combination.
It also forms compact granular aggregates, rounded grains and
water-worn pebbles. It has an imÂperfect dodecahedral cleavage.
Fracture: Conchoidal to uneven. Hardness = 6-1/2 to 7-1/2. Specific
gravity = 3.4 to 4.3 depending upon the composition. Colours vary. All
colours except blue are met with, e.g. red, brown, yellow, green and
black are the more common colours and also colourless. The colours are
due to the elements that enter into the composition of the minerals of
the group, and also due to impurities present in small amounts. The
colourless garnets may be transparent to translucent, whereas the dark
coloured varieties are translucent to opaque. Lustre is vitreous to
resinous. Refractive index of gem garnets vary from 1.70 to 1.94, and
the dispersion ranges from 0.024 to 0.028, but in one instance
andradite goes up to 0.057, which is comparable with that of diamond.
When the colour is not dark, the garnets have considerable brilliancy
and fire. Garnets are isotropic and have no dichroism—this fact enables
rubies to be distinguished from garnet, although refractive index and
specific gravity may be more or less of the same order.
Composition For the whole group, the generalized formula M'3M*2 (SiO4)3,
in which M' represents the bivalent elements Mg, Ca, Mn, or Fe and M"
for trivalent elements as alumina, ferric iron or chromium. Six
varieties of garnets have been identified and also their physical and
chemical properties. Their composition grades one into the other, and
so also the physical and chemical properties.
