IDENTIFICATION OF OPAQUE SULFIDE INCLUSIONS IN RUBIES FROM MOGOK, MYANMAR AND MONTEPUEZ, MOZAMBIQUE

Opaque, metallic inclusions are common in rough rubies, but most scientific studies do not have access to gem-quality rough samples. During the polishing and cutting of gems, opaque inclusions are routinely removed because they are very obvious and distract from the beauty of the gem. This makes it challenging to study them, even though they potentially hold valuable geological information.

The appearance of ruby is controlled by two elements: Cr which causes the red body color as well as the red fluorescence and Fe which can quench the fluorescence.

This study aims to identify opaque, metallic inclusions in rubies using electron probe microanalysis (EPMA). We selected samples from the two sources that provide the majority of high quality gems nowadays: Mogok in Myanmar and Montepuez in Mozambique. These rubies form in very different environments but the opaque inclusions look very similar.

The inclusions were identified as pyrrhotite and sphalerite in Mogok, marble-related rubies. Their paragenesis is fairly well understood, and pyrrhotite is so far the only identified Fe-rich inclusion in these rubies. This suggests that the formation of these sulfides is an important factor during the growth of Fe-free, strongly fluorescent rubies found in marbles.

Opaque inclusions in Mozambican rubies are a complex mix of Fe-Cu-Ni sulfides with exsolution textures. We interpret those as small amounts of sulfide melt trapped during corundum formation. During cooling, the different sulfide phases solidified eventually developing the exsolution textures.

The formation of these amphibole-related rubies is not understood. The Fe-content and fluorescence in these rubies varies more than in marble-related rubies. It is clear that very different process are at work in this environment and that the sulfides play a different role.