Aragonite

Aragonite is one of the naturally occurring forms of calcium carbonate, CaCO₃. Although Aragonite and Calcite have exactly the same chemical formula, their atoms are arranged differently. Aragonite crystallises in the orthorhombic crystal system, whereas Calcite crystallises in the trigonal system. Minerals that share a chemical composition but have different crystal structures are known as polymorphs. Aragonite commonly forms as slender needles, radiating clusters, columnar growths, stalactitic formations and distinctive cyclic twins. Those repeated twins can create apparently six-sided crystals even though the mineral itself is orthorhombic. This pseudo-hexagonal appearance is one of Aragonite’s most recognisable features. The famous radiating clusters often sold as Sputnik Aragonite or Star Aragonite consist of crystals growing outward in multiple directions from a central point, giving them a starburst appearance. The mineral was named in 1797 by Abraham Gottlob Werner after Molina de Aragón in Spain, the area associated with its type locality. Interestingly, later writers sometimes assumed the name referred to the wider Aragón region, but Mindat notes that Werner specifically named it after Molina de Aragón.