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G r a n i t e

Granite is an igneous rock of visible crystalline formation and texture. It is composed of feldspar (usually potash feldspar and oligoclase) and quartz, with a small amount of mica (biotite or Muscovite) and minor accessory minerals, such as zircon,  apatite,  magnetite,  limenite,  and sthene.

Granite is usually whitish or grey with a speckled appearance caused by the darker crystals. However potash feldspar imparts a red or flesh colour to the rock. In more recent years, more colourful and vivid varieties of granite have been discovered which have increased the choices available to architects. Granite crystallizes from magma that cools slowly, deep below the Earth's surface. Exceptionally slow rates of cooling give rise to a very coarse-grained variety called pegmatite. Granite, along with other crystalline rocks, constitutes the foundation of the continental masses, and it is the most common intrusive rock exposed at the Earth's surface.

The relative density of granite ranges from 2.63 to 2.75. Its crushing strength is from 1,050 to 14,000 kg per sq cm (15,000 to 20,000 lb per sq in). Granite has greater strength than sandstone, limestone, and marble and is correspondingly more difficult to quarry. It is an important building stone, the best grades being extremely resistant to weathering.


L i m e s t o n e

Limestone is a sedimentary rock consisting largely of calcium carbonate (CaCO3), usually in the form of the mineral calcite. It may be produced biologically, chemically, or physically. Most of the world’s ocean floors contain limestone that formed from the shells of dead marine organisms (such as foraminifera) that drifted downwards through the water and settled on the sea floor. Coquina and oolite are also organic forms of limestone. However, limestone may also be produced chemically, being forced to precipitate out from saturated seawater that can dissolve no more carbonate. In rarer instances it may also be produced physically, by the deposition of pre-existing limestone particles that have been washed down by rivers, although rivers would probably dissolve much of the limestone that entered them.

Limestone may contain a small percentage of the calcium-magnesium carbonate mineral dolomite, CaMg(CO3)2, and still be called a limestone, or sometimes dolomitic limestone. Moreover, unlike chalk, a particularly pure form of limestone, “limestone” may contain significant amounts of non-carbonate material such as silica, feldspar, clay, or pyrite.

Metamorphosed limestone is known as marble, however, not all the so-called marble is of true limestone origin.

The simple beauty and warmth imparted by limestone makes it an ideal medium for exterior facades as well as internal wall cladding and paving.


M a r b l e

Marble is a crystalline, compact variety of metamorphosed limestone, capable of taking a high polish. Commercially the term is extended to include any rock composed of calcium carbonate that takes a polish, and includes some ordinary limestones; the term is further extended in the loose designation of stones such as alabaster, and serpentine.

The surface of marble crumbles readily when exposed to a moist, acid atmosphere, but marble is durable in a dry atmosphere and when protected from rain. The purest form of marble is statuary marble, which is white with visible crystalline structure. The distinctive lustre of statuary marble is due to the effect caused by light penetrating a short distance into the stone and then being reflected from the surfaces of inner crystals. Carrara marble, occurring abundantly in the Apuan Alps of Italy and quarried in the region about Carrara, Massa, and Serravezza, was used in Rome for architectural purposes in the time of Augustus, the first emperor, but the finer varieties of sculptural marble were discovered later. The greatest works of Michelangelo are made of this marble; it is used extensively by contemporary sculptors.

Other varieties of marble contain varying amounts of impurities, which cause the variegated patterns of colours for which many marbles are prized. They are used in building, particularly for interiors, and also in small ornamental works, such as lamp bases, tabletops, desk sets, and various novelties. Statuary and building varieties of marble are distributed over the world in thick and extensive deposits.


S a n d s t o n e

Sandstone is a durable coarse-grained, sedimentary rock consisting of consolidated masses of sand deposited by moving water or by wind. The chemical constitution of sandstone is the same as that of sand; the rock is thus composed essentially of quartz. The cementing material that binds together the grains of sand is usually composed of silica, calcium carbonate, or iron oxide. Largely the cementing material often determines the colour of the rock, iron oxides causing a red or reddish-brown sandstone, and the other materials producing white, yellowish, or greyish sandstone. Sandstones of various geological ages and of commercial importance are widely distributed. Yorkstone is a particularly durable sandstone and as such can be used in internal and external paving application.


S l a t e

Slate is a dense, fine-grained, fissile rock, formed by the metamorphism of shale or clay, or more rarely of igneous rocks.

The process of metamorphism results in consolidation of the original rock and in formation of new cleavage planes along which slate characteristically splits into thin, broad sheets. Many rocks that show “slaty cleavage” are by extension loosely called slate. True slate is hard and compact and does not undergo appreciable weathering.

The basic minerals comprising slate are quartz and muscovite, a kind of mica; biotite, chlorite, and haematite are often present as accessory minerals, and apatite, graphite, kaolin, magnetite, tourmaline, and zircon may occur as minor accessory minerals.

Slate is commonly bluish-black or grey-black in colour, but red, green, purple, and variegated varieties are known; it is quarried in Wales, France, Germany, and the United States. Slate is quarried usually in open pits and rarely in underground workings. The stone splits best when it is “green”, or freshly taken from the quarry. Slate, whilst at one time was almost exclusively employed as a roofing material, is today used for paving stones, flooring, and wall cladding, both internally and externally.