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. The point group symbols are given following the Shoenflies and the Hermann-Mauguin symbols as given in the International Tables for Crystallography, Vol. Select a point group symbol from the table in order to get the information about the Wyckoff positions of the chosen three-dimensional crystallographic point group.

Wyckoff Positions of 3D Crystallographic Point Groups Wyckoff Positions 3D Crystallographic Point Groups Wyckoff positions of three-dimensional crystallographic point groups 3D Point Groups WP Bilbao Crystallographic Server

Crystalmaker wyckoff symbol