Identification of Opaque Sulfide Inclusions in Rubies from Mogok, Myanmar and Montepuez, Mozambique

Abstract

The red variety of corundum owes its color and strong fluorescence to the presence of Cr, as well as traces of Fe. The latter can reduce the fluorescence and thus impact the appearance of the final gem. Gem quality rubies are rarely available for scientific study and even less common in their rough form. Opaque inclusions in rubies are often removed during faceting and remain unidentified. This study aims to identify opaque inclusions in rubies from the two most common origins seen in the high end market today: Mogok, Myanmar and Montepuez, Mozambique. Using electron probe microanalaysis (EPMA) the inclusions were identified as sphalerite and pyrrhotite in Mogok rubies. The paragenesis of Myanmar, marble-related rubies is fairly well understood and no Fe-rich minerals apart from sulfides have been identified. Opaque inclusions in Mozambican rubies are a complex mix of Fe-Cu-Ni sulfides with exsolution textures. These inclusions are interpreted to be small amounts of sulfide melt trapped during corundum formation. The different sulfide phases crystallized from this entrapped melt and some phases experienced later exsolution during cooling. The formation of amphibole-related, Mozambican rubies is not well understood, but it is obvious that very different processes are at work compared to the marble-related Myanmar ruby deposits.

Editorial for Special Issue “Mineralogy and Geochemistry of Ruby”

“…An earlier suggested predominance of pyrite for Mogok sulfide inclusions had misidentified the phase, which is now identified as pyrrhotite. Similarly, claimed chalcopyrite in Montepuez rubies has proved to be largely Fe-Ni-Cu sulfides [13].…”

“…This not only provides a solid foundation on the mineralogy and geochemistry involved in this gem varietal, but also presents an encompassing survey of its global deposits through historic and geological time. From this supporting synthesis, the satellite papers present their own particular points of interest that carry the ruby theme into new research territory [8][9][10][11][12][13]. Research into gem corundum, however, is a continuing process, and new aspects are already appearing since publication of the papers in the Special Issue.…”

“…The rarer sphalerite inclusion within Mogok rubies was linked to low grade metamorphism of the host marble platform. In contrast, the more complex Montepeluez sulfide inclusions are accompanied by greater ranges in Fe contents within the rubies, giving less noticeable fluorescence effects [13].…”

“…A further Mogok ruby study by Vertriest and Palke [13] targeted the identity and genetic implications of opaque sulfide inclusions in the marble-hosted rubies. The authors then compared the Mogok inclusion features against those of opaque sulfide inclusions in Mozambique rubies, which formed in a very different host lithology and genetic growth processes.…”

Mineralogy and Geochemistry of Ruby

“…Mineral inclusions are also key features in the debate about metamorphic versus magmatic origin of rubies. The most common solids are clinopyroxene (Al-rich diopside), plagioclase (bytownite to andesine), K-feldspar-spinel association [148,149], garnet (pyrope, ± almandine), sapphirine, scapolite, Cr-spinel, meionite, phlogopite, anatase, apatite, sillimanite, and sulphides [5,68,77,105,145,146,154,156]. All of these minerals are described in rubies found in metamorphic environment, although some of them are also found in magmatic assemblages.…”