Aragonite

aragonite or aragonite is one of the crystalline forms of calcium carbonate (CaCO₃), along with calcite and vaterite. It can be found in the form of stalactites, and also in the shell of almost all mollusks and in the skeleton of corals. Among the varieties of aragonite, the so-called flos-ferri (iron flower) stands out, which resembles a beautiful coral.

The aragonite/calcite pair was the first recognized case of mineral polymorphism. This means that both have identical chemical composition, but different crystal structure. Because of this difference, aragonite is more soluble in water (and unstable at room temperature and pressure) than calcite. In fact, for geological periods of time (from 10 million to 100 million years), aragonite tends to transform into calcite. This last property can be used to determine the age of certain rock formations.

The aragonite also belongs to an isomorphic series, that is, a group of minerals that belong to the same class and present the same crystalline structure, but whose composition is different. Aragonite contains isomorphic substitutions of barium (witherite), lead (cerussite), or strontium (strontianite). These minerals form the aragonite group.

Regarding the applications of aragonite, they are very limited, since calcite, with the same chemical composition, is much more common. Aragonite is only used as an ornamental or collector's stone.

Crystallography

Aragonite crystallizes in the orthorhombic system, in the rhombic dipyramidal class (2/m 2/m 2/m or mmm). Its space group is Pmcn, indicating that the cell Aragonite unit is simple or primitive.

The dimensions of the aragonite unit cell are:

a = 4.96Å, b = 7.97Å, c = 5.74Å; Z = 4.

Macla of three aragonite crystals forming a pseudohexagonal prism (columnar habit).

In nature, aragonite usually occurs as multiple twins, which give it the appearance of a hexagonal prism (although aragonite does not have a hexagonal crystalline structure, which is why it is called pseudohexagonal). It is also found as pointed crystals, often twinned as well, as hyaline coralloid (flos-ferri), fibrous or fibroradiate, stalactitic, oolitic, and pisolitic forms.

Formation and deposits

Added of multiple spragonite (about 4 cm) pots popularly known as "piña de aragonito".

The most common form in which aragonite appears is as individual or grouped prismatic twins in continental Triassic (Keuper) gypsum and marl. In Spain there are many dozens of deposits, some of which are very important on a global scale. First of all, we must highlight the type locality of the mineral, located next to the banks of the Gallo River, near Molina de Aragón (Guadalajara), a municipality from whose name the word "aragonite" comes from. The best quality specimens, present in museums around the world, appear in various sites around Minglanilla, in this municipality and in Enguídanos. They are also found in other localities in Cuenca, Guadalajara and in the Valencian Community. Pseudohexagonal crystals can be found in Italy and Sicily, pineapple aggregates in Morocco, and stalactitic and coralloid varieties in Arizona, Chihuahua (Mexico), France (blue in color)., Austria and Italy.

Large deposits of high-purity aragonite sands are found on the seabed in various areas around the islands of the Bahamas archipelago, which are extracted on an industrial scale by dredging for use in construction and as a source of carbonate Calcium for soil amendments. Its formation has probably been due to chemical precipitation due to changes in water temperature, as deep waters warm up as they approach the surface.

Aragonite crystals. Corta Azcárate, Eugui, Esteribar (Navarra) Spain

Aragonite is also formed from hot springs or geysers, filtered waters that have come into contact with very hot rocks located at great depths and that have re-emerged to the surface. These waters dissolve minerals from the rocks in their path, including calcium. As the hot springs evaporate, the calcium it contains precipitates and, when it comes into contact with air, combines with oxygen and carbon dioxide to form aragonite crystals.

Aragonite can be found forming some stalactites in caves. It can also be located in metamorphic rocks or sedimentary rocks on the ocean floor, as well as in the skeletons of many living or fossilized marine organisms. In addition, it is common in oxidized zones of metallic deposits.

Aragonito inside the cave Ochtinská.

In Slovakia there is an entire cave made of aragonite, called the Ochtinská aragonite cave. This cave is declared a World Heritage Site by Unesco. The Cueva de El Soplao in Cantabria (Spain) is equally remarkable in terms of its eccentric formations of aragonite.

Etymology and history

Hexagon crystals (now Aragonite) by Molina de Aragón (Guadalajara), according to Torrubia (1754)

The first reference to this mineral, although without giving it a name, is found in the book by Torrubia (1754), titled “Aparato para la Historia Natural Española”, in which the existence of large amounts of “hexagon” crystals is indicated. ”, locally called “torrecillas”, on a hillock near the Gallo river, in Molina de Aragón. The name aragonite is due to the mineralogist Abraham Gottlob Werner, who in 1788 defined it based on some specimens coming from Molina de Aragón, in Guadalajara, which he erroneously attributed to Aragón.

In some iron deposits aragonite appears in coraloid form, sometimes forming very beautiful coraloid specimens that the miners called flos ferri, iron flower.