Calcite (mineral)
Calcite is a prominent calcium carbonate mineral found in sedimentary, igneous, and metamorphic rocks, making it one of the most abundant minerals on Earth. Its most common forms are limestone and marble, both of which have significant historical and contemporary uses in construction and decoration. Natural calcite typically appears white or colorless with a dull luster, but impurities can give it various colors. Notable characteristics of calcite include its softness, allowing it to be scratched easily, and its unique rhombic cleavage, which permits it to break in specific geometric shapes.
Calcite plays a vital role in environmental applications, such as acid neutralization, and is commonly included in products like antacids, animal feed, and various forms of construction materials. Additionally, calcite is responsible for the formation of stunning geological features such as stalactites and stalagmites in caves, which take thousands of years to develop. Its versatility extends to everyday products, including cosmetics, paint, and even paper. Overall, calcite’s widespread presence and diverse applications underscore its importance in both nature and human industry.
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Calcite (mineral)
Calcite is a calcium carbonate mineral present in sedimentary, igneous, and metamorphic rocks, the three main types of rocks on Earth. This makes calcite one of the most abundant minerals in the world. Carbonate minerals are salts formed from carbon dioxides. Calcite is found most commonly in Earth's crust, where it acts as a major component of both limestone and marble. In its natural state, calcite is white or colorless and dull in luster.
Calcite's ubiquity throughout the world had led people to discover a multitude of uses for the mineral. One of its most common uses is as construction material, as limestone and marble are widely used to make cement and concrete. Calcite also serves a range of other purposes in its pure form or as part of limestone or marble. Pure calcite is used in antacid tablets, animal feed, glass, and tiles, while calcite in its limestone and marble forms is used for sculptures, paint, soap, and toothpaste.
Background
Calcite is one of the most abundant minerals in the world. It can be found in all three primary types of rock—sedimentary, igneous, and metamorphic—in Earth's crust. Sedimentary rocks are composed of many layers of rocks and minerals that have settled on top of one another after being deposited by water or air. Igneous rocks form from the solidification of magma, molten rock that escapes from beneath Earth's surface and hardens. Metamorphic rocks are sedimentary, igneous, or existing metamorphic rocks that have been changed by pressure, heat, or natural chemicals.
Limestone, a sedimentary rock, is one of the most well-known examples of calcite rocks. It forms usually in warm, shallow ocean waters from the gradual buildup of shells, coral, and other marine material. These substances take millions of years to form limestone. People have known the rock to be consistently durable since ancient times. This is what has made limestone a mainstay in construction for thousands of years.
For example, the ancient Egyptians built the Great Pyramid of Giza entirely of limestone blocks in about 2550 BCE. Many other bygone peoples around the world, including the ancient Romans and some indigenous South American groups, used limestone in the construction of their buildings. Limestone continued to be used for a variety of construction purposes in later centuries, such as for restoring the Tower of London in the thirteenth century CE and the Great Wall of China from the fourteenth to the seventeenth centuries CE.
Another type of calcite limestone is commonly found in rock formations in caves. The long, slender cones descending from cave roofs are called stalactites. They form from water that has penetrated the roof of the cave from the ground above. The water dissolves some calcite rock as it travels and then drips from the roof to the ground. The water droplets deposit the calcite they were carrying when they fall. Over time, the calcite deposits on the roof form stalactites, which resemble icicles made of rock. Additionally, water droplets that fall from cave roofs may still contain some calcite. After enough calcite water has fallen in the same place on the ground over an extended period, stalagmites will form on the cave floor. They are tall, slender columns of calcite, essentially the inverse of stalactites. Both stalactites and stalagmites can take hundreds of thousands of years to form. Given enough time, they often meet and form single calcite columns in caves.
The other common form of calcite is marble. This is a metamorphic rock created from limestone that has been naturally subjected to heat or pressure. Some of the layers of the limestone are eliminated as sediment forms between them and links them together. Pure limestone will form white marble after being changed in these ways. Marble is also used in construction, home décor, and sculpting.
Overview
As an abundant material, calcite can form in a diverse range of locations on Earth, including caves, hot springs, and coral reefs. Some small marine animals even use calcium carbonates such as calcite to make their shells.
Pure calcite appears as a white or colorless crystal, although various impurities can make the mineral one of a variety of colors. Calcite's luster, or shine, is classified as dull. Calcite is one of the softest minerals in the world and can be scratched with a knife blade. The mineral possesses what is known as rhombic cleavage. Cleavage is the ability of a crystal to be split along weak places in its body. Calcite's rhombic cleavage means the crystal can be split in three places to form a rhombus shape.
Calcite looks almost exactly like quartz, an even more abundant mineral. The two are chemically distinct, however, as calcite is calcium carbonate, while quartz is silicon dioxide. Calcite and quartz can be distinguished by dropping small amounts of diluted hydrochloric acid, lemon juice, or vinegar onto samples of each of them and observing which one develops bubbles on its surface. Calcite always reacts to diluted acid this way. Quartz does not react at all to these substances.
Calcite has been a valuable tool for people for millennia, and it continues to benefit humans in a variety of ways in the twenty-first century. The mineral is a hallmark of construction, as limestone and marble blocks themselves can be used to build structures or can be ground down and mixed into concrete or cement. Concrete is widely used to build infrastructure such as buildings, bridges, and roads.
Acid neutralization is another benefit of calcite. The mineral can eliminate harmful acid from the environment simply by being spread in affected areas. Crushed pieces of limestone can be scattered in fields to rid the soil of acid. The same can be done in water that has been polluted with acid. This property of calcite also makes the mineral useful as an ingredient in chewable antacid tablets. Other commercial products that use calcite include dough, baking powder, animal feed, and some paper.
The limestone and marble forms of calcite are useful in their own ways. In addition to construction materials, limestone is used for cosmetics, dyes, floor or wall tiles, paint, pesticides, rubber, soap, and toothpaste. Marble, meanwhile, is used for countertops, sinks, tiles, tombstones, and blocks for sculptures.
Bibliography
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"Calcite (and Aragonite)." University of Minnesota, www.esci.umn.edu/courses/1001/minerals/calcite.shtml. Accessed 23 June 2017.
Chan, Candice, et al. "Carbonates." LibreTexts, chem.libretexts.org/Core/Inorganic‗Chemistry/Descriptive‗Chemistry/Main‗Group‗Reactions/Compounds/Carbonates. Accessed 23 June 2017.
"The Interesting History of Limestone." Stoneworth Warehouse, www.stoneworthwarehouse.com/blogs/news/89532550-the-interesting-history-of-limestone. Accessed 23 June 2017.
King, Hobart. "Calcite." Geology.com, geology.com/minerals/calcite.shtml. Accessed 23 June 2017.
Snyder, Kelly, and Peter Russell. "Calcite, Limestone and Marble." University of Waterloo, uwaterloo.ca/earth-sciences-museum/resources/detailed-rocks-and-minerals-articles/calcite-limestone-and-marble. Accessed 23 June 2017.
"Three Types of Rock." American Museum of Natural History, www.amnh.org/exhibitions/permanent-exhibitions/rose-center-for-earth-and-space/david-s.-and-ruth-l.-gottesman-hall-of-planet-earth/how-do-we-read-the-rocks/three-types-of-rock. Accessed 23 June 2017.