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The Mineral aragonite

Spanish Aragonite Trilling

Calcium carbonate forms as both Aragonite and Calcite, and these two minerals only differ in their crystallization. Calcite, the more common mineral, forms in trigonal crystals, whereas Aragonite forms orthorhombic crystals. On occasion, crystals of Aragonite and Calcite are too small to be individually determined, and it is only possible to distinguish these two minerals with optical or x-ray testing. The true identity of microcrystalline forms of Aragonite or Calcite may also not be known without complex testing, and this can also cause a confusion between these species.

Most large Aragonite crystals are twinned growths of three individual crystals that form pseudohexagonal trillings. Although Aragonite crystallizes in the orthorhombic system, most prismatic crystals are hexagonally shaped due to the twinning. Trillings can be identified by their multi-directional basal striations from each individual member crystal.

Other minerals may form pseudomorphs after Aragonite. A peculiarity of the mineral world is Calcite after Aragonite, which is a pseudomorph after an existing paramorph. Some Aragonite crystals available to collectors are actually Calcite pseudomorphs after Aragonite. A rare but popular pseudomorph is Copper after Aragonite. Aragonite may also contain sand inclusions, which give a specimen a brown color.

A particularly interesting formation of Aragonite is as a deposition product of hot, mineral-rich springs. The water releases calcium upon emerging from the spring, and forms growing mounds and thick crusts around the springs. When these are banded, they may be carved and given trade names such as "Onyx Marble", and "California Onyx".

Aragonite is the main component of many organic substances, such as pearl and coral. The iridescent surface of Pearl and mother-of-pearl is actually a layer of Aragonite secreted by mollusks and related invertebrates. Some forms of Aragonite, especially the Flos Ferri variety, are brittle and very fragile, and may easily break when touched. Such specimens need to be much care.

Aragonite was named by Abrahan Gottlieb Werner after Molina de Aragón, Spain, the type locality where this mineral was first described.

Chemical Formula

CaCO₃

Color

Colorless, white, brown, gray, yellow, red, pink, purple, orange, blue, green

Crystal System

Orthorhombic

Properties

Streak
White

Hardness
3.5 - 4

Transparency
Transparent to opaque

Specific Gravity
2.9 - 3.0

Luster
Vitreous, dull

Cleavage
3,1 - prismatic ; indiscernible,2

Fracture
Subconchoidal

Tenacity
Brittle

Other ID Marks
1) May fluoresce blue, pink, yellow, or cream.
2) Clear specimens display a visible double refraction.

Crystal Habits

The most prevalent crystallized habit is pseudohexagonal trillings, in the form of elongated prismatic crystals or short tabular ones. Less commonly in individual, untwinned crystals. Many aggregates exist, including acicular, radiating, fibrous, columnar, stalactitic, botryoidal, pisolitic, oolitic, tuberose, granular, encrusting, and ball-like protrusions of pseudohexagonal crystals.

3D Crystal Atlas

Additional Information

Composition
Calcium carbonate, sometimes with some strontium, lead, and zinc.

In Group
Carbonates; Aragonite group

Striking Features
Poor cleavage, twinning habits, strong effervescence, and low hardness

Environment
Sedimentary formations and evaporite deposits, hot spring deposits, hydrothermal ore veins, igneous traprock environments, and metamorphic schists.

Rock Type
Igneous, Sedimentary, Metamorphic

Varieties

Alabaster

- Massive, fine grained variety of Gypsum. Occasionally may also refer to a translucent, banded type of Aragonite in the form of Travertine or Tufa.

Flos Ferri

- Filiform variety of Aragonite composed of wormlike intergrowths resembling branching coral.

Mossottite

- Strontium rich variety of Aragonite, or mixture of Aragonite and Strontianite. (Ca,Sr)CO₃. Also known as Strontian Aragonite.

Nicholsonite

- Zinc rich variety of Aragonite. (Ca,Zn)CO₃. Also known as Zincian Aragonite.

Onyx Marble

- Travertine or Tufa in the mineral form of Aragonite or Calcite that exhibits color banding.

Satin Spar

- Fibrous variety of Gypsum. May occasionally also describe a fibrous form of Calcite or Aragonite.

Tarnowitzite

- Lead rich variety of Aragonite, or mixture of Aragonite and Cerussite. (Ca,Pb)CO₃. Also known as Plumboan Aragonite.

Travertine

- Mounds of calcium carbonate formed from hot springs that contain calcium-rich water that bubbles up to the earth and cools down, and its capability to hold calcium is reduced. The water eliminates the calcium, and the calcium forms a growing mound of calcium carbonate, which is porous. Travertine is usually Aragonite, although it may also be Calcite.

Tufa

- Aragonite (or Calcite) formed from precipitating water that traps in organic matter, such as leaves, twigs, and moss. Also calcareous mounds formed from deposition of hot springs that trap in organic matter.

Uses

Aragonite does not have many practical uses, although interesting specimens are popular collector minerals. "Onyx Marble" formed from mineral springs is sometimes carved into ornamental objects.

Noteworthy Localities

The namesake locality for Aragonite is Molina de Aragon, Spain, where excellent trillings occur at the Gallo river. Spain has also produced some of the finest trillings, in locations such as Los Molinillos (Minglanilla), the Retamal Ravine, and the Salt Mine in Cuenca. Excellent Aragonite has also come from the famous Sulfur mines at Agrigento Province in Sicily, Italy

Aragonite has come from Austria in a Siderite mine in Eisenerz, Styria (mainly in the Flos Ferri variety), and in the salt mines of Salzburg. The old iron mines at Frizington, Cumbria, England; as well as the locality of Vitosov, Moravia, Czech Republic; have both been good specimen producers.

A recent producer of specimen-grade Aragonite is Liupanshui, Guizhou Province, China. One of the most prolific occurrences for this mineral, in large trillings and in aggregates of reddish-brown crystals, is the Tazouta Mine near Sefrou, Morocco.

In the U.S., New Mexico has provided many fine Aragonite specimens at the Kelly Mine, Magdalena, Soccoro Co.; at Lake Arthur, near Roswell, Chaves Co; and at Las Cruces, Dona Ana Co. Fine Aragonite has also come from Bisbee, Cochise Co., Arizona; the Grand Deposit Mine, White Pine Co., Nevada; and the Northern Lights Mine, Hussman Spring, Mineral Co., Nevada. Other Western occurrences are Morro Bay, San Luis Obispo Co., California; the Oquirrh Mountains, Tooele Co., Utah; and Fort Collins, Larimer Co., Colorado. Midwestern and Eastern localities include Cave-in-Rock, Hardin Co., Illinois; Sterling Hill, Ogdensburg, Sussex Co., New Jersey; and Paterson and Prospect Park, Passaic Co., New Jersey.

Common Mineral Associations

Quartz, Calcite, Gypsum, Albite, Azurite, Chalcopyrite, Bornite

Distingushing Similar Minerals

Calcite - Distinguished by perfect rhombohedral cleavage, and is slightly softer. Additionally, crystal habits are often different.
Cerussite, Witherite, and Strontianite - Heavier.
Hemimorphite - Harder, heavier.