The mineral aragonite

Chemical Formula

CaCO₃

Composition

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

Variable Formula

(Ca,Sr,Pb,Zn)CO₃

Color

Colorless, white, brown, gray, yellow, reddish, pink, purple, light blue, light green

Streak

White

Hardness

3½ - 4

Crystal Forms and Aggregates

(Orthorhombic) Very rarely occurs in single, untwinned crystals. The most common crystallized form is in pseudohexagonal trillings, which can be in the form of long, slender, prismatic crystals or short stubby ones. Many other forms exist in Aragonite, such as acicular, radiating, fibrous, columnar, stalactitic, botryoidal, pisolitic, oolitic, tuberose, granular, and ball-like protrusions of pseudohexagonal crystals.

Transparency

Transparent to opaque

Specific Gravity

2.9 - 3.0

Luster

Vitreous, resinous, dull

Cleavage

3,1 - prismatic ; indiscernible,2

Fracture

Subconchoidal

Tenacity

Brittle

Other ID Marks

1) May fluoresce blue pink, or yellow
2) Clear specimens exhibit a strong double refraction

Varieties

Commonly used variety names
Flos Ferri - Variety composed of wormlike intergrowths resembling branching coral

Varieties classified by impurities
Mossottite - Strontium rich variety, or mixture of Aragonite and Strontianite. (Ca,Sr)CO₃
Tarnowitzite - Lead rich variety, or mixture of Aragonite and Cerussite. (Ca,Pb)CO₃
Nicholsonite - Zinc rich variety. (Ca,Zn)CO₃

Microcrystalline forms of Aragonite
Tufa, Calcareous Tufa, or Calcareous Sinter - 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.
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.
Onyx Marble, Suisan Marble, California Onyx, Oriental Alabaster, and Mexican Onyx - Travertine or Tufa with impurities causing it to exhibit color banding.
(Note: Other formations of calcium carbonate, such as stalactites, stalagmites, and flowstone may also be Aragonite, but are usually Calcite. See "microcrystalline forms of Calcite" for information regarding these other forms)

Variety names usually applied to other minerals

Alabaster
Aragonite connotation = translucent, banded, Travertine or Tufa
Usual connotation = massive, fine-grained variety of Gypsum

Satin Spar
Aragonite connotation = fibrous Aragonite (or Calcite)
Usual connotation = fibrous Gypsum

Onyx
Aragonite connotation = translucent, banded, Travertine or Tufa with black and white bands
Usual connotation = Agate banded with black and white bands

Polymorphs

Calcite, Vaterite

In Group

Carbonates ; Aragonite group

All About

Almost all Aragonite crystals are twinned growths of three individual crystals that form pseudohexagonal trillings . Therefore, although Aragonite crystallizes in the orthorhombic system, virtually all its crystals are hexagonal shaped. The trillings can be noted from the basal striations on each of the three individual crystals running in different directions, causing a hexagonal crystal to have three striation directions on each base.

Aragonite frequently contains sand inclusions, which give a specimen a brown color. It forms in many environments, but a particularly interesting one is its formation from deposition of hot, mineral-rich springs. The water releases the calcium it contains upon reaching the air, and forms mounds and thick crusts around the springs. When these deposited mounds and crusts are banded, they are sometimes carved and termed "Onyx Marble", "Suisan Marble", "California Onyx", or "Mexican 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 a layer of Aragonite secreted by mollusks and related invertebrates.

Aragonite and Calcite are polymorphous to each other. Although Aragonite and Calcite contain the same elements, they differ in crystal structure. Calcite forms trigonal crystals, whereas Aragonite forms orthorhombic crystals. Sometimes, the crystals of Calcite and Aragonite are too small to be detected, and it is only possible to distinguish these two minerals from each other by conducting complex scientific tests. The true identity of these microcrystalline forms of Calcite or Aragonite is not known, so these forms of CaCO3 are labeled as "Calcium Carbonate". This definition accurately includes both Calcite and Aragonite.

Uses

Aragonite does not have many practical uses, but interesting specimens are popularly collected by mineral collectors. The "Onyx Marble" formed from mineral springs is cut into cabochon and ornaments.

Striking Features

Poor cleavage, twinning habits, strong effervescence, and hardness

Complex Tests

Effervesces in hydrochloric acid, even if cold and diluted.

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Calcite - Distinguished by perfect rhombohedral cleavage and by the fact that it is slightly softer. In addition, many Aragonite aggregate types don't exist in Calcite.
Cerussite, Witherite, and Strontianite - heavier

Commonly
Occurs With

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

Noteworthy
Localities

There are numerous localities where fine Aragonite specimens were obtained, so some locations may have been left out:
The name Aragonite is derived from the location of Molina de Aragon, Spain. Aragonite occurs in Agrigento, Sicily, Italy, in the famous sulfur mines. It has also been found on Mount Vesuvius, Italy, at Monte Somma. It occurs with Siderite in Styria, Austria. Much Aragonite has come from various places in Cumbria and Cumberland, England. Giant masses were found in Ota City, Shimane Perfecture, Honshu, Japan. Interesting reddish crystal growths are abundant in Tazoula, near Serfou, Morocco. Australia has also produced much material, in practically all its states.
New Mexico has provided many fine specimens in Kelly and Magdalena, Soccoro Co. and Lake Arthur, near Roswell, Chaves Co, and Las Cruces, Dona Ana Co. It also occurs in Bisbee, Cochise Co., Arizona; the Grand Deposit Mine, White Pine Co., Nevada; Morro Bay, San Luis Obispo Co., California; Fort Collins, Larimer Co., Colorado; and Berks Co., Pennsylvania. Spiky aggregates were found in San Luis Obispo, San Luis Obispo Co., California.

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Picture Links

1. Pseudohexagonal prismatic trilling
2. Cluster of penetrating trillings
3. Tuberose "blobs"
4. Rounded cluster of pseudohexagonal trillings
5. Spray of prismatic crystals