Garnets


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Garnets

 

minerals related to the silicates that form a complex group of variable composition, with the general chemical formula R32+R23+[SiO4]3, where R2+ is Mg, Fe, Mn, or Ca, and R3+ is Al, Fe, or Cr. The CaFe3+ type can be replaced by NaTi4+. In the (SiO4) radical, the Si part can be replaced by Al3+ and the oxygen by (OH)-1(for example, in hydro-garnets).

The crystal structure of garnets is complex. The elementary cubic cell contains eight formula units. The isolated SiO 4tetrahedrons are connected by oxygen atoms to octahedral R3+O6 groups, forming something like a complex skeleton whose interstices contain R2+ surrounded by eight oxygen atoms. The entry of the large Ca2+ cation into the composition of the garnet greatly influences the crystal chemistry of individual minerals of the group—for example, the capacity for isomorphism with other members of the garnet group. Hence, at present the garnet group may be broken down as follows: (I) the Mg-, Mn-, and Fe2--garnets—pyrope Mg3Al2[SiO4]3, almandite Fe3Al2[SiO4]3, and spessartite Mn3Al2[SiO4]3; (2) the Ca-garnets—grossularite Ca3AI2-[SiO4]3, andradite Ca3Fe2[SiO4]3 uvarovite Ca3Cr2[SiO4]3 and the very rare goldmanite Ca3V2[SiO4]3.

The hydrogarnets—for example, plazolite Ca3Al2[SiO4] 2(OH)4, in which the SiO4 groups are replaced by four (OH) groups—are also distinguished. Mixed garnets are widespread in nature within the series that have been distinguished. Isomorphism among the various series of garnets is quite limited.

Garnets crystallize in the cubic system, forming isometric crystals of various shapes, but they more often form granular aggregates, concretions of crystals, and spherical grains. Optical anomalies—anisotropy—are sometimes observed in garnets, which testifies to the ability of these compounds to crystallize in other systems (such as the rhombic system). The hardness of garnets on the mineralogical scale depends on their composition and varies from 6.5 for hydrogarnets to 7.7 for Mg-, Fe-, and Mn-garnets. The density also varies, from 3,100 to 4,300 kg/m3.

Garnets vary from colorless, greenish yellow, green, brown, and black (in the case of Ca-garnets) to pink, red, and reddish brown in the case of Mg-, Fe-. and Mn-garnets. There are many names for the various varieties of garnet —for example, hessonite (Fe3+). which contains grossularite. Recently many artificial compounds with the structure of garnets have been produced, such as the Cal2Mn2[GeO4]3type, which are acquiring great industrial importance as semiconductors.

In nature garnets are formed in contact-metasomatic rock and ore bodies, or so-called garnet skarns (andradite and grossularite); they are also encountered in pegmatite veins (spessartite and almandite) and are the rock-forming minerals for gneisses and many metamorphic shales, metamorphosed basic rocks (pyrope and demantoid garnet), and plutonic rocks such as eclogites and kimberlites (pyrope) and are also widespread in placers of various origins. They sometimes form in hydrothermal veins. Varieties of garnets that are transparent and have beautiful colors (demantoid garnet, pyrope, and almandite) are used as gems. Opaque garnets of great hardness are used industrially as abrasives. The best almandites are found in deposits in India; pyropes. in Czechoslovakia; and demantoid garnets, in the USSR (in the Urals). In the USSR hydrogarnets are found in Transcaucasia.

REFERENCES

Merenkov, B. Ia. Dragotsennye, tekhnicheskie i podelochnye kamni. Moscow-Leningrad, 1936.
Strunz, C. Mineralogicheskie tablitsy. Moscow, 1962. (Translated from German.)
Betekhtin, A. G. Kurs mineralogii, 3rd ed. Moscow. 1961.

G. P. BARSANOV

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