The results of x-ray diffraction and IR-spectroscopic studies of samples of nonstoichiometric aluminomagnesium spinel Mg 1 -x Al 2 -y O 4 -z (OH) z formed after treating aluminomagnesium spinel synthesized by the sol-gel method at temperatures of 800 and 1100°C with concentrated acids, are reported. It was found that three phases of nonstoichiometric aluminomagnesium spinel Mg 1 -x Al 2 -y O 4 -z (OH) z with tetragonal lattices and a different type of superstructure are formed. The x-ray patterns of the two phases correspond to body-centered lattices with parameters a = 5.712 (3) Å, c = 8.092(3) Å, and a = 5.714(3) Å. The x-ray pattern of the third phase was indexed in a primitive tetragonal lattice: a = 5.718(3) Å, c = 24.261(27) Å.The noble spinel MgAl 2 O 4 -a mineral of high pressures and temperatures [1] -is encountered in the form of single, developed, octahedral crystals in metamorphic, highmagnesium rocks, has high hardness (7.5 -8.0 on the Moos scale), and despite the low atomic mass of magnesium and aluminum, high density (3.59 g/cm 3 ). Due to the possibility of substituting Mg 2+ and Al 3+ with transition metal ions (Mg 2+ ® Fe 2+ , Fe 3+ , Mn 2+ , Co 2+ ; Al 3+ ® Fe 3+ , Cr 3+ , V 3+ , Mn 3+ , Fe 2+ ), many varieties are found in nature [2]. The best known are ceylonite -a spinel of intermediate composition between MgAl 2 O 4 , and hercynite FeAl 2 O 4 ; chlorspinel -a bright green variety of intermediate composition between noble spinel and magnesioferrite MgFe 2 O 4 containing up to 7.2 wt.% Fe 2 O 3 ; chromspinel (composition intermediate between spinel and picrochromite MgCrO 4 ); gahnospinelwith a composition intermediate between spinel and gahnite ZnAl 2 O 4 . Aluminomagnesium spinels enriched with vanadium, nickel, cobalt, and manganese have been found. With respect to the variety of tints and shades of color, noble spinel is as good as multicolored corundum. Ruby spinelbright red, pink, and pinkish-red, purple-red, and orangered -is especially distinct among jewelry spinels. An alexandrite-like spinel whose color varies from violet in daylight to reddish-violet in artificial light, and "star spinel," whose asterism is due to microscopic rutile needle inclusions have been described. Pigments based on solid solutions of (Mg, Fe) [Al, Ti, Cr, Fe] 2 O 4 are widely used for decorating ceramic ware. The structure of noble spinel is of the type of structures incorporated in very close packing of atoms of the same size. The first studies of the structure of MgAl 2 O 4 (1915) showed that it is a derivative of cubic three-layer very close packing with filled tetrahedral and octahedral vacancies. There are eight formula units (Z = 8) -Mg 8 Al 16 O 32 -per unit cell. The structure of noble spinel is described by Fedorov symmetry group Fd3m. Structurally equivalent positions 8(a) A, 8(b) A * , 48(f) A ** correspond to 64 tetrahedral vacancies in it; 16(d) B, 16(c) B * correspond to 32 octahedral vacancies. Magnesium ions occupy octuple position 8(a) with point symmetry 43m; aluminum ions occupy centrosym...
