Granites in the commercial sense are hard natural stones which are polishable
and they must be worked on by harder tools than for marbles for cutting,
shaping and polishing.
They are usually suitable for internal and external use. Commercial granites
have different geological origin and minerals. Petrographically, they
are either magmatic or metamorphic rocks. True granites, i.e. granites
in the scientific sense, are only their one group. They are light magmatic
rocks formed by crystalization from magma under the earth‘s surface.
COMMERCIAL SENSE SCIENTIFIC SENSE
GRANITES MAGMATIC ROCKS: GRANITES, SYENITES, DIORITES, GAbbROS, CHARNOCKITES
METAMORPHIC ROCKS: GNEISSES, MIGMATITES, GRANULITES, QUARTZITES (polishable),
Commercial granites are mixtures of minerals and are composed of visible
multicoloured mineral grains. Grain of one colour is typically encircled
with grains of other colours, e.g. grey quartz is closed to pink orthoclase,
white plagioclase and dark mica in true granite.
On the contrary, marbles in the commercial sense are either without visible
grains of calcite (in limestones) or are composed of grouped calcite grains
of similar colour (in true marbles).
In commercial granites, the larger the grain size, the lower is the strength,
and the greater is the brittleness, because mineral cleavage can manifest
better in larger grains. A homogenous structure or a mildly-oriented one
is a feature of magmatic rocks. Magmatic rocks have a similar appearance
(structure, pattern) even over a large area of a slab.
Characteristic feature of metamorphic rocks is pattern with bands, schlierens
(streaks) or waves. Every part of a slab has usually a different appearance.
Every commercial granite contains feldspars of 6 Mohs Hardness of various
colours – white, pink, red, yellow, brown, green and grey. Feldspar
grains are typically not translucent and are with cleavage. Many granites,
especially of lighter colours, contain quartz of 7 Mohs Hardness with
grey (sometimes bluish, brownish) colour and grains are glassy translucent
without cleavage. Further, there are dark minerals such as hornblende,
pyroxene and biotite with black, dark green or dark brown colours. These
minerals have larger specific gravity and lower hardness than feldspars
and quartz. Some granites (e.g. KASHMIR WHITE) contain garnet of almost
round shape and brown to dark-red colour.
The more the quartz, the lighter is the granite in colour, the larger is
the microcrack porosity (because of quartz volume decrease during crystalization),
and so the lower is the strength, and the lower is the resistance to fire.
The darker the granite, the more are the dark minerals, and the heavier
is the granite.
Granites are hard and cannot be scratched by a nail, knife or glass piece
unlike marbles. Depending on the feldspars and quartz portions, the total
Mohs Hardness of granite is from 5.5 to 7. The darker the granite, the
lesser is the quartz, and thus the lower is the hardness.
Commercial granites are not affected by common organic acids such as a
lemon acid and vinegar, contrary to marbles.