Corundum - a mineral, crystalline α- alumina (Al 2 O 3 ), trigonal syngony , ditrigonal-scalenohedral. It has the following crystal-chemical structure: aluminum cations are located in the octahedral voids between six oxygen ions . Each oxygen ion is surrounded by six oxygen ions, and each oxygen ion is bonded to four aluminum ions. The crystals have a prismatic or bipyramidal shape. Due to the development of the faces of bipyramids, crystals often take on a characteristic barrel-shaped appearance [1] . The density of corundum is 4000 kg / m³.
| Corundum | |
|---|---|
 | |
| Formula | Al 2 O 3 |
| Molecular mass | 101.96 |
| Physical properties | |
| Colour | Blue, Red, Yellow, Brown, Gray, Pink |
| Trait color | Missing |
| Shine | Glass, frosted |
| Transparency | Transparent, opaque |
| Hardness | 9 |
| Fragility | Fragile |
| Cleavage | Not |
| Kink | Uneven |
| Density | 3.9-4.1 g / cm³ |
| Melting temperature | 2050 ° C |
| Crystallographic properties | |
| Point group | 3m (3 2 / m) |
| Space group | R3c (R3 2 / c) |
| Syngonia | Trigonal |
| Number of formula units (Z) | 6 |
| Optical properties | |
| Optical type | Anisotropic |
| Refractive index | n α = 1.767 - 1.772 n β = 1.759 - 1.763 |
| Birefringence | Low, primarily gray and white |
| Optical relief | Tall |
| Color in reflected light | Colorless |
| Optical axis dispersion | strong |
| Pleochroism | Invisible |
Content
Origin
Typical hypogenic mineral . It is found in igneous rocks , poor in silica (such as syenites , nepheline syenites ). Large crystals are found in pegmatites genetically related to these rocks. Grains and metacrystals , reaching 10 cm in diameter, are characteristic of metamorphic rocks of the deep stage of metamorphism - gneisses , garnet amphibolites , granulites . It is also formed during contact-metamorphic changes in sedimentary rocks rich in alumina - emery . The bulk of the precious varieties are found in placers .
Technical corundum is obtained by heat treatment of bauxite .
Crystal Shape
It occurs in the form of separate or intergrown groups of barrel-shaped, dipyramidal and tabular crystals of pseudo-hexagonal shape, isolated inclusions and grains, dense granular aggregates , fine-grained solid masses.
Varieties
Corundum as a mineral species has the following varieties [2] :
- Ruby , “red yacht ” - red; a gem of the first category, the price of transparent, well-colored specimens is more than that of diamonds.
- Sapphire , “blue yacht” - of blue color of different intensity. With a moderately intense cornflower blue, this is a gem of the first category, but is appreciated much lower than ruby. Too dark or too light sapphires are pretty cheap.
- green ( obsolete. eastern emerald ).
- purple ( obsolete. eastern amethyst ).
- Padparaja - yellow or orange-yellow.
- Star ruby - an exotic gem with the effect of asterism , processed in the form of a cabochon .
- Leucosapphire or "Oriental Diamond" - a colorless and completely transparent corundum; inexpensive gem.
- Common corundum is opaque, coarse or fine-grained, grayish in color. Sometimes in large opaque crystals. Due to its high hardness it is used as an abrasive material, due to the high melting point it is used as a refractory material, as well as in the manufacture of enamels .
- Emery is a solid granular corundum rock of gray-black color with admixtures of hematite , magnetite , etc. [3]
- Synthetic corundum - is produced by heat treatment of various high alumina raw materials. Its lilac-pink varieties are called the French rose or the rose of France [4] .
Associated Minerals
Andalusite , sillimanite , rutile , diaspora , gibbsit , boehmite , spinel , serpentine , nepheline , feldspars , chlorite , mica , magnetite .
Deposits
The main world deposits: Zanskar (pc. Kashmir ), India ; near Balangoda, Rakvana and Ratnapura - Sri Lanka ; Adam's bridge - a chain of rocks, beams and sandbanks between India and about. Sri Lanka ; Ambusitra, about. Madagascar Marbles of Mogok, Burma ; Thailand about. Naxos , Greece ; Bancroft, Craigmont, Renfrew (Ontario) - Canada ; Norway Cortland (pc. New York ), USA . In Russia: Sigangoyskoye ( Krasnoyarsk Territory ), Ilmen Mountains and Cherry Mountains ( Chelyabinsk Region , Urals ), the Rai-Iz massif, the Polar Urals , Kosoy Brod, the Middle Urals , the Kola Peninsula , Karelia.
Usage
Granular masses of corundum ( emery ) and artificial corundum are used as abrasive material . Artificial corundum was previously used in the manufacture of corundum needles (instead of expensive diamond) for electro- playing devices for reproducing gramophone records . Transparent varieties of corundum (sapphire , ruby ) are precious stones and are widely used in jewelry. Large, well-formed crystals are mined as collection artifacts. High-quality corundums are used in the manufacture of precise mechanisms, lasers, and other optical devices [3] . The pink vacuum corundum ceramic is used as an insulator with high thermal conductivity in the production of military tubes and microchip housings in military design. From leucosapphire, produced artificially, make high pressure sodium lamp burners.
There is a known case of finding a processed stone with traces of corundum abrasive in Egypt, dating back to the New Kingdom [5] .
See also
- Electrocorundum
Notes
- ↑ V.V.Dobrovolsky. Geology, mineralogy, dynamic geology, petrography .. - Moscow: VLADOS, 2001. - P. 72. - 320 p.
- ↑ Corundum and its varieties
- ↑ 1 2 M.I.Kadenskaya. Guide to practical exercises in mineralogy and petrography. - Moscow: Enlightenment, 1976. - S. 193. - 240 p.
- ↑ B. F. Kulikov, V. V. Bukanov Dictionary of gem stones, Leningrad, "Nedra", 1988, p. 92
- ↑ Evidence for the use of corundum abrasive in Egypt from the Great Aten Temple at Amarna .