Granite

Granite, from igneous rock, is a very hard, crystalline, and primarily composed of feldspar, quartz accompanied by one or more dark minerals. It is visibly homogeneous in texture.

The term "Granite" means "grain" in Latin word "Granum" because of its granular nature.

Granite is the hardest building stone, and granite slabs and granite tiles occupy a prominent place among dimensional stones. Due to its hardness, resistance to weathering, capability to take mirror polish, fascinating colors and textural patterns, granite slabs and granite tiles are extremely popular.

The principal characteristics of granite also include high load bearing capacity, crushing strength, abrasive strength, amenability to cutting and shaping without secondary flaws, ability to yield thin and large slabs and - above all - durability.

Due to highly dense grain, it is impervious to stain. Polished granite slabs and granite tiles have achieved a special status as building stones globally. Granite is also used for wall cladding, roofing, flooring, and a variety of other interior and exterior applications.

India is one of the largest producers of granites blocks and processed granite slabs and granite tiles. Granites from leading Indian exporters are listed here.

Physical Properties of Granite

In terms of its physical properties granite is a unique material. These properties lending uniqueness to granite are:

Porosity/permeability

Granite has almost negligible porosity ranging between 0.2 to 4%.

Thermal Stability

Granite is thermally highly stable, so it shows no changes with the changing temperature. Granite is impervious to weathering from temperature and even from the air borne chemicals. It is the highly resistance to chemical erosion which makes granite useful for making tanks to store highly caustic material.

Co-efficient of expansion

The co-efficient of expansion for granite varies from 4.7x10-6 - 9.0x10-6(inch x inch).

Variegation

Granite shows consistency in color and texture.

Hardness

It is the hardest building stone and hardness of granite that lends it excellent wear.

Granite Composition

Granite is formed of quartz and orthoclase or microcline. The principal constituents of granite are :

  • Feldspar
  • Quartz
  • Biotite

The percentage composition of each varies and accordingly impart different color and texture to the final product. The percentage composition of feldspar varies between 65-90%, of quartz can extend from 10 to 60% and that of biotite lies between 10 to 15%.

Chemical Properties of Granite

Granite is primarily composed of feldspar, quartz along with various other minerals in varying percentage, which are stated as follows:

Nominal chemical composition of various oxides
Silica(SiO2)

70-77%

Alumina(Al2O3)

11- 14%

Potassium Oxide(P2O5)

3 - 5%

Soda(Na2O)

3 - 5%

Lime

1%

Iron(Fe2O3)

1-2%

Iron(FeO)

1 - 3%

Magnesia(Mg0)

5 - 1%

Titina

Less than 1% (.38%)

Water(H2O)

0.03%
Uses of Granite
  • Roadstone, building blocks, but has poor resistance to fire as it crumbles when exposed to intense heat.
  • Traditionally the main use of black granite is in tombstones and other monumental items, and to a lesser extent for various types of counters, especially in the financial world, decorative applications, elegant trimmings and luxury floorings.
  • Stones on bolo cord slides and pendants, paperweights, bookends, lazy susan turntables, etc.

Marble

Marble, formed from limestone with heat and pressure over years in the earth's crust. These pressure or forces cause the limestone to change in texture and makeup. The process is called recrystallization. Fossilized materials in the limestone, along with its original carbonate minerals, recrystallize and form large, coarse grains of calcite.

Impurities present in the limestone during the recrystallization period affect the mineral composition of the marble which is formed. At relatively low temperatures, silica impurities in the carbonate minerals form masses of chert or crystals of quartz. At higher temperatures, the silica reacts with the carbonates to produce diopside and forsterite. At a very high temperatures, rarer calcium minerals, such as larnite, monticellite, and rankinite, forms in the marble. If water is present, serpentine, talc, and certain other hydrous minerals may be produced. The presence of iron, alumina, and silica may result in the formation of hematite and magnetite.

The minerals that result from impurities give marble a wide variety of colors. The purest calcite marble is white in colour. Marble containing hematite are reddish in color. Marble that has limonite is yellow, and marble with serpentine is green in colour.

Marble does not split easily into sheets of equal size and must be mined with care. The rock may shatter if explosives are used. Blocks of marble are mined with channeling machines, which cut grooves and holes in the rock. Miners outline a block of marble with rows of grooves and holes. They then drive wedges into the openings and separate the block from the surrounding rock. The blocks are cut with saws to the desired shape and size.

Physical Properties of Marble

Colour

White, Pink, Block, Green, Shaded, Striped

Luster/Gloss

Glassy

Reflectivity

80-95%

Transparency

Translucent

Hardness

2.8-3.5

Abrasive Index

8-42

Solubility

Soluble residue negligible

Sp. gravity

2.7 - 2.9

Absorption

Negligible

Strength

Capacity to resist stress, depends upon rift, hardness, degree of cohesion & interlocking (56-190mpa)

Transverse Strength

4.2 - 28 mpa

Compressive Strength

50 - 90 mpa
Chemical Composition of Marble

Indian marble has the following main chemical constituents in its composition:

  • Soluble residue - 0.89%
  • Fe2o3 - 0.28%
  • CaCo3 - 97.74%
  • MgCo3 - 1.22%
  • Phosphoric Acid - 0.04%
Uses of Marble

Marble has always been highly valued for its beauty, strength, and resistance to fire and erosion. The ancient Iranian & Greeks were good user of marble in their buildings and statues. The Italian artist Michelangelo used marble from Carrara, Italy, in a number of sculptures. Marble from Tennessee was used in parts of the National Gallery of Art in Washington, D.C. The Lincoln Memorial, also in Washington, was built of marble from Alabama, Colorado, and Georgia.

Very pure calcite marble is used for most statues. They are translucent. Large blocks of colored marble are used for columns, floors, and other parts of buildings. Smaller pieces of marble are crushed or finely ground and used as abrasives in soaps and other such products. Crushed or ground marble is also used in paving roads and in manufacturing roofing materials and soil treatment products.

Textures of Marbles

Marbles show variety of textures on account of existing minerals & re-crystallization patterns. Texture depends upon form, size, uniformity of grain arrangements. Marbles can be classified on the basis of the following factors :

  • Calcite Marble - Mostly CaCo3; MgCo3<0.50%
  • Dolomite Marble - Having > 40% MgCo3
  • Magnesium Marble - MgCo3 between 5 to 40%
  • Serpentine Marble - remobilised marble due to the effect of Thermodynamic metamorphic wherein serpentine is prominent
  • Onyx Marble - Lime carbonate deposition on account of cold water solution activity
Impurities in Marble

The following are the major mineral impurities in marble:

  • Quartz
  • Tremolite Actinolite
  • Chert
  • Garnet
  • Biotite
  • Muscovite
  • Microline
  • Talc
  • Fosterite

The following are the major chemical impurities in marble:

  • SiO2
  • Fe2O3
  • 2Fe2O3
  • 3H2O
  • Limonite
  • Manganese
  • Al2O3
  • FeS2(pyrite)

Colour variations occur as a result of the difference in mineral compositions.

Onyx

The name comes from the Greek word onyx which means nail of a finger or claw. Legend says that one day while Venus was sleeping Eros/Cupid cut her fingernails and left the clippings scattered on the ground. Because no part of a heavenly body can die, the gods turned them into stone which later became known as onyx.

It is a member of the chalcedony family which also includes carnelian, agate, bloodstone, etc. Occuring naturally in a variety of colors, black onyx is polished to enhance appearance, a member of the quartz family.

Originally, almost all colors of chalcedony from white to dark brown and black were called onyx (SiO2 with impurities). Today when we think of onyx we often preface the word with black to distinguish it from other varieties of onyx that come in white, reddish brown, brown and banded. A variety of onyx that is reddish brown with white and lighter reddish bands is known as sardonyx.

Physical Properties of Onyx

Onyx is a form of chalcedony, part of the quartz group. Like the other stones in the chalcedony family, onyx has a smooth, waxy luster. Onyx can be found in a wide range of colors, often layered.

  • Specific Gravity : 2.58 - 2.64
  • Refractive Index : 1.55 - 1.54
  • Hardness : 6.5 - 7.0 on Moh's scale
Chemical Properties of Onyx
  • Formula : SiO2
  • Elements : Si and O
Uses of Onyx

Onyx is often used as a back splash material, also in vanities and bathroom walls. It is more of a decorative stone, not something suitable for kitchen countertops; too easy to scratch, too porous.

Enhancements of Onyx

Black and colored onyx is always dyed. Onyx is easily dyed, so beware of the more "unnatural" looking colors, such as bright blue, etc. Onyx with red and white bands is sometimes referred to as "Sardonyx". Onyx is the black variety of agate. However some parts are more or less translucent. This is why it is often heated to accentuate the intensity of its color.