The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, and petrology is an essential component of geology.

Rock classification

Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. They are further classified according to particle size. The transformation of one rock type to another is described by the geological model called the rock cycle.

Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (example granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt) .

Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% (mudstone, shale and siltstone); sandstones 20 to 25% and carbonate rocks 10 to 15% (limestone and dolostone).

Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation).

The three classes of rocks — the igneous, the sedimentary and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.

English: Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

Sandstone

Banded gneiss

Impact on society

Rocks have had a huge impact on the cultural and technological advancement of the human race. Rocks have been used by Homo sapiens and other hominids for more than 2 million years. Lithic technology marks some of the oldest and continuously used technologies. The mining of rocks for their metal ore content has been one of the most important factors of human advancement, which has progressed at different rates in different places in part because of the kind of metals available from the rocks of a region.

The prehistory and history of civilization is classified into the Stone Age, Bronze Age, and Iron Age. Although the stone age has ended virtually everywhere, rocks continue to be used to construct buildings and infrastructure. When so used, rocks are called dimension stone.

Dimension stone

is natural stone or rock that has been selected and fabricated (i.e., trimmed, cut, drilled, ground, or other) to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements. Another important selection criterion is durability, the time measure of the ability of dimension stone to endure and to maintain its essential and distinctive characteristics of strength, resistance to decay, and appearance.

Quarries that produce dimension stone or crushed stone (used as construction aggregate) are interconvertible. Since most quarries can produce either one, a crushed stone quarry can be converted to dimension stone production. However, first the stone shattered by heavy and indiscriminate blasting must be removed. Dimension stone is separated by more precise and delicate techniques, such as diamond wire saws, diamond belt saws, burners (jet-piercers), or light and selective blasting with Primacord, a weak explosive.

Although a variety of igneous, metamorphic, and sedimentary rocks are used as dimension stone, the principal rock types are granite, limestone, marble, travertine, quartz-based stone (sandstone, quartzite) and slate. Other varieties of dimension stone that are normally considered to be special minor types include alabaster (massive gypsum), soapstone (massive talc), serpentine and various products fashioned from natural stone. ^[1]

The commonest finish mentioned below is polished. A polished finish is one having a surface with high luster and strong reflection of incident light (almost mirror-like). The rougher finishes are bush-hammered, honed, sandblasted, and thermal. A bush-hammered finish is one with a rough uniformly patterned surface produced by an impact tool. A honed finish is one with a superfine, smooth, satinlike, nonreflective surface. A sandblasted surface is one with an irregular pitted surface produced by impacting sand particles at high velocity against a stone surface. A thermal (or flamed) finish is one with a rough nonreflective surface with only a few reflections from cleavage faces, produced by applying a high-temperature flame. This finish may change the natural color of the stone. ^[2]

The most easily-accessible general references are the latest (2006) Minerals Yearbook Chapter (production and foreign trade, with statistics), and the latest (Issue 30) Dimension Stone Advocate News (new "building green" developments and demand statistics); see below.

Major applications

Rough cut slabs of granite dimension stone.

While common colors used in some of the major applications are listed below, there is an extraordinarily wide range of colours, available in thousands of patterns. These patterns are created by geological phenomena such as mineral grains, inclusions, veins, cavity fillings, blebs, and streaks. In addition, rocks and stones not normally classed as dimension stone are sometimes selected for these applications. These can included tiles made of jade, agate, and jasper.

Stone (usually granite) countertops and bathroom vanities both involve a finished slab of stone, usually polished but sometimes with another finish (such as honed or sandblasted). Industry standard thicknesses in the United States are 3/4" (2 cm) and 1.25" (3cm). Often 2 cm slabs will be lamintated at the edge to create the appearance of a thicker edge profile. The slabs are cut to fit the top of the kitchen or bathroom cabinet, by measuring, templating or digital templating. Countertop slabs are commonly sawn from rough blocks of stone by reciprocating gangsaws using steel shot as abrasive. More modern technology utilizes diamond wire saws which use less water and energy. Multi-wire saws with as many as 60 wires can slab a block in less than two hours. The slabs are finished (i.e., polished, honed), then sealed with resin to fill micro-fissures and surface imperfections typically due to the loss of poorly bonded elements such as biotite. The fabricators shop cuts these slabs down to final size and finishes the edges with equipment such as hand-held routers, grinders, CNC equipment, or polishers. There has been a recent (2008) hubbub about radon emission from granite countertops; the National Safety Council states that the contributions of radon to inside air come from the soil and rock around the residence (69%), the outdoor air and the water supply (28%), and only 2.5% from all building materials-including granite countertops. A concerned homeowner can employ radon mitigation and removal techniques the ASTM covers.^[3] The stone for countertops or vanities is usually granite, but often is marble (especially for vanity tops), and sometimes limestone or slate. The majority of the stone for this application is produced in Brazil, Italy, and China.

Slate tile covers this entire structure in Germany.

Tile is a thin modular stone unit, commonly 12 in. square (30.5 cm) and 3/8 in. (10 mm) thick. Other popular sizes are 15 in. square (38 cm), 18 in. square (46 cm), and 24 in. square (61 cm); these will usually be thicker than the 12 in. square. The majority of tile has a polished finish, but other finishes such as honed are becoming more common. Almost all stone tile is mass-produced by automated tile lines to identical size, finish, and close tolerances. Exceptions include slate flooring tile and special orders: tile with odd sizes or shapes, unusual finishes, or inlay work. In summary, the automated tile line is a complicated complex of cutting and calibrating machines, honing-polishing machines, edging machines that put on flat or rounded edges, and interconnecting conveyors to move the stone from the slab input to the final tile product. The stone for tiles is most commonly marble, but often is granite, and sometimes limestone, slate, or quartz-based stone. Common colors are white and light earth colors. Much of the stone for this application is produced in Italy and China.

Stone monuments include tombstones, grave markers or as mausoleums. After being gangsawed into big thick (up to 10 ft (3.0 m). long and over 6 in. thick) slabs, smaller saws or guillotines (they break the granite and make the rough edges commonly seen on monuments) shape the monuments. The fronts and backs are usually polished. The individual monuments are then carved, shaped, and further defined by hand tools and sandblasting equipment. The stone for monuments is most commonly granite, sometimes marble (as in military cemeteries), and rarely others. A rose quartz tombstone stands in a Harpers Ferry cemetery. Sandstone was common in the nineteenth century but is no longer used due to rapid rates of erosion. Most common monument colors for granite are gray, then black, then mahoghany; for marble it is white. Today, the majority of the stone used in North America in this application is imported from such countries as India and China. This has depressed traditional North American monument centres such as Georgia and Quebec.

Dimension stone has been used in the construction of buildings for centuries. Due to costs, today stone veneers are usually used in place of solid stone blocks. This courthouse was built of dimension stone quarried in Berea, Ohio.

There are a number of smaller applications for buildings and traffic-related uses. Building components include stone used as veneer (exterior), ashlar, or other shapes. Veneer is a nonload-bearing facing of stone attached to a backing, of an ornamental nature though it protects and insulates. Ashlar is a square block of stone, often brick-sized, for facing of walls (primarily exterior). The other shapes are rectangular blocks used for stair treads, sills, and coping (coping is sometimes nonrectangular). The shapes subject to foot traffic will usually have an abrasive finish such as honed or sandblasted. The stone is mostly limestone, but often is quartz-based stone (sandstone), or even marble or granite. Roofing slate is a thin-split shingle-sized piece of slate, and when in place forms the most permanent kind of roof; slate is also used as countertops and flooring tile. Traffic-related stone is that which is used for curbing (vehicular) and flagstone (pedestrian). Curbing is thin stone slabs used along streets or highways to maintain the integrity of sidewalks and borders. Flagstone is a thin naturally irregular-edged slab of stone, sometimes sawed into a rectangular shape, used as paving (almost always pedestrian). For curbing, the stone is almost always granite, and for flagstone the stone is almost always quartz-based stone (sandstone or quartzite).^[4]

There are several other applications resembling flagstone in using rough dimension (or crushed) stone, usually as quarried, sometimes made smaller (i.e. by a jackhammer), often simply put in place: dry stone and riprap.

The stone used in these applications usually has to have certain properties, or meet a standard specification. The American Society for Testing and Materials (ASTM) has such specifications for granite, marble, limestone, quartz-based dimension stone (C616), slate (C629), travertine (C1527), and serpentine (C1526).

Production

Marble quarry in Carrara, Italy.

The majors producers of dimension stone include Brazil, China, India, Italy, and Spain, and each have annual production levels of nine to over twenty-two million tons. Portugal produces 3 million tons of dimension stone each year.

According to the USGS, 2006 U.S. dimension stone production was 1.33 million tons valued at $265 million, compared to 1.36 million tons (revised) valued at $269 million in 2005. Of these, granite production was 428,000 tons valued at $105 million in 2006 and 416,000 tons valued at $106 million in 2005, and limestone was 559,000 tons valued at $96.1 million in 2006 and 581,000 tons valued at $95.8 million (revised) in 2005. The United States is at best a mid-level dimension stone producer on the world scene; Portugal produces twice as much dimension stone annually.^[8]

World comparison for dimension stone demand: The DSAN World Demand for (finished) Granite Index showed a growth of 15% annually for the 2000-2006 period, compared to 14% annually for the 2000-2005 period. The DSAN World Demand for (finished) Marble Index showed a growth of 12% annually for the 2000-2006 period, compared to 10.5% annually for the 2000-2005 period. Other DSAN indexes for 2007 (preliminary) indicate that the 2000-2007 growth probably will be down from the 2000-2006 growth. ^[9]

The DSAN U.S. Ceramic Tile Demand Index shows a growth of 5.0% annually for the 2000-2006 period, compared to 5.5% annually for the 2000-2005 period. The "traditional" major ceramic tile suppliers, Italy and Spain, have been losing markets to new entrants Brazil and China. The same thing has been happening with dimension stone with increasing supplies from Brazil, China and India.

The Chinese Government has made a policy shift of long-term worldwide significance for dimension stone production and demand by eliminating the 15% export tax rebate on all dimension stones. So far, it has not replaced it with an export tariff on it, as it has on other industrial minerals. In addition, the Chinese Government sometimes strongly discourages its domestic firms from buying rough dimension stone overseas.

"Building green" with dimension stone

Marble cladding on a building.

Green building or environmentally friendly construction with natural materials, is an idea that has been around for several decades. Energy price increases and the need for energy conservation when heating or cooling buildings have recently brought it to the fore. This resulted in the formation in 1993 of the U.S. Green Building Council (USGBC), which has developed a building rating system called Leadership in Energy and Environmental Design (LEED). Educational institutions (colleges, universities, grade, and high schools) are often requiring new buildings to be green, and a few jurisdictions (i.e., some cities) have some rules pushing green building. When "building green", dimension stone has a big advantage over concrete, aluminum, and steel, whose productions are all highly energy intensive and create much air and water pollution. As an entirely natural product, dimension stone also has an advantage over quartz surface artificial stone (resin-agglomerated stone) made from mixed quartz sand or ground stone and a resin (i.e., acrylic). Dimension stone rates very well in terms of the criteria on the ASTM checklist for sustainability of building products: there are no toxic materials used in its processing, there are no direct greenhouse gas emissions during processing, the dust created is controlled, the water used is almost completely recycled (per OSHA/MSHA regulation), and it is a perpetual resource (virtually inexhaustible in a human time scale). Dimension stone in use can last many generations, even centuries, so the dimension stone manufacturer haven't needed a product recycling program.^[10] One LEED requirement provides that the dimension stone used in a green building be quarried within a 500-mile (800 km) radius of the building being constructed. This gives a clear advantage to domestic dimension stone, plus some quarried near the U.S. borders with Canada and Mexico. A current problem is how to consider stone quarried domestically, sent to China or Italy for finishing, and shipped back to be used in a project. Dimension stone also has the advantage from a green perspective of being recyclable and can often be recycled and not sent to a landfill. There are also "green" ways of cleaning stone being developed; for example, removing the black gypsum crusts that form on marble and limestone by applying sulfate-reducing bacteria to the crust to gasify most of it, breaking up the crust. See DSAN for updates on "building green" and dimension stone recycling. ^[9]

The Federal Trade Commission (FTC) is reexamining and will probably update its "Green Guides" which it uses to regulate green advertising claims. The FTC's updating will emphasize green building, including the products it involves, such as dimension stone. When the new requirements are finalized, the FTC will go after firms that violate the new requirements, in order to establish legal precedents.

The Natural Stone Council has an extensive amount of information on building green with dimension stone, including a life-cycle inventory for each major dimension stone (i.e. granite, limestone), giving the amount of energy, water, and other inputs required per ton of stone extracted and produced, plus the amount of emissions to the air and water that occur during processing. Some best practice studies are available, for example, on water consumption, treatment, and reuse while extracting and processing dimension stone, including dust mitigation, sludge management, and maximizing water recycling.

Stone recycling and reuse

Reconstruction of the Charles Bridge in Prague showing numbered dimension blocks.

Recycling dimension stone can occur when structures are demolished, along with recycling timber and recycling construction aggregate in the form of concrete. The material most likely to be recycled is concrete, and this represents the largest volume of recycled construction material. Not too many structures incorporate dimension stone, and even less of them have dimension stone worth saving. Stone recycling is usually done by specialists that monitor local demolition activity, looking for stone-containing houses, buildings, bridge abutments, and other dimension stone structures scheduled for demolition. Particularly treasured are old hand-carved stone pieces with the chisel marks still on them, local stones no longer quarried or that are quarried in a different shade of color or appearance. There is no national or regional trade in reclaimed stone, so a large storage yard is required, since the recovered stone may not be quickly sold and reused. The recycled dimension stone is used in old stone buildings being renovated (to replace deteriorated stone pieces), in fireplace mantels, benches, veneer, or for landscaping (like for retaining walls).

The Parthenon in Athens underwent a major reconstruction prior to the 2004 Olympics.

Related to stone recycling and stone reuse is the deconstruction and reconstruction of a stone building. The building is taken apart stone block by stone block and the location and orientation of each block is carefully noted. Any roofing slate and interior stone in place is catalogued and moved in the same fashion. After transporting the blocks, slate, and other stone used to the new location, they are put back in place where and how they were originally, thus reassembling the building. This has been a very uncommon occurrence, but will probably become more common in the future.

Dimension stone is also reused. Buildings immediately spring to mind, but such things as the ornate stone walls, arches, stairways and balustrades alongside a boulevard can also be renovated and reused. Sometimes the old interior of the building is kept as is, after repair. Sometimes the old building is gutted, leaving only a shell or facade and the space inside reconfigured and modernized. The stone work will usually need attention too. ^[12]

The old stone work may only need cleaning or sandblasting, but it may need more. Firstly, the building exterior (facade) needs to be inspected for unsafe conditions.^[13] Next, the building walls need to be inspected for water leakages.^[14] The most likely needs are mortar restoration (repointing), applying consolidants to the old stone, or replacing pieces of stone that are deteriorated (damaged) beyond the point of any repair. The repointing is the removal of existing damaged mortar from the outer portion of the joint between stone units and its replacement by new mortar matching the appearance of the old.^[15] The consolidants re-establish the original natural bonding between the stone particles that weathering has removed.^[16] Deteriorated pieces of stone work are replaced with pieces of stone that match the original as much as possible. Exterior dimension stone will often change color after exposure to weather over time. For example, Indiana Limestone will weather from a tan to an attractive light yellow. Interior dimension stone can sometimes change its shade a little over time too. For both, it may not be possible to find an exact match, even from the original quarry. Stone will often change its appearance from location to location in the same quarry. If the dimension stone renovationist is truly fortunate, the original builder put aside some spare pieces of the stone for future need.

Stone selection and cleaning

The selector of dimension stone begins by considering stone color and appearance, and how the stone will match its surroundings. The selector has literally thousands of options to choose from, and should examine many options. In addition to many hundreds of different stones with different colors and patterns, each stone can change radically in color and appearance when a different finish is put on it. A polished finish accentuates the color and makes any pattern more vivid, and the rougher finishes (i.e. honed, thermal) lighten the color and make the patterns more subdued. With thousands of possibilities, the selector must start by looking at many stones in many different finishes, or photos of them. Such photos can be found on some dimension stone websites, and on DSAN's Architects Stone Selection Helper.

In addition to selecting a stone color and pattern, the suitability of its properties for the intended use must be considered. Stone being chosen for countertops or vanities should be nonabsorptive, resist stains, and be heat and impact resistant. Stone being used in tiles should be sealed in order to resist staining by spilled liquids. Stone being used for flooring, paving, or surfaces subject to foot or vehicular traffic ought to have a semiabrasive finish for slip resistance, such as bush-hammered or thermal. A glossy polished finish will be slick. Most flagstone surfaces are rough enough to be naturally slip-resistant. The ASTM document C1528 Standard Guide for Stone Selection is very helpful, and covers topics not mentioned here. ^[17]

Dimension stone requires some specialized methods for cleaning and maintenance. Abrasive cleaners should not be used on a polished stone finish because it will wear the polish off. Acidic cleaners can not be used on marble or limestone because it will remove (i.e. dissolve) the finish. Textured finishes (thermal, bush-hammered) can be treated with some mildly abrasive cleaners but not bleach or an acidic cleaner (if marble or limestone). Stains are another consideration; stains can be organic (food, grease, or oil) or metallic (iron, copper). Stains require some special removal techniques, such as the poultice method. A new method of cleaning stone on ancient buildings (midaeval and renaissance) has been developed in Europe: sulfur-reducing bacteria are used on the black gypsum-containing crusts that form on such buildings to convert the sulfur to a gas that dissipates, thus destroying the crust while leaving the patina produced by aging on the underlying stone. This method is still in development and not commercially available yet. The ASTM document C1515 Standard Guide for Cleaning of Exterior Dimension Stone is also very helpful, and covers problems and remedies not mentioned here ^[18]; if masonry, concrete or stucco walls are also involved

Marble

Marble is a nonfoliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite (a crystalline form of calcium carbonate, Ca C O₃). It is extensively used for sculpture, as a building material, and in many other applications. The word "marble" is colloquially used to refer to many other stones that are capable of taking a high polish.

Etymology

The word "marble" derives from the Greek μάρμαρον (marmaron)^[1] and that from μάρμαρος (marmaros), "crystalline rock", "shining stone"^[2]^[3], perhaps from the verb μαρμαίρω (marmairō), "to flash, sparkle, gleam"^[4]. This stem is also the basis for the English word "marmoreal" meaning "marble-like".

[edit] Origins

Marble is a metamorphic rock resulting from regional or rarely contact metamorphism of sedimentary carbonate rocks, either limestone or dolomite rock, or metamorphism of older marble. This metamorphic process causes a complete recrystallization of the original rock into an interlocking mosaic of calcite, aragonite and/or dolomite crystals. The temperatures and pressures necessary to form marble usually destroy any fossils and sedimentary textures present in the original rock.

Pure white marble is the result of metamorphism of very pure limestones. The characteristic swirls and veins of many colored marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which were originally present as grains or layers in the limestone. Green coloration is often due to serpentine resulting from originally high magnesium limestone or dolostone with silica impurities. These various impurities have been mobilized and recrystallized by the intense pressure and heat of the metamorphism.

Types of marble

White marbles, like Carrara in Italy, Royal White and Beijing White in China and Malagori of Pakistan, have been prized for sculpture since classical times. This preference has to do with the softness and relative isotropy and homogeneity, and a relative resistance to shattering. Also, the low index of refraction of calcite allows light to penetrate several millimeters into the stone before being scattered out, resulting in the characteristic "waxy" look which gives "life" to marble sculptures of the human body.

Construction marble

In the construction, specifically the dimension stone trade, the term "marble" is used for any crystalline calcitic rock (and some non-calcitic rocks) useful as building stone. For example, "Tennessee marble" is really a dense granular fossiliferous gray to pink to maroon Ordovician limestone that geologists call the Holston Formation.

Marble is also used in coolers and kitchen construction because it stays 5°C (9°F) cooler than the air that surrounds it at room temperature and pressure.

Industrial use of marble

Colorless or light-colored marbles are a very pure source of calcium carbonate, which is used in a wide variety of industries. Finely ground marble or calcium carbonate powder is a component in paper, and in consumer products such as toothpaste, plastics, and paints. Ground calcium carbonate can be made from limestone, chalk, and marble; about three-quarters of the ground calcium carbonate worldwide is made from marble. Ground calcium carbonate is used as a coating pigment for paper because of its high brightness and as a paper filler because it strengthens the sheet and imparts high brightness. Ground calcium carbonate is used in consumer products such as a food additive, in toothpaste, and as an inert filler in pills. It is used in plastics because it imparts stiffness, impact strength, dimensional stability, and thermal conductivity. It is used in paints because it is a good filler and extender, has high brightness, and is weather resistant. However, the growth in demand for ground calcium carbonate in the last decade has mostly been for a coating pigment in paper.

Calcium carbonate can also be reduced under high heat to calcium oxide (also known as "lime"), which has many applications including being a primary component of many forms of cement.

Production

According to the United States Geological Survey, U.S. dimension marble production in 2006 was 46,400 tons valued at $18.1 million, compared to 72,300 tons valued at $18.9 million in 2005. Crushed marble production (for aggregate and industrial uses) in 2006 was 11.8 million tons valued at $116 million, of which 6.5 million tons was finely ground calcium carbonate and the rest was construction aggregate. For comparison, 2005 crushed marble production was 7.76 million tons valued at $58.7 million, of which 4.8 million tons was finely ground calcium carbonate and the rest was construction aggregate. U.S. dimension marble demand is about 1.3 million tons. The DSAN World Demand for (finished) Marble Index has shown a growth of 12% annually for the 2000-2006 period, compared to 10.5% annually for the 2000–2005 period. The largest dimension marble application is tile.

Artificial marble

Faux marble or faux marbling is a wall painting technique that imitates the color patterns of real marble (not to be confused with paper marbling). Marble dust can be combined with cement or synthetic resins to make reconstituted or cultured marble.

Cultural associations

As the favorite medium for Greek and Roman sculptors and architects (see classical sculpture), marble has become a cultural symbol of tradition and refined taste. Its extremely varied and colorful patterns make it a favorite decorative material, and it is often imitated in background patterns for computer displays, etc.

Places named after the stone include Marblehead, Ohio; Marble Arch, London; the Sea of Marmara; India's Marble Rocks; and the towns of Marble, Minnesota; Marble, Colorado; and Marble Hill, Manhattan, New York. The Elgin Marbles are marble sculptures from the Parthenon that are on display in the British Museum. They were brought to Britain by the Earl of Elgin.