Corundum

The corundum , or “Alumina alpha”, is a Minéral composed of smoothed Alumine Anhydre Cristal, of formula Al2O3. It is noted also α-Al2O3.

Certain natural varieties of corundum are invaluable stones. The Ruby and the Saphir are the varieties of Sesquioxyde of aluminum most frequently used in jewelry. The variety emery of this stone is a Céramique very much used in industry, primarily like Abrasif because of its Dureté, and like Réfractaire.

The hardness of corundum is of 9 on the scale of hardness Mohs, that the stone is natural or artificial, which makes of it the second natural mineral hardest after the Diamant. This hardness is explained by covalent nature atomic connections.

Uses

Abrasive, the corundum is used in certain stones to sharpen, but especially in all kinds of Meule S. This use seems to date from Antiquity, in particular in Minoan Crete and China.

Crystalline structure

The corundum has a noted structure D51 in Notation Strukturbericht. It is a rhomboedric structure, of group of space R {\ bar {3}} c (n°167). A reason is composed of two reversed Al2O3 pentahedrons which are repeated with the nodes of the rhombohedron. Its parameters of mesh are:

  • has = 5,13 Å;
  • α = 55,47° = 55°28'.
A pentahedron makes 2,7 Å top (Al3+-Al3+ distance) and 2,49 Å on side (O2--O2- distance).

The O2- ions form a network compact Hexagonal, with thus an alternation of plans A-B; the Al3+ ions occupy the two-thirds of the octahedral sites intersitiels, with three types of plans has, B and C in alternation. There is thus a A-a-b-B-c-a-A-b-c-B-a-b-A-c-a-B-b-c-A-a alternation… The parameters of mesh in this hexagonal description are:

  • has = 4,76 Å;
  • C = 12,97 Å.
Compared to preceding description, the distance between the plans of O2- of the pentahedrons is C /2.

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