![]() In igneous rocks it occurs both as dispersed very small grains and as large crystals in pegmatites and it is also found as a detrital mineral in sediments. It occurs in metamorphic rocks such as amphibolites, eclogites and kimberlite xenoliths. Rutile (TiO 2) has a tetragonal structure (space group P4 2/ mnm.). It has low reflectance in reflected light and the high hardness can cause difficulties during polishing that leave many surface scratches that are nevertheless an aid to identification. Corundum has a high hardness (9 on Mohs scale). There is often some zonation and alteration lamellae may be present. In thin section corundum has a high relief and it is generally colorless although tinges of the red of ruby or the blue of sapphire can sometimes be apparent. These unstable phases are β-Al 2O 3 (hexagonal), γ-Al 2O 3 (cubic), δ-Al 2O 3 (tetragonal or orthorhombic), κ-Al 2O 3 (orthorhombic or hexagonal), ε-Al 2O 3 (hexagonal), η-Al 2O 3 (cubic) and σ-Al 2O 3 (cubic). In addition to corundum (α-Al 2O 3) there are natural and synthetic Al 2O 3 phases that are not considered as polymorphs but are transitional stages between corundum and the Al-hydroxides. The effect of these impurities is to color the corundum, causing the red of ruby, the blue of sapphire and green, yellow, orange, pink, and brown varieties ( Fig. Most of these elements are usually only present in amounts up to a few hundred ppm but Cr can reach 2–3 wt% and Fe up to 1 wt%. Ĭorundum consists of nearly pure Al 2O 3 with trace to minor amounts of Cr, V, Fe, Ni, Ga, Ca and Mn whilst Na, Mg, Si, Co, Cd, Ga, S and Zr can also occur. Weldon, owned by J Ambalu, Amba Gem Corp, reproduced with permission from Hughes RW, Manorotkul W, and Hughes EB (2017) Ruby & Sapphire: A Gemologist's Guide. Oxford, England: CrystalMaker Software Ltd, (C) Reproduced with permission from Bowles JFW, Howie RA, Vaughan DJ, and Zussman J (2011) The Rock Forming Minerals, vol. Crystal Structure Images Generated Using CrystalMaker®: A Crystal and Molecular Structures Program for Mac and Windows. ![]() ![]() (C) the color of corundum is determined by the proportions of the trace elements. (A) The unit cell of corundum viewed down the z axis, (B) the unit cell viewed down the x axis. ![]()
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