Introducing Semi-gem quality blue corundum from the Alvand complex, Hamedan, west Iran

Abstract

Semi-gem quality corundum crystals are formed in a syenite pegmatite dyke in the Alvand igneous-metamorphic complex in west Iran. The dyke is intruded the igneous rocks and the surrounding hornfelses. Corundum is mainly associated with K-feldspar in the rocks and makes crystals 2 to 60 mm across. It shows gray, bluish-gray, pale blue, ink blue and dark blue colours. The crystals show colour zoning and trapiche-like zoning in hand specimens and under the microscope. The main mineral phase inclusions in corundum crystals are muscovite, phlogopite, hercynite and Nb-Ta oxides. LA-ICP-MS and electron microprobe analyzes indicate wide range of Fe/Mg, Fe/Ti, Ti/Mg and Ga/Mg ratios for the analyzed crystals while variation of Cr/Ga ratio is limited. The high Fe and Ga and low Mg and Cr contents suggest a magmatic origin for these corundums. Ti content was found as the main factor controlling the colour.