Kalugin. In 1879, after the study of the chemical composition of the demantoid by mining engineer A.A. Loesch, the mineral was identified as the andradite garnet. Political events in Russia at the beginning of the 20th century stopped the development of the demantoid deposits in the Urals for many years
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