Calcite

Calcite is a carbonate mineral, one of the most abundant on the Earth surface. It is the main component of chalk, limestone, stalactites and stalagmites.


Calcite
Calcite

image source: http://www.sandatlas.org/calcite/


What is calcite?

The name calcite derived from the Greek word chalix, meaning “lime.” This mineral is a common constituent of sedimentary rocks and it is present in lots of hot springs. It is the main component of stalactites and stalagmites. Moreover, calcite is the primary constituent of the shell of marine organisms.

It is present in various geological settings, in particular, Italy is a great source of calcite. It has different colours: pink and green calcites are found around Germany, Cumberland, England, and the United States. Golden yellow and amber varieties of calcite occur in Joplin, Missouri and Pitcher, Oklahoma, but it also presents in Romania, Namibia, and Mexico.

David by Michelangelo is carved in Carrara Marble.
David by Michelangelo is carved in Carrara Marble.

image source: https://uniquewendy.wordpress.com/category/carrara-marble/


How is calcite formed?

The lime cycle is the process by which calcite is changed to quicklime, then to the slaked line by hydration and finally turns in calcium carbonate by carbonation. The conditions present during each phase have a huge influence on the final product.

For instance, when slaked lime is mixed into a thick slurry with sand and water to form mortar for building purposes, the slaked lime in the mortar slowly starts to react with carbon dioxide to turn on calcium carbonate. So according to the reaction: Ca(OH)2 + CO2 → CaCO3 + H2O; The carbon dioxide present in this reaction is principally available in the air or dissolved in rainwater.

The lime cycle for high-calcium lime.
The lime cycle for high-calcium lime.

image source: http://bridgettconservation.co.uk/hot-lime-building-restoration-and-conservation/


What are its characteristics?

Pure calcite is colourless or white, the different scades of colour are given by the present of impurities. It can be transparent or opaque.

Calcite is softer than the stone, porcelain, and plastic surfaces found in kitchens and bathrooms but more lasting than dried food.

One of its well-known properties is birefringence or double refraction because when light passes through, it is divided into two rays that travel at different speeds and in different directions. This way objects appear doubled.
First produced in the 17th century from the east coast of Iceland, has been used for optical instruments. Later, Polaroid sunglasses will use this same property to cut down glare on a sunny day.

Iceland spar is a transparent crystalline form of calcite (calcium carbonate) that exhibits the property of double refraction, or birefringence.
Iceland spar is a transparent crystalline form of calcite (calcium carbonate) that exhibits the property of double refraction, or birefringence.

image source: http://www.gemstonesadvisor.com/iceland-spar/


calcite types

Calcite has two naturally occurring polymorphs: aragonite and vaterite.
Aragonite changes to calcite at 470°C, while Vaterite, if exposed to water, turns in calcite (at low temperature) or aragonite (at about 60°C). Crystals of calcite may be hexagonal-rhombohedral, but also tabular forms, prisms, and various scalenohedrons.
Moreover may occur in fibrous, granular, lamellar, and compact forms.

The size of the calcite crystals depends on the level of metamorphism. For instance, Marble which has been subjected to higher levels of metamorphism will generally have larger calcite crystals.

The mineral is typically light-colored and large crystals have well-developed crystal faces.
The mineral is typically light-colored and large crystals have well-developed crystal faces.

image source: http://www.sandatlas.org/calcite/


how was it used THROUGHOUT history ?

The earliest use of lime as a construction material was approximately 4000 BC in Egypt to build pyramids. It is also well documented, that the Roman Empire used lime-based mortars widely. Vitruvius, an important Roman architect, made guidelines for lime mortar mixes. Examples of applications included cisterns, fish-ponds, and aqueducts.

James Parker made a product called Roman cement or natural cement in 1796. It was produced by burning a mixture of limestone and clay together in kilns similar to those used for lime. The ending product had better strength and was used in construction.

Afterwards, Joseph Aspdin an English mason/builder patented the Portland Cement in 1824. It consisted of a blend of limestone, clay and other minerals which were calcined into fine particles. Its consistency and higher strength levels allowed it to replace natural cement in mortars. However, Portland cement had poor workability, but if it was combined with lime provided an excellent balance between strength and workability.

Lime products have played a significant role in the construction for thousands of years and excellent performances such as the Great Wall of China, which have lasted for hundreds of years. For this reason, it is still used today in modern masonry construction.

Eddystone Lighthouse.
Eddystone Lighthouse.

image source: http://www.minervaconservation.com/articles/abriefhistoryoflime.html


where can find calcite today?

The construction industry is the primary consumer of calcite in the form of limestone and marble. Marble is an easily worked rock that has long been used for monuments and sculptures. Its lack of significant porosity allows it to resist well to freeze-thaw action outdoors.

Calcite has also lots of uses as a neutralizer of acids. For hundreds of years, it was used as an acid-neutralizing soil treatment.

In Medicine, calcite is mixed with sugar to create calcium carbonate that therefore it is made into chewable tablets used in the neutralization of stomach acids and to treat digestive and other ailments.

Finally, Limestone is often treated and used as sorbent material during the burning of fossil fuels because Calcium carbonate reacts with gases in the combustion emissions, absorbs them, and prevents them from escaping to the atmosphere.

Calcite in concrete used in a high-rise building.
Calcite in concrete used in a high-rise building.

image source: http://geology.com/minerals/calcite.shtml

info sources:

http://geology.com/minerals/calcite.shtml

http://www.jewelsforme.com/gem_and_jewelry_library/calcite

http://www.newworldencyclopedia.org/entry/Calcite

http://www.minervaconservation.com/articles/abriefhistoryoflime.html