Commercial granites and marbles

Granites in the commercial sense are hard natural stones that are polishable and they must be worked on by harder tools than marbles for cutting, shaping, and polishing.

They are usually suitable for internal and external use. Commercial granites have different geological origins and minerals. Petrographically, they are either magmatic or metamorphic rocks. True granites, i.e., granites in the scientific sense, are only one group. They are light magmatic rocks formed by crystallization 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) AND METACONGLOMERATES.

Commercial granites are mixtures of minerals and are composed of visible multicolored mineral grains. A grain of one color is typically encircled with grains of other colors, e.g. grey quartz is close 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 color (in true marbles).

In commercial granites, the larger the grain size, the lower the strength and the greater 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.

The characteristic feature of metamorphic rocks is a pattern with bands, schlieren (streaks), or waves. Every part of a slab usually has a different appearance.

Every commercial granite contains feldspars of 6 Mohs Hardness of various colors – white, pink, red, yellow, brown, green, and grey. Feldspar grains are typically not translucent and are with cleavage. Many granites, especially of lighter colors, contain quartz of 7 Mohs Hardness with grey (sometimes bluish, brownish) color, 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 colors. These minerals have larger specific gravity and lower hardness than feldspars and quartz. Some granites (e.g., KASHMIR WHITE) contain garnets of almost round shape and brown to dark-red color.

The more the quartz, the lighter the granite in color, and the larger the microcrack porosity (because quartz volume decreases during crystallization), and so the lower the strength, and the lower the resistance to fire. The darker the granite, the more the dark minerals, and the heavier 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 the quartz, and thus the lower the hardness.

Commercial granites are not affected by common organic acids such as lemon acid and vinegar, contrary to marbles.

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