Corundum
[Chemical composition] Sometimes a trace amount of Fe, Ti, Cr, Mn, V, Si, etc. is present in corundum in the form of isomorphous substitution or mechanical mixing.
[Crystal Structure] Trigonal System; 
; a 0 = 0.477 nm, c 0 = 1.304 nm; Z = 6. The crystal structure is shown in Figure Y-3. O 2- into the hexagonal closest packing along the vertical three-axis direction, while Al 3+ is between the two O 2- layers, filling the octahedral void. The octahedron is coextensively layered in the parallel {0001} direction (Fig. Y-3(a)). In the parallel c-axis direction, the coplanar structure is formed into two solid [AlO 6 ] octahedrons (Fig. Y-3 ( b) The middle slanted square) and a hollow octahedron enclosed by O 2- (the blank square in Figure Y-3 (b)) are arranged in a column. The [AlO 6 ] octahedron pair is three-axis helically symmetric along the c-axis (Fig. Y-3(c)). Since the Al—O bond has the property of transitioning from a ionic bond to a covalent bond (covalent bond accounts for about 40%), the corundum has a characteristic of a covalent bond compound. The repulsive forces occur in the two adjacent Al 3+s , so the Al 3+ between the two O 2 layers is not in the same horizontal plane.
Figure Y-3 Crystal structure of corundum
(quoted from Pan Zhaoyu et al., 1993)
[Form] The crystal is usually in the shape of a lumbar drum or a column, and a few are plate-shaped or sheet-like (Fig. Y-4). Often according to rhombohedron {10 1}, less {0001} into a polycrystalline twin (Fig. Y-5), so that there are often several sets of intersecting stripes on the crystal plane. The crystal morphology of corundum is related to the composition of the medium during its formation: corundum produced in rocks with low SiO 2 content (such as syenite, plagioclase, etc.) has a long columnar shape and a nearly three-dimensional crystal form; The corundum in the rock with an increased SiO 2 content is characterized by a plate shape. The aggregate is granulated or densely packed.
Figure Y-4 Crystal of corundum
(quoted from Pan Zhaoyu et al., 1993)
[Physical properties] is generally gray, yellowish gray, those containing black Fe; Cr-containing red person who said ruby; containing Ti and said blue sapphire; {0001} can look at the surface of some of ruby and sapphire to exit into six acicular rutile directional distribution of the inclusions as a star-like color, said Leder star sapphire or ruby; glass luster. No cleavage; often produces {0001} or {10 due to polycrystalline twin or fine inclusions 1} split (Figure Y-6). Hardness 9. The relative density is 3.95 to 4.10. The melting point of 2 000 ~ 2030 ° C, chemical stability, not easy to corrode.
Figure Y-6. The corundum crystal is cracked by {0001} due to the polycrystalline twin or fine inclusions.
[genesis and occurrence] corundum can be formed in the process of magma, contact metamorphism and regional metamorphism. In the magma, corundum is formed under the condition of rich Al 2 O 3 and SiO 2 deficiency, so it is more common in corundum syenite and plagioclase or corundum syenite pegmatite. Corundum formed by contact metamorphism is found in the contact zone between igneous rocks and limestone. In the regional metamorphism, the clayey rock can be formed into a corundum chip by metamorphism. Corundum is often transferred into sand mines when weathered and damaged by various corundum deposits or rocks.
[Identification characteristics] The crystal form, double crystal stripe and high hardness are used as identification features.
[Main use] It mainly uses its high hardness as a bearing for abrasive materials and precision instruments. The crystal form is good, thick, beautiful and flawless. It is a high-grade gemstone, such as ruby, sapphire, star ruby, star sapphire and so on. Synthetic rubies can be used as laser materials.
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