Corundum, 4.16

Corundum has a trigonal structure. The oxygen ions are arranged in approximately hexagonal closed packing. Between the oxygen layers there are sites 

The emission of natural spinel exhibits a great deal of fine structure, which was ascribed to Cr luminescence (Tarashchan 1978 Mohler and White 1995). The natural spinel in our study consisted of three samples. The laser-induced time-resolved technique enables us to detect Cr and possibly Mn emission centers (Fig. 4.51). 

The dominant features in the structure are the hexagonal rings of six Si-0 tetra-hedra, these rings forming hollow columns parallel to the z-axis. Within the 

The steady-state emission of beryl has been previously studied. The broad band at 720 nm is connected with Fe , while the relatively narrow bands at 480 and 570 nm are ascribed to Mn in tetrahedral and octahedral coordination, respectively. Cr emission was connected with narrow i -lines at 680 and 682 nm (Tarashchan 1978 Kuznetsov and Tarashchan 1988). 

The main colored varieties are ruby (red) and sapphire (blue). The corresponding color centers may present in different quantitative and qualitative proportions with resulting varieties of colors (up to 2000 different tints). The main color center in ruby is impurity of Cr (up to 4 %). Ion Cr generates two strong absorption bands in the visible part of the spectrum, which explain the red color. Different tints of red, which influence strongly on the commercial value, are connected with impu- 

An interesting feature exists in classical Cr luminescence in corundum. The weaker peaks from the long wavelength side from those lines are also known and ascribed to so call N-lines, namely Cr pairs. The lines from the short wavelength side are less famous (Fig. 4.118). The peak at 658 nm is corrected to combination of R3 and R2 lines from Ti level, at 668 nm to Rj from the same level, at 675 and 

680 nm are anti-Stokes phonon repetitions as result of superposition of the energies of electron transitions connected to R-lines with the energies of atomic vibrations (Nelson and Sturge 1965). 

The spinel structure is a cubic close packing of anions bound together by interstitial cations. The lattice is face-centered cubic, space group Fd3m, with eight MgAl204 

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Aluminium is not found free but its compounds are so widespread that it is the most abundant metal in the earth s crust. Aluminosilicates such as clay, kaolin (or china clay), mica and feldspar are well known and widely distributed. The oxide. AI2O3. occurs (anhydrous) as corundum and emery, and (hydrated) as bauxite. Cryolite. Na,AlF. (sodium hexafluoroaluminate). is found extensively in Greenland. 

Aluminium oxide occurs naturally as emery (an impure form) and as corundum. Corundum is a crystalline form which may be coloured by traces of impurity, for example as ruby (red) and sapphire (blue). Small synthetic rubies and sapphires have been made by heating alumina with the colouring oxide in an oxy-hydrogen flame. 

The compounds of greatest importance are aluminum oxide, the sulfate, and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery, and is used in glassmaking and refractories. Synthetic ruby and sapphire are used in lasers for producing coherent light. 

Corundum. Comndum [1302-75-5] (see Aluminum compounds) is a naturally occurring massive crystalline mineral composed of aluminum oxide. It is an impure form of the gems mby and sapphke. Prior to 1900 comndum was an important abrasive for the production of grinding wheels. Today it is mainly employed as a loose abrasive for grinding and polishing optical lenses. Almost all the world s supply of comndum now comes from Africa, primarily from Zimbabwe. 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

Hard Materials (7) Quartz, granite (8) topaz (9) corundum, sapphire, emeiy (10) diamond. 

Corundum (AljOj) Vegetable oil mists (except castor cashew nut, or similar irritant oils) Sucrose, Tin Oxide, Titanium Dioxide Silicoh Carbide... 

On ihe Mohs scale diamond is 10 and quartz 7. An abemative measure is the Knoop hardness (kgmm ) as measured with a 100-g load typical values on this scale are diamond 7000, boron carbide 2750, corundum 2100. topaz 1340, quartz 820. hardened tool steel 740. 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

A completely different structural motif has very recently been found in the red-brown phosphide CasPg, formed by direct fusion of Ca metal and red P in the correct atom ratio in a corundum crucible at 1000 C. The structure comprises Ca + cations and Pg anions, the latter adopting a staggered ethane conformation. (Note that P+ is isolobal with C and P with H so that C2H6 = [(P+)2(P"")6] = Ps - -) The internal P-P distance is 230.1pm and the terminal P-P distances 214.9-216.9 pm, while the internal PPP angles are 104.2-106.4° and the outer angles are 103.4-103.7°. 

When is approximately the same size as Ti (i.e. M = Mg, Mn, Fe, Co, Ni) the stmcture is that of ilmenite, FeTi03, which consists of hep oxygens with one-third of the octahedral interstices occupied by and another third by Ti This is essentially the same structure as corundum (AI2O3, p. 243) except that in that case there is only one type of cation which occupies two-thirds of the octahedral sites. 

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