As the duration and intensity of metamorphism continues, the calcite crystals increase in size. Starting at a microscopic scale, the calcium carbonate in the rock begins to crystallize or recrystallize into fine-grained calcite crystals. This is the beginning of the process known as metamorphism. When limestone is subjected to heat, pressure, and chemical activity, the calcite in the rock begins to transform. The rock shown here is about four inches (ten centimeters) across. If you view this rock closely by eye, or better, with a hand lens, you will clearly see cleavage faces of calcite intersecting at rhombic angles. Some might call this material "crystalline limestone" - however, the proper name is marble. Here are some of the more commonly encountered types of limestone.Ĭrystalline Limestone: A specimen of limestone that has been subjected to metamorphism. These names are often based upon how the rock formed, itsĪppearance, its composition, or its physical properties. There are many different types of limestone - each with its own name. See our article about the " acid test" for identifying carbonate rocks and minerals. The calcium carbonate content of limestone gives it a property that is often used in rock identification - it effervesces in contact with a cold solution of 5% hydrochloric acid. Particles of chert, pyrite, siderite, and other minerals can form in the limestone by chemical processes. These can be small particles of quartz, feldspar, or clay minerals delivered to the site by streams, currents and wave action. All limestones contain at least a few percent other materials. Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. Related: The "Acid Test" for Carbonate Minerals Composition of Limestone A rock known as "tufa" is a limestone formed by evaporation at a hot spring or on the shoreline of a lake in an arid area. The limestone that makes up these cave formations is known as "travertine," a chemical sedimentary rock. If droplets fall to the floor and evaporate there, stalagmites could eventually grow upwards from the cave floor. Over time, this evaporative process can result in an accumulation of icicle-shaped calcium carbonate on the cave ceiling. When the water evaporates, any calcium carbonate that was dissolved in the water will be deposited. There they might evaporate before falling to the cave floor. In a cave, droplets of water seeping down from above enter the cave through fractures or other pore spaces in the cave ceiling. Stalactites, stalagmites, and other cave formations (often called "speleothems") are examples of limestone that formed through evaporation. Limestone can also form through evaporation. This is producing an extensive deposit of calcium carbonate sediment that has already converted to limestone at depth. There, abundant corals, shellfish, algae, and other organisms produce vast amounts of calcium carbonate skeletal debris and fecal matter that completely blanket the platform. One of these areas is the Bahamas Platform, located in the Atlantic Ocean about 100 miles southeast of southern Florida (see satellite image). Limestone is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean islands, and within the Indonesian archipelago. Most of them are found in shallow parts of the ocean between 30 degrees north latitude and 30 degrees south latitude. Many limestone-forming environments are active on Earth today. The amount of precipitated calcium carbonate in a biological limestone can be as low as a few percent of the rock by volume, or it can be higher than 50% of the rock by volume. If the biological grains are not cemented together, a rock will not be formed. "Cementation" is an important step in the transformation of a sediment into a rock. Calcium carbonate, precipitated directly from solution, forms as a "cement" that binds the biological grains together. After the biological grains have accumulated and are buried, water that is saturated with dissolved materials moves slowly through the sediment mass. Most biological limestones contain significant amounts of directly precipitated calcium carbonate. They are thought to be less abundant than biological limestones. Limestones formed this way are chemical sedimentary rocks. Some limestones form by direct precipitation of calcium carbonate from marine or fresh water.
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