The term granite comes from the Latin root word
granum, meaning "grain". The geological definition of granite is "any
plutonic rock in which the mineral quartz makes up 10 to 50 per cent of
the felsic components, and the ratio of alkali to total feldspar is
between 65 and 95 per cent." Commercially, any holocrystalline
quartz-bearing plutonic rock is generally included in the granite group.
The granite group is one of the most versatile stone types available.
Granite, and granite-like materials, are capable
of taking a wide variety of finishes which allow the designer to
custom-tailor the stone to the aesthetic or performance requirements of
a specific application.
Resistance to scratching and durability in foot
traffic areas are largely dependent upon the hardness of the minerals
that make up the stone. In most granites, the primary minerals are
quartz and feldspars, accounting for approximately 90% of the stone.
The hardness of a mineral is oftentimes defined by use of Moh's Scale
of Relative Hardness, developed in 1822 by the Austrian Mineralogist
Friedrich Moh. This scale lists 10 minerals in ascending order of
scratch resistance:
1. Talc 2. Gypsum 3. Calcite 4. Fluorspar 5. Apatite
6. Feldspar 7. Quartz 8. Topaz 9. Corundum 10. Diamond
This scale can be further expanded by adding other
minerals or common materials with scratch resistance that is similar to
those minerals originally cited by Moh:
1. Talc, Sulpher
2. Gypsum, Amber 2½ Fingernail
3. Calcite, Coral (3-4), Pearl (3-4) 3½ Copper penny
4. Fluorspar, Fluorite, Rhodochrosite
5. Apatite, Turquoise (5-6)5½ Opal, Steel knife blade
6. Feldspar 6½ Hardened steel file, Common window glass
7. Quartz, Garnet, Beryl 8. Topaz 9. Corundum 10. Diamond
It should be noted that the above scales are of
"relative" hardness, and not linear. As example, there is significantly
less difference between 7 and 8 on the list than there is between 9 and
10. What the scale does tell us is that a mineral that can be scratched
with a fingernail has a hardness of less than 2½. A mineral that can be
scratched with a pocketknife, but not with a penny, has a hardness of
between 3½ and 5½. Feldspar and quartz, with hardness of 6 & 7
respectively, are the minerals that give granite its exceptional
abrasion resistance. This abrasion resistance contributes to its long
service life in high traffic areas of public buildings.
The dimensional stability of granite is very good,
so good in fact, that granite is the material of choice for high
precision applications such as surface plates, machine mounts and press
rolls, where tolerances can be measured in micro-inches (millionths of
an inch). Granite, like any solid, will expand and contract with
changes in temperature. This change is relatively small. The
coefficient of linear thermal expansion of granite is typically in the
neighborhood of 4.4 x 10-6 inches per inch per degree Fahrenheit. In
the perspective of common dimension stone panels, this means that a 5'
0" [1524 mm] panel would change dimension by approximately 0.026" [0.67
mm] in a 100 [56] temperature change. Granite will typically return to
its original dimension when the original temperature is reestablished.
Permanent strain, or failure to return to its original dimension will
not normally occur unless the material has been heated to excessive
temperatures (above 480 [250]).
Industrial processing vats containing sulfuric
acid, hydrochloric acid, nitric acid, and bromine are commonly lined
with granite panels, taking advantage of the material's natural
resistance to these caustic chemicals. This level of chemical
resistance contributes to the ability of granite to resist attack from
airborne pollutants associated with acid rain and/or snow-melting
chemicals. Certainly there are chemicals that will attack granite, but
exposure to them in a typical building environment would be extremely
rare.
Flexural strength, or the ability to resist
bending force, is a factor that determines the allowable span of a
dimension stone panel in a given thickness subjected to given loads.
Flexural strength varies amongst different types of granite, and
typically is between 1,000 and 2,000 lbs/in². This allows the use of
"thin" (30 mm) panels for many applications, minimizing both
curtainwall cost and dead load for the building frame. Thicker granite
panels (15/8" [40 mm], 2" [50 mm] or greater) are available where spans
or loads necessitate their use.
For applications that are below grade or in
contact with soil, water absorption is an important property.
Absorption rates of granites range from 0.10% and 0.40% by weight.
Furthermore, most granite materials will effectively allow water to
evacuate during freezing cycles to prevent surface damage from the
freezing water. Repetitive freeze/thaw cycles, particularly saturated
cycles, will result in a reduction of strength in the granite panel.
This loss can be significant, perhaps 20%. Laboratory experiments have
shown that the strength loss occurs most aggressively in the first 100
cycles, after which the strength loss is much slower paced.
(cn-stonenet.com)
