Aragonite
A metastable calcium carbonate mineral forming in diverse environments.
Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (CaCO₃), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments. The mineral is named after Molina de Aragón in Spain, though the naming geologist mistakenly believed that location was in Aragon.
- field
- Mineralogy
- type
- Carbonate mineral
- crystal_system
- Orthorhombic
- common_polymorphs
- Calcite, vaterite
- notable_locations
- Molina de Aragón (Spain), Ochtinská Aragonite Cave (Slovakia), Cave of El Soplao (Spain), Bahamas (seabed), Carlsbad Caverns (USA)
Lore & Background
The mineral occurs there as cyclic twins inside gypsum and marls of the Keuper facies of the Triassic. This type of deposit is common in Spain and also found in France. Aragonite forms in many settings: as speleothems in caves such as the Ochtinská Aragonite Cave in Slovakia and the Cave of El Soplao in Spain; as stalactites and 'cave flowers' in Carlsbad Caverns; as massive oolitic sand deposits on the seabed in the Bahamas; and in high-pressure metamorphic rocks at subduction zones. It also forms biologically in mollusk shells, coral skeletons, serpulid tubes, calcareous sponges, and the endocarp of Celtis occidentalis. The nacreous layer of some extinct ammonite shells produces iridescent ammolite. Aragonite is metastable at Earth's surface pressures and temperatures, tending to alter to calcite over geological timescales. It can be synthesized by adding calcium chloride to sodium carbonate at temperatures above 60 °C or in water-ethanol mixtures at ambient temperatures.
Reader's Guide
Aragonite holds significance as a key calcium carbonate polymorph that forms under a wide range of conditions, from biological precipitation in shells and corals to inorganic precipitation in caves, marine sediments, and high-pressure metamorphic rocks. Its metastability at surface conditions makes it a subject of interest in geochemistry and mineral physics, illustrating Ostwald's step rule where a less stable phase may form first. The mineral's presence in mollusk shells and coral skeletons is critical for marine ecosystems, and its use in aquaria helps replicate reef conditions by buffering pH and providing materials for sea life. Aragonite has also been tested for removing pollutants like zinc, cobalt, and lead from wastewater. Notable occurrences include the Ochtinská Aragonite Cave in Slovakia and the Cave of El Soplao in Spain, both of which are tourist attractions. The mineral's name, derived from a geographic misattribution, reflects the history of mineralogical classification. Its transformation to calcite over millions of years means aragonite older than the Carboniferous is essentially unknown, providing constraints on fossil preservation and diagenesis.
Did You Know?
- Aragonite is named after Molina de Aragón in Spain, but the naming geologist mistakenly thought that location was in Aragon.
- Aragonite is metastable at Earth's surface and tends to alter to calcite over 10⁷ to 10⁸ years; aragonite older than the Carboniferous is essentially unknown.
- The nacreous layer of aragonite in some extinct ammonite shells forms an iridescent material called ammolite.
- Aragonite can be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above 60 °C.
Frequently Asked Questions
What is Aragonite?
Aragonite is a carbonate mineral made of calcium carbonate (CaCO₃) that crystallizes in the orthorhombic system. It is one of three naturally occurring polymorphs of calcium carbonate, the others being calcite and vaterite. It forms through both biological activity and physical precipitation in marine and freshwater settings.
How is Aragonite different from calcite?
Although both share the same chemical formula, aragonite crystallizes in the orthorhombic system while calcite is trigonal. Aragonite is also metastable, so it gradually transforms into calcite over long geological timescales, whereas calcite is the thermodynamically stable form.
Where can I find Aragonite in the wild?
Famous occurrences include the Ochtinská Aragonite Cave in Slovakia, the Cave of El Soplao in Spain, and vast deposits on the Bahamas seabed. It also appears in Carlsbad Caverns in the United States and in the type locality of Molina de Aragón, Spain.
How does Aragonite form in nature?
It precipitates from calcium-rich water in both marine and freshwater environments, often scaffolded by organisms such as corals, mollusks, and certain algae. Purely physical crystallization also produces the characteristic needle-like and prismatic crystals seen in cave formations and seabed deposits.
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