Calcite is a carbonate mineral and is among the most widely distributed minerals on the Earth’s surface. Calcite is also one of the most widely distributed minerals. It is the principal component of chalk, marble, limestone and stalactites and stalagmites. Some geologists consider it to be a “ubiquitous mineral” – one that is found everywhere.
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What is calcite?
The name calcite is derived from the Greek word chalix, meaning “lime.” This mineral is a common constituent of sedimentary rocks, particularly limestone, and is the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs. In caverns, it is the main component of formations such as stalactites and stalagmites.
Calcite is often the primary constituent of the shells of marine organisms. It is found, for instance, in plankton (such as coccoliths and planktic foraminifera), the hard parts of red algae, some sponges, brachiopods, echinoderms, most bryozoa, and parts of the shells of some bivalves, such as oysters and rudists.
Formed in various geological environments, constitutes about fourpercent of the Earth’s crust.
Italy is a great source of calcite. The finest marble available comes from Carrara. In fact, there are many locations around Italy where stunning translucent calcite crystals can be found. Pink and green calcite crystals specifically, occur around Germany, Cumberland, England, and the United States. Golden yellow and amber varieties of calcite occur in Joplin, Missouri and Pitcher, Oklahoma. Calcite also occurs in Romania, Namibia, and Mexico. It is found in association with iron, manganese, zinc and other minerals.
How is calcite formed?
The process by which lime (calcium carbonate) is converted to quicklime by heating, then to slaked lime by hydration, and naturally reverts to calcium carbonate by carbonation is called the lime cycle. The conditions and compounds present during each step of the lime cycle have a strong influence of the end product, thus the complex and varied physical nature of lime products.
An example is when slaked lime (calcium hydroxide) is mixed into a thick slurry with sand and water to form mortar for building purposes. When the masonry has been laid, the slaked lime in the mortar slowly begins to react with carbon dioxide to form calcium carbonate (limestone) according to the reaction: Ca(OH)2 + CO2 → CaCO3 + H2O. The carbon dioxide that takes part in this reaction is principally available in the air or dissolved in rainwater so pure lime mortar will not recarbonate under water or inside a thick masonry wall.
The lime cycle for dolomitic and magnesium lime is not well understood but more complex because the magnesium compounds also slake to periclase which slake slower than calcium oxide and when hydrated produce several other compounds thus these limes contain inclusions of portlandite, brucite, magnesite, and other magnesium hydroxycarbonate compounds. Magnesium sulfate salts may damage the mortar when they dry and recrystalize due to expansion of the crystals as they form which is known as sulfate attack.
What are its characteristics?
Calcite is colourless or white when pure, but it may be of almost any colour – reddish, pink, yellow, greenish, blueish, lavender, black, or brown, owing to the presence of diverse impurities. It may be transparent, translucent, or opaque. Its luster ranges from vitreous to dull; many crystals, especially the colourless ones, are vitreous, whereas granular masses, especially those that are fine-grained, tend to be dull.
Calcite has a hardness of three on the Mohs scale, and that makes it suitable as a low-hardness abrasive. It is softer than the stone, porcelain, and plastic surfaces found in kitchens and bathrooms but more durable than dried food and other debris that people want to remove. Its low hardness makes it an effective cleaning agent that does not damage the surface being cleaned.
One of its well-known properties is called birefringence or double refraction, because of which objects viewed through a clear piece of calcite appear doubled. When light passes through some minerals, it is split into two rays that travel at different speeds and in different directions. Calcite consequently exhibits double refraction that can be observed with the naked eye. Iceland spar, first produced in the 17th century from the east coast of Iceland, has been used for nearly two centuries for optical instruments. William Nicol in 1828 found that by cutting the crystals in the appropriate direction one could make an optical device which eliminated one of the rays and permitted the other to emerge as plane polarised light. Polaroid sunglasses use this same property to cut down glare on a sunny day. This technology was used in microscopes which are utilized in the study of rocks and minerals.
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Calcite represents the stable form of calcium carbonate. It has two naturally occurring polymorphs: aragonite and vaterite. Aragonite changes to calcite at 470°C. When exposed to water, vaterite converts to calcite (at low temperature) or aragonite (at about 60°C).
Crystals of calcite are hexagonal-rhombohedral, though actual calcite rhombohedrons are rare in nature. They show a remarkable variety of habits, including acute to obtuse rhombohedrons, tabular forms, prisms, and various scalenohedrons. Moreover, calcite exhibits several twinning types, adding to the variety of observed forms. It occurs in fibrous, granular, lamellar, and compact forms. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal but difficult to obtain.
Limestone is a sedimentary rock that is composed primarily of calcite. It forms from both the chemical precipitation of calcium carbonate and the transformation of shell, coral, fecal and algal debris into calcite during diagenesis. Limestone also forms as a deposit in caves from the precipitation of calcium carbonate.
Marble is a metamorphic rock that forms when limestone is subjected to heat and pressure. A close examination of a broken piece of marble will usually reveal obvious cleavage faces of calcite. The size of the calcite crystals is determined by the level of metamorphism. Marble that has been subjected to higher levels of metamorphism will generally have larger calcite crystals.
One variety, called Iceland spar, is valuable for optical equipment; another, called Mexican onyx (different from the quartz variety of onyx), is used as an ornamental stone. Another crystalline form, called dogtooth spar for its dogtooth appearance, is composed of acute scalenohedral crystals.
image source: http://www.sandatlas.org/calcite/
how was it used THROUGHOUT history ?
The first mortars were made from mud or clay. These materials were used because of availability and low cost. The Egyptians utilized gypsum mortars to lubricate the beds of large stones when they were being moved into position. Each of these materials, however, did not perform well in the presence of high levels of humidity and water. It was discovered that limestone, when burnt and combined with water, produced a material that would harden with age. The earliest documented use of lime as a construction material was approximately 4000 BC when it was used in Egypt for plastering the pyramids. The beginning of the use of lime in mortars is not clear. It is well documented, however, that the Roman Empire used lime based mortars extensively. Vitruvius, a Roman architect, provided basic guidelines for lime mortar mixes:
“… When it [the lime] is slaked, let it be mingled with the sand in such a way that if it is pit sand three of sand and one of lime is poured in; but if the same is from the river or sea, two of sand and one of lime is thrown together. For in this way there will be the right proportion of the mixture and blending.”
Mortars containing only lime and sand required carbon dioxide from the air to convert back to limestone and harden. Lime/sand mortars hardened at a slow rate and would not harden under water. The Romans created hydraulic mortars that contained lime and a pozzolan such as brick dust or volcanic ash. These mortars were intended be used in applications where the presence of water would not allow the mortar to carbonate properly. Examples of these types of applications included cisterns, fish-ponds, and aqueducts.
The most significant developments in the use of pozzolans in mortars occurred in the 18th century. It was discovered that burning limestone containing clays would produce a hydraulic product. In 1756, James Smeaton developed perhaps the first hydraulic lime product by calcining Blue Lias limestone containing clay. An Italian pozzolanic earth from Civitavecchia was also added to provide additional strength. This mortar mixture was used to build the Eddystone Lighthouse. James Parker patented a product called Roman cement or natural cement in 1796. Natural cement was produced by burning a mixture of limestone and clay together in kilns similar to those used for lime. The resulting product was ground and stored in waterproof containers. Typically natural cements had higher clay contents than hydraulic lime products which allowed for better strength development. Natural cement mortar was used in construction where masonry was subjected to moisture and high levels of strength were needed.
Joseph Aspdin an English mason/builder patented a material called Portland Cement in 1824. Portland cement consisted of a blend of limestone, clay and other minerals in carefully controlled proportions which were calcined and ground into fine particles. Though some portland cement was imported from Europe, it was not manufactured in the United States until 1871. The consistency and higher strength levels of portland cement allowed it to replace natural cements in mortars. Portland cement by itself had poor workability. Portland cement combined with lime provided an excellent balance between strength and workability. The addition of portland cement to lime mortars increased the speed of the construction process for masonry building due to faster strength development. Mix designs incorporating different amounts of lime and portland cement were developed.
Until approximately 1900, lime putty was used in construction applications. Limestone was burned in small kilns often built on the side of a hill to facilitate loading. Wood, coal and coke were used as fuel. The quicklime produced from these kilns was added to water in a pit or metal trough and soaked for an extended period of time. The time required for soaking was dependent on the quality of the quicklime and could range from days to years. It was generally thought that the longer the quicklime was soaked, the better it would perform.
Lime products have played a significant role in the masonry construction for thousands of years. Prior to 1930, most masonry construction utilized lime based mortars. Lime has proven performance that is demonstrated by structures, such as the Great Wall of China, which have lasted for hundreds of years. The reasons for using lime in mortar 2000 years ago, still remain valid today in modern masonry construction.
where can find calcite today?
The construction industry is the primary consumer of calcite in the form of limestone and marble. These rocks have been used as dimension stones and in mortar for thousands of years. Limestone blocks were the primary construction material used in many of the pyramids of Egypt and Latin America. Today, rough and polished limestone and marble are still an important material used in prestige architecture.
Modern construction uses calcite in the form of limestone and marble to produce cement and concrete. These materials are easily mixed, transported, and placed in the form of a slurry that will harden into a durable construction material. Concrete is used to make buildings, highways, bridges, walls, and many other structures.
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Marble is an attractive and easily worked rock that has long been used for monuments and sculptures. Its lack of significant porosity allows it to stand up well to freeze-thaw action outdoors, and its low hardness makes it an easy stone to work. It has been used in projects as large as the pyramids and as small as a figurine. It is widely used as cemetery markers, statues, mantles, benches, stairways, and much more.
Calcite has numerous uses as a neutralizer of acids. For hundreds of years, limestones and marbles have been crushed and spread on fields as an acid-neutralizing soil treatment. They are also heated to produce lime that has a much faster reaction rate in the soil.
Calcite is used as an acid neutralizer in the chemical industry. In areas were streams are plagued with acid mine drainage, crushed limestone is dispensed into the streams to neutralize their waters.
Calcium carbonate derived from high-purity limestones or marbles is used in medicine. Mixed with sugar and flavoring, calcium carbonate is made into chewable tablets used in the neutralization of stomach acids. It is also an ingredient in numerous medications used to treat digestive and other ailments.
Sorbents are substances that have the ability to “capture” another substance. Limestone is often treated and used as sorbent material during the burning of fossil fuels. Calcium carbonate reacts with sulfur dioxide and other gases in the combustion emissions, absorbs them, and prevents them from escaping to the atmosphere.
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