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CRYSTALLOCHEMICAL CHARACTERISTICS AND COLOR ORIGIN OF JEWELRY DEMANTOIDS AT THE SKALNY MANIFESTATION (POLAR URALS, RUSSIA)

A. G. Nikolaev

Abstract

This paper presents geology aspects of jeweler demantoid new manifestation, which was recently discovered in the Polar Urals. The mineralogical characteristics of the manifestation are described, demantoids chemical analysis results are shown, and comparative analysis of demantoids with samples from classical Ural deposits is given. The crystal chemical characteristics of demantoids from this manifestation are studied and color origin of these garnets is investigated. Demantoid mineralization is presented in two types: single rounded grains and granular discharges from 2 to 5 mm, which form small chains up to 1-2 cm; edges of the garnet. The garnet purity is low; there are numerous inclusions of tremolite, gas-liquid inclusions, hollow channels and cracks. Crystals are categorized as cleanliness 2. The presence of byssolite is the most characteristic sign for demantoids under study (the ?horse tail? inclusion). The nature of byssolite distribution, morphology of its aggregates allows us to talk about several types: clustered secretions; needle inclusions; matted-fibrous aggregates. The Skalny manifestation is confined to the western endocontact of the ultra-massif Cyum-Keu massif and is represented by veins zone, in which there are numerous veins of jadeite, albite-jadeite and plagioclase-jadeite bodies. The host rocks for garnets are serpentized ultrabasic rocks. Aggregates are found in cracks (rupture) which filled by tremolite. Demantoids comparative analysis of the Skalny manifestation and famous Ural field the Novo-Karkodinskoe deposits (Middle Urals) with following comparison criteria were used: geology, deposit, structure of veins, morphology, gemological characteristics. Garnet mineralization was studied in the Skalny manifestation and garnet group mineralogical series was established: andradite, demantoid, topazolite and uvarovite. Together with garnet, talc, antigorite, tremolite and chromite are found. The noble varieties of andradite (topazolite, demantoid) can be used as a material for obtaining gemstones of average quality.

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