aragonite

n.
An orthorhombic mineral form of crystalline calcium carbonate, dimorphous with calcite.

[After ARAGON.]

One of three naturally occurring mineral forms of calcium carbonate (CaCO₃). The other forms (or polymorphs) are the abundant mineral calcite and the relatively rare mineral vaterite. Still other forms of calcium carbonate are known, but only as products of laboratory experiments. The name aragonite comes from Aragon, a province in Spain where especially fine specimens occur. See also Calcite.

Aragonite has an orthorhombic crystal structure in which layers of calcium (Ca) atoms alternate with layers of offset carbonate (CO₃) groups. A common crystallographic feature of aragonite is twinning, in which regions of crystal are misoriented as though they were mirror images of each other. This can give rise to a pseudohexagonal symmetry which is readily identified in large crystals (see illustration). Aragonite crystals are usually colorless or white if seen individually; however, aggregates of small crystals may exhibit different colors. Most aragonites are nearly pure calcium carbonate; however, small amounts of strontium (Sr) and less commonly barium (Ba) and lead (Pb) may be present as impurities.

Aragonite. Pseudohexagonally twinned specimen from Girgenti, Sicily. (American Museum of Natural History specimens)

At the low temperatures and pressures found near the Earth's surface, aragonite is metastable and should invert spontaneously to calcite, which is stable at these conditions. This, in part, explains why calcite is far more abundant than aragonite. However, at low temperatures the transformation of aragonite to calcite effectively occurs only in the presence of water, and aragonite may persist for long periods of geologic time if isolated from water. Increased temperature also promotes the transformation to calcite. Despite being metastable, aragonite rather than calcite is sometimes the favored precipitate from certain solutions, such as seawater, in which magnesium (Mg) ions inhibit precipitation of calcite.

Aragonite occurs most abundantly as the hard skeletal material of certain fresh-water and marine invertebrate organisms, including pelecypods, gastropods, and some corals. The accumulated debris from these skeletal remains can be thick and extensive, usually at the shallow sea floor, and with time may transform into limestone. Most limestones, however, contain calcite and little or no aragonite. The transformation of the aragonite to calcite is an important step in forming limestone and proceeds by the dissolution of aragonite followed by the precipitation of calcite in the presence of water. This process may take more than 100,000 years. See also Limestone.

Other occurrences of aragonite include cave deposits (often in unusual shapes) and weathering products of calcium-rich rocks.

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CaCO
Orthorhombic -- Rhombic bipyramidal

Environment

Deposits around warm and hot springs, some ore veins, and sedimentary formations. Also secreted by mollusks.

Crystal description

Single crystals, most often long slender needles, also in tabular plates. Trillings (three intergrown individuals) common, looking like short hexagonal prisms or even hexagonal plates. Re-entrant angle visible in fresh, sharp crystals in the center of each apparent prism of these pseudohexagons. Striations on the apparent base suggest the three individuals. Also in crusts and stalactites.

Physical properties

Colorless, white, pale violet, light blue, or light yellow. Luster vitreous; hardness 3Ɖ-4; specific gravity 2.9-3.0; fracture subconchoidal; cleavage poor, parallel to prism and side pinacoids. Brittle; translucent to transparent; commonly fluorescent and phosphorescent.

Composition

Calcium carbonate, like calcite (56.0% CaO, 44.0% CO ₂ ) with strontium (mossottite), lead (tarnowitzite), and sometimes zinc (nicholsonite).

Tests

Dissolves with bubbles in cold hydrochloric acid, like calcite. Strong solutions give a precipitate of white needles or granules of calcium sulfate when sulfuric acid is added to the hydrochloric acid solution.

Distinguishing characteristics

Recognizable as a carbonate by its effervescence in hydrochloric acid; as the aragonite group by lack of a conspicuous cleavage; as a calcium carbonate by its sulfate precipitate (only in concentrated solutions) and by the weak red-orange coloration of the flame, and aragonite by the pink-violet color it assumes when a powder is boiled in a test tube with a cobalt nitrate solution (calcite stays white). Also, it crumbles more on heating than calcite does and loses its fluorescence, whereas calcite tends to remain intact and to gain fluorescence.

Occurrence

Aragonite is far rarer than calcite, apparently being deposited from warmer solutions than calcite (but not too warm). It is found around hot springs in crusts and thick beds (which are cut as onyx marble). Fine needlelike elongated and pointed crystals were found in the Cumberland (England) iron mines. Good pseudohexagonal trillings common in the Sicilian sulfur mines. Also found as isolated six-sided rods and plates in central Spain, the former in crystal clusters with sulfur, gypsum, and celestite, the latter with gypsum in red and green shale beds often colored by red-stained clay inclusions. Trillings and sandy calcite pseudomorphs of this type have been found at Chaves (near Roswell, New Mexico), in s. France, Spain, Morocco, and at the Matsushiro gypsum mines, Ota City, Shimane Prefecture, Honshu, Japan, in medium to giant spherical crystal balls. Violet pseudohexagonal rods cross gas cavities in basalt at Suzu Gun, Ishikawa Prefecture, in the Noto Peninsula, Japan.

A white coral-like branching growth, called flos ferri (iron flowers), has been found in Austrian iron mines (and lesser examples in New Mexico and Mexico).

Remarks

The iridescent inner surfaces of shells (and pearls) are composed of aragonite secreted by mollusks. The light violet cast that is common in some aragonites (in cores of those from green shale in Spain and from Japanese basalt) seems to be due to iron plus a natural irradiation. It fades from crystals exposed on outcrops but can be restored (and the balance made greenish) by irradiation in the laboratory.

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