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stone introduction
Comparing Granite to OtherBuilding Stones
Natural stones are considered to be the oldest building material known to man. The use of natural stone materials adds both permanence and elegance to modern construction. Any natural stone, or combination of natural stones, will enhance your building space with an aesthetically pleasing image while providing a faade of high performance with low maintenance.
By geological definition, there are hundreds of stone types commonly used as dimension stone products. The commercial definitions of stones are much broader, allowing stones with similar mineralogy, workability, performance, and behavior to be combined into one classification, discounting the fact that they may be scientifically classified as different stone types. For example, stones such as gabbro, diabase, diorite, anorthosite, etc., are marketed commercially as granites because their properties are similar, even though they are not true granites by geological definition.
Most stones used in dimension stone applications will fall under one of five commercial definitions: Granite, Marble, Limestone, Quartz-Based, or Slate.

The Granite Group
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. 1Talc
  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
    2 Fingernail
  3. Calcite, Coral (3-4), Pearl (3-4)
    3 Copper penny
  4. Fluorspar, Fluorite, Rhodochrosite
  5. Apatite, Turquoise (5-6)
    5 Opal, Steel knife blade
  6. Feldspar
    6 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 2. A mineral that can be scratched with a pocketknife, but not with a penny, has a hardness of between 3 and 5. 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 100F [56C] 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 480F [250C]).
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.

The Marble Group
Like granite, the term "marble" has both a geological and commercial definition. Geologically, marble is a "metamorphic rock consisting of fine to coarse-grained recrystallized calcite and/or dolomite." Commercially, the term marble is used to describe any crystallized carbonate rock including true marble and certain limestones (orthomarble) that is capable of taking a polish. Travertine and serpentine, while not true marbles, are usually included in the commercial definition of marble.
A geologic marble of pure calcite or dolomite would be white. Marbles are available in a vast array of colors, and the variety of colors is the result of impurities found in the marbles, such as iron oxides, carbonaceous minerals, mica, chlorite or silicate.
The Marble Institute of America defines 4 classes of marbles in descending order of soundness as A, B, C, & D. Research of a particular marble is necessary to determine its suitability for the application, particularly exterior applications. Marbles in the C & D soundness classifications have the most limitations in fabrication and application, yet many of the highly decorative marbles with exotic veining fall into these groups.
There is a wide variation of physical properties in the stones that make up the marble group. Generally, the flexural strength, compressive strength, and abrasion resistance are lower than what is typically found in the granite group. This may result in either reduced spans or increased thicknesses for cladding, or increased wear in high traffic areas. As long as the material is properly selected for the application, the performance should be satisfactory.
Some marbles exhibit hysteretic behavior after repeated heating and cooling cycles. This hysteresis is typically evident by a bowing of the marble panels, and can normally be eliminated if the panels are of sufficient thickness. Research of the particular marble should be done to determine its vulnerability to this phenomenon.
The Limestone Group
Limestone is a sedimentary rock consisting mainly of the mineral calcite (calcium carbonate) with or without dolomite (magnesium carbonate). The color of limestone is altered by the presence of impurities, which broaden the color spectrum of limestone to include white, brown, gray, buff, yellow, red, block, or mixtures of these colors.
Limestone, being of sedimentary origin, is usually quite anisotropic, or "directionally specific" in its behavior. This accounts for a pronounced "rift", or plane of easiest splitting, within most limestone types.
The flexural strength of limestone usually necessitates the use of thicker panels for cladding applications, where 3" or 4" thicknesses are not uncommon.
Due to high absorption and susceptibility to staining, Limestone is not generally used is applications where it comes into contact with soil. A limestone clad building utilizing another stone type for the exterior base course is traditionally an attractive solution to this limitation.
The composition of this stone type allows for the cutting of profiles by means of "planing". The use of a plane to shape the stone makes profiles pieces (e.g. cornice or moulding pieces) more economical than in other stone types.

The Limestone Group
Limestone is a sedimentary rock consisting mainly of the mineral calcite (calcium carbonate) with or without dolomite (magnesium carbonate). The color of limestone is altered by the presence of impurities, which broaden the color spectrum of limestone to include white, brown, gray, buff, yellow, red, block, or mixtures of these colors.
Limestone, being of sedimentary origin, is usually quite anisotropic, or "directionally specific" in its behavior. This accounts for a pronounced "rift", or plane of easiest splitting, within most limestone types.
The flexural strength of limestone usually necessitates the use of thicker panels for cladding applications, where 3" or 4" thicknesses are not uncommon.
Due to high absorption and susceptibility to staining, Limestone is not generally used is applications where it comes into contact with soil. A limestone clad building utilizing another stone type for the exterior base course is traditionally an attractive solution to this limitation.
The composition of this stone type allows for the cutting of profiles by means of "planing". The use of a plane to shape the stone makes profiles pieces (e.g. cornice or moulding pieces) more economical than in other stone types.

The Quartz-Based Group
Sedimentary stones that have high contents of quartz and silica are included in the commercial definition of "quartz-based" stones. Stone types such as sandstone, bluestone, and brownstone are included in this group. The quartz grains in such stones are cemented together by another material, commonly silica, iron oxide, or calcium carbonate. A wide variety of oxidized trace minerals will account for varying colors from light buff, to dark blue-gray, to reddish brown.
Being sedimentary in origin, all quartz-based stones have specific bedding planes and a pronounced anisotropic behavior.
Quartz based stones have been used for centuries as flagging materials, ashlar, copings, sills, hearths, and mantles. Modern fabrication capabilities allow its use in cladding applications, although it will generally be of 3" or greater thickness.

The Slate Group
Slate is a compact, fine grained metamorphic rock, commonly derived from shale. The presence of carbonaceous materials and/or iron compounds are responsible for producing the wide variety of colors available in slates, including gray, black, green, red, purple, yellow, brown, and buff.
Of all the dimension stone types, slate is perhaps the most anisotropic in its behavior and properties with a very pronounced rift. These cleavage planes allow slate to be split into very thin sheets with exceptionally high flexural strength. The combination of easy splitting characteristics and exceptionally high flexural strength allow its use in 3/16" (5 mm) thick roofing materials. It is the only dimension stone type to be used in this application.
Not all slates exhibit permanence of color in exterior exposures, therefore slates are usually classified as being either "fading" or non-fading" varieties.

Measurement of Physical Properties
The various physical properties of dimension stones are tested by means of the procedures documented by ASTM. ASTM also publishes standards for the major stone types, listing the minimum/maximum values to be expected from a particular stone type in a particular test. It should be noted that there are many stones that do not meet these values, yet have demonstrated satisfactory performance in a variety of applications. This table should then be considered to be more of a general guide than an absolute pass/fail gauge.
Table of Dimension Stone Testing Values per ASTM Standard Specifications
  Absorption (max) per ASTM C 97 Density (min) per ASTM C 97 Modulus of Rupture (min) ASTM C 99(9)(10) Compressive Strength (min) ASTM C 170 Abrasion Resistance (min) ASTM C 241 Flexural Strength (min) ASTM C 880
Stone Type ASTM Standard % lbs/ft3 kg/m3 lbs/in2 Mpa lbs/in2 Mpa Ha lbs/in2 Mpa
Granite ASTM C 615 0.40% 160 2,560 1,500 10.34 19,000 131 25 1,200 8.27
Marble ASTM C 503 0.20% 162 2,590 1,000 6.89 7,500 52 10 1,000 6.89
Limestone (1) ASTM C 568 12.00% 110 1,760 400 2.76 1,800 12 10 n/a n/a
Limestone (2) ASTM C 568 7.50% 135 2,160 500 3.45 4,000 28 10 n/a n/a
Limestone (3) ASTM C 568 3.00% 160 2,560 1,000 6.89 8,000 55 10 n/a n/a
Quartz - Based(4) ASTM C 616 8.00% 125 2,000 350 2.41 4,000 28 2 n/a n/a
Quartz - Based(5) ASTM C 616 3.00% 150 2,400 1,000 6.89 10,000 69 8 n/a n/a
Quartz - Based(6) ASTM C 616 1.00% 160 2,560 2,000 13.79 20,000 138 8 n/a n/a
Slate (7) ASTM C 629 0.25% n/a n/a 9,000 62.05 n/a n/a 8 n/a n/a
Slate (8) ASTM C 629 0.45% n/a n/a 7,200 49.64 n/a n/a 8 n/a n/a
Notes:
1) Low Density Limestone
2) Medium Density Limestone
3) High Density Limestone
4) Sandstone
5) Quartzitic Sandstone
6) Quartzite
7) For Exterior Use
8) For Interior Use
9) Test Procedure for Modulus of Rupture Testing of Slate is C 120.
10) Modulus of Rupture values for slate are taken across the grain.
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