Aragonite

Definition

Aragonite is a mineral comprising calcium carbonate (CaCO₃), it is polymorphous to calcite, meaning that they both contain the same main elements but have different crystal structures. Aragonite forms orthorhombic crystals, whereas calcite is trigonal. Aragonite is characterized not only by pseudohexagonal crystals but also acicular habits, often in the form of radiating groups of needles, which are also common. Aragonite has an imperfect to poor cleavage, a subconchoidal fracture, and a hardness on Moh’s scale of 3.5–4. Aragonite tends to be colorless or white, and most are relatively pure, with strontium and lead, acting as the most common substitutions for calcium. Aragonite is highly soluble in cold dilute hydrochloric acid, resulting in significant effervescence. Feigl’s solution stains aragonite black, whereas calcite remains unaffected.

Aragonite most commonly precipitates in seawater and so is found in marine sediments and forms the shells of some marine organisms....