The name garnet is derived from Latin “granatus” meaning “like a grain” (c.f., granite
derived from “granum” meaning “grain”; also the fruit called pomegranate the seeds of which
are similar to garnet grains). Garnet is the collective name for a group of silicates of two (one
bivalent and one trivalent) of the metals calcium, aluminium, magnesium, iron, manganese
and chromium. The individual minerals under this group are:
(i) Grossularite or hessonite [Ca3Al2(SiO4)3]
(ii) Pyrope [Mg3Al2(SiO4)3]
(iii) Almandite [Fe3Al2(SiO4)3]
(iv) Spessartite [Mn3Al2(SiO4)3]
Iron garnet Andradite [Ca3Fe2(SiO4)3]
Chromium garnet Uvarovite [Ca3Cr2(SiO4)3]
Out of these, the iron-containing garnets, almandite and andradite are the most common.
Many garnets occurring in the nature are admixtures of the various varieties .Garnet may
occur as a primary mineral in schist, gneiss and pegmatite, and as a secondary mineral in
beach and river placers.
The most important uses of garnet are:
- Coarse polishing: The most important industrial use of garnet is in the manufacture of
coated abrasive-like paper (garnet paper), cloth (garnet cloth) and discs for removing rust or
worn-out paint of metal surfaces, sizing of metals and wood, for smoothening leather, etc.
Sodium silicate, gelatin or cattle hide glue is generally used as the bonding material for
mounting garnet grains on the base. For making garnet paper/cloth garnet grains are sprayed
or gravity-dropped on sheets of paper/cloth coated with heated bonding material (shellac,
gelatin etc.). Hardness (6.5-7.5 on Mohs scale), and subconchoidal fracture with sharp edges
are the key criteria. Crushed grains of primary garnet crystals are more effective than rounded
grains of secondary placer garnet, but the latter can also be used for less hard surfaces like
those of wood, plastic, rubber etc.
- Fine polishing: Micronized garnet mounted on paper/cloth is used for fine polishing of
delicate surfaces like TV picture tubes (for achieving high picture resolution), for fine
190 Kaulir Kisor Chatterjee
polishing wood surface etc. Here also, hardness (6.5-7.5 on Mohs scale), and subconchoidal
fracture with sharp edges are the key criteria and crushed grains of primary garnet crystals are
used. In this application, grain size, consistency of the size and angularity of the grains are the
most important quality parameters. If the size is too large then the grains will be very angular
and will scratch the surfaces. On the other hand, very small-sized grains will be rounded and
will be ineffective. The industries use a minimum of 10 micron size, because grains finer than
this tend to be too rounded. The most preferred size range, however, is 20-35 micron.
- Sand blasting: Garnet sand is fired under pressure usually in the presence of water to
suppress the dust against the material to be polished. For this purpose the chemical
composition of the sand is not important, but absence of clay and other water-soluble
clogging materials is very important. Moderate specific gravity of garnet contributes to the
force of the blast and is an important criterion in this usage.