Abstract
In this paper, Raman spectroscopy experiments were used to distinguish the characteristics of inclusions (calcite, anatase, graphite etc.) between natural and heat-treated tanzanite. These characteristics were preliminarily divided according to their pleochroism. In natural unheated tanzanite (N5), calcite inclusion is often interspersed with dolomite and has Raman shifts around 156, 283, 710, and 1087 cm−1. In other high temperature treatment samples, the baseline of calcite increased and their Raman peaks gradually shifted towards lower frequencies. Anatase inclusions in natural tanzanite (N5) have four characteristic Raman peaks around 146, 394, 514, and 641 cm−1. Because of the longer Ti-O bond and the wider bond angle distribution caused by high temperature, fewer Raman peaks were observed and the peaks’ intensities were weakened in the heat-treated T7 sample. The black graphite inclusions are often scattered or have a dotted distribution. The most obvious difference between natural and heat-treated samples is that the latter lack the characteristic 1350 cm−1 Raman peak of graphite, thus representing the order and structural incompleteness of graphite. In addition, there are other inclusions in natural unheated tanzanite, such as lead-grey molybdenite with strong metallic luster, randomly scattered prehnite with white dots, orange-yellow rounded rutile, and metallic luster hematite.
Highlights
Tanzanite is a blue-violet vanadium-containing zoisite, which was discovered in1967 [1] and is a form of calcium-aluminum silicate, with an idealized formula ofCaAl3 Si2 O7 [SiO4 ]O(OH) [2]
These deposits are mainly located in metamorphic rocks, marble, and schist of the Neoproterozoic Mozambique Metamorphic Belt (NMMB) [3]
Tanzanite and tsavorite are the results of vanadium replacing aluminum in crystal structures, and the formation of tanzanite seems to be related to tsavorite
Summary
Tanzanite is a blue-violet vanadium-containing zoisite, which was discovered in1967 [1] and is a form of calcium-aluminum silicate, with an idealized formula ofCaAl3 Si2 O7 [SiO4 ]O(OH) [2]. The tanzanite mineralization occurs in pressure shadows created by the filling of hydrothermal fissures and boudinage, and the whole process is controlled by the boudinage in the graphite gneiss [8]. According to their fission track dating study, Naeser and Saul determined that tanzanite crystals were formed 585 ± 28 million years ago [9]. Some minerals are frequently associated with tanzanite, e.g., other colored zoisite (brown and yellow), graphite, diopside, pyrite, quartz, grossular (tsavorite), dolomite, and calcite [11]
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