Ruby is mineralogically the chromiferous variety of corundum gemstone, in other words an aluminium oxide in which some of the aluminium ions have been substituted by chromium. Chromium contributes, along with vanadium, another metal constituent of ruby, to the crystal’s red colour. The most prized ruby deposits are those of Central and South-East Asia, like in the celebrated Mogok deposit in Myanmar (ex-Burma), from which the highest gem-quality rubies are extracted, reputed for their intense “pigeon blood” colour and their transparency.
In spite of their commercial interest, these deposits have attracted little geological research. However, they possess a special feature that have stimulated the interest of the IRD and CRPG/CNRS scientists for several years. The ruby crystals they hold are always present as inclusions in marble formations, which are calcareous rocks altered by high temperature. Research projects have been launched to determine in particular the origin of certain major constituents of the ruby which are usually absent from marbles—mainly aluminium, chromium and vanadium—, unravel the mechanisms of formation of these deposits, their age and their significance in the functioning of deep zones of the Earth’s crust. Combination of field data and results of laboratory geochemical analyses on samples taken from different deposits recorded from Afghanistan to Viet Nam, the team succeeded in establishing a new genetic model, valid for the whole of these deposits where ruby is associated with marble.
Ruby crystals form at high temperature, between 620 and 670°C. Dating of minerals contemporary with their growth, such as zircon and mica, performed in the different deposits gave the age of the ruby crystallization. Depending on the deposits, this was found to be 40 to 5 million years B.P., in the Cenozoic. The ruby is thus an excellent geological marker of the collision between the Indian and the Eurasian plates which set off the Himalayan uplift.
Marie Guillaume | alfa
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