Description
The mineral calcite (also called limespar) consists primarily of the salt calcium carbonate (CaCO3) and is one of the most common minerals in the Earth's crust. Calcium carbonate is formed when soluble calcium ions come into contact with CO2 (carbon dioxide forms a carbonate ion when a positive ion is present in a solution). Calcium carbonate is a poorly soluble salt, causing the resulting CaCO3 to precipitate; this occurs according to the following reaction: Ca2+(aq) + CO32-(aq) → CaCO3(s). Calcite is a common constituent of sedimentary rocks, veins, and deposits in hot springs and caverns in karst regions (as stalactites). It is the mineral that forms limestone and thus the most common mineral of biological origin. Calcite is the main constituent of limestone, chalk, and marble, and is also an important component of marl, calcareous sandstone, and chalk shales. Calcite causes chemical weathering in igneous and metamorphic rocks, creating sedimentary rocks. Calcite is soluble in CO2-containing water under slight pressure. As it falls through the air, rainwater absorbs the gaseous carbon dioxide, making it acidic. This acidic rainwater can dissolve lime. Underground currents can carry this material in solution until, upon contact with atmospheric pressure, CO2 escapes and the calcite precipitates. This is how stalagmites and stalactites are formed in caves. Historically, the transparent Icelandic form of calcite, called Icelandspar, was used to make Nicol prisms. Some researchers hypothesize that the Vikings navigated the sea using crystals of this mineral, which allowed them to accurately determine the position of the sun even in cloudy weather. Today, limestone is primarily used in construction as a building element, for the production of lime and cement, while marble is used as a building and ornamental stone.
Phosphorescence is a special case of luminescence. It is the phenomenon that a substance continues to glow in the dark for a while after being illuminated. The term roughly means "luminous like phosphorus." Now, white phosphorus does indeed glow in the dark, but in this substance, this is caused by oxidation reactions of the phosphor with oxygen in the air (it can also ignite spontaneously), and the light therefore has a different origin. Phosphorescence can also occur when such a material is bombarded with accelerated electrons, as in a cathode ray tube. In common luminescent substances, it results from the slow decay of electrons excited by light irradiation. The fact that this occurs slowly is because the return of electrons to the ground state is a forbidden transition in quantum mechanics. While studying phosphorescence, Henri Becquerel discovered the phenomenon of radioactivity in 1896. The phenomenon of phosphorescence was based on the use of certain paints on the hands of some clocks and watches. Phosphorescence must be distinguished from fluorescence. In fluorescence, no forbidden transition is involved in the electron fallback, so it will emit light in a much shorter time.
