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Corundum crystals of semi-gem quality (sapphire) formed in a syenitic dyke around the plutonic rocks in the Alvand igneous-metamorphic complex were studied by Sheikhi Gheshlaghi et al. (2020). These authors studied field relations and petrography of a corundum-bearing pegmatite, and proposed a genetic model for corundum formation. Although studies on whole rock chemistry of corundum-bearing pegmatites and oxygen isotope data for corundum by these authors are valuable, some of their reports and findings – such as the sharp contact between corundum-bearing pegmatite and Alvand syenite, the lack of tourmaline group minerals in corundum-bearing pegmatite, the presence of syngeneic quartz inclusion in corundum and the genesis of corundum as a result of kyanite break down – are in contrast with our own observations and findings. Neither the P-T conditions, nor the volatile-bearing minerals present, are in favour of formation of corundum from kyanite breakdown. Based on present authors' studies, mineral chemistry of corundum in the Alvand area clearly indicates magmatic origin. [ABSTRACT FROM AUTHOR]

In the Hamadan region, Sanandaj-Sirjan Zone (SaSiZ), W Iran, a pegmatite-aplite vein occurs that contains up to 15 vol% sapphire. The Hamadan sapphire is black to blue, and translucent to opaque, which reveals its semi-gem and gem quality. On the basis of petrographic studies, the Hamadan sapphire-bearing pegmatite-aplite vein represents alkali syenitic to syenitic composition and its adjacent granitoids have a range of composition from syenitic to quartz monzonite and granodiorite. New petrographic, whole-rock geochemical data and δ18O values [9.30–12.45‰VSMOW (Vienna Standard Mean Ocean Water)] suggest crustal origin for the peraluminous, S-type and alkaline sapphire-bearing pegmatite-aplite vein. The presence of syngenetic quartz inclusions in the sapphire, and some similar physical-gemological signatures (e.g., parting patterns and angles) of the Hamadan sapphire and kyanite support the generation of the sapphire at the expense of kyanite. In addition, a secondary metasomatic stage may have played a minor role in formation of the sapphire grains. The results indicate an important role for the mafic magmas in triggering crustal anatexis and gem crystallization in continental subduction zone settings. [ABSTRACT FROM AUTHOR]