The mineral calcite (also called calcite) mainly consists 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 there is a positive ion in a solution). Calcium carbonate is a poorly soluble salt, causing the CaCO3 formed to precipitate; this proceeds according to the following reaction: Ca2+(aq) + CO32-(aq) -→ CaCO3(s) Calcite is a common constituent of sedimentary rocks, of veins and in deposits in hot springs and caverns in karst areas (as stalactite). It is the mineral that forms limestone and thus the most common mineral of biological origin. Calcite is the main component of limestone, chalk and marble and it is also an important component of marl, calcareous sandstone and lime shales. Calcite is responsible for the chemical weathering of igneous and metamorphic rocks, resulting in sedimentary rocks. Calcite is soluble in CO2-containing water under light pressure. As it falls into the air, rainwater absorbs the gaseous carbon dioxide, making it acidic. This acidic rainwater is able to dissolve lime. In underground flows, this material can transport in solution until, after contact with atmospheric pressure, CO2 escapes and the calcite precipitates. For example, stalagmites and stalactites are formed in caves. In the past, the transparent Icelandic form of calcite, called Iceland spar, was used to make Nicolprisms. Some researchers hypothesize that the Vikings navigated the seas using crystals of this mineral, which can accurately determine the position of the sun even in cloudy weather. Today, limestone is mainly used in construction as a building element, for the manufacture of lime and cement, while marble is used as a building and ornamental stone.