A carbonate mineral, the stable form of calcium carbonate (CaCO3) at high pressures. It is somewhat harder and has a slightly higher specific gravity than calcite. Aragonite is found in recent deposits formed at low temperatures near the surface of the earth, as in caves as stalactites, with ore minerals, in serpentine and other basic (see acid and basic rocks) rocks, and in sediments. Aragonite is the mineral normally found in pearls, and it occurs in some animal shells. It is polymorphous (same chemical formula but different crystal structure) with calcite and vaterite, and, with geologic time, changes to calcite even under normal conditions.
Learn more about aragonite with a free trial on Britannica.com.
Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone.
Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years. The young age of the California blueschists has been famously demonstrated by the finding therein of aragonite not yet reverted to calcite. The mineral vaterite, also known as μ-CaCO3, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.
Aragonite from Cicov Hill in the Czech Republic: new discoveries have recently been made at the well-known aragonite occurrence at Cicov Hill, near Horenec, near Bilina, Czech Republic. Development work at the locality between 1990 and 1995 has yielded loose gemmy crystals of aragonite to record sizes.
Mar 01, 2004; INTRODUCTION Aragonite from Cicov Hill (pronounced "Chee-choff near the town of Horenec which is near the larger town of...
Aragonite production in calcite seas: effect of seawater Mg/ Ca ratio on the calcification and growth of the calcareous alga Penicillus capitatus
Jul 01, 2005; Abstract.-Previous studies have shown that secular variation in the Mg/Ca ratio of seawater throughout the Phanerozoic would have...