Calcite on Aragonite

Object/Artifact

-

Crater Rock Museum

Name/Title

Calcite on Aragonite

Entry/Object ID

2004.1.36

Description

Assemblage Zone: pegmatite dikes Chemical Composition: CaCO3, Calcium Carbonate Crystal System: Orthorhombic System Description: Common Name: Calcite on Aragonite Group Name: Aragonite Chemistry: CaCO3, Calcium Carbonate Location: Dalnegorsk, Russia Description: Whitish bae with thin white crystals and light pink crystals on top. PHYSICAL CHARACTERISTICS: Color can be white or colorless or with usually subdued shades of red, yellow, orange, brown, green and even blue. Luster is vitreous to dull. Transparency: Crystals are transparent to translucent. Crystal System is orthorhombic; 2/m 2/m 2/m Crystal Habits include twinned hexagonal prismatic crystals as well as a diverse assortment of thin elongated prismatic, curved bladed, steep pyramidal (spiked) and chisel shaped crystals. A branching tree, coral or worm-like delicate form is called "flos ferri". Can also be compact, granular, radially fibrous and massive. Its massive forms can be layered, coralloid, pisolitic, oolitic, globular, stalachtitic and encrusting. Aragonite is a constituent of many species' shell structures. A layered sedimentary marble like formation is called Mexican Onyx and is used for carvings and ornamental purposes. Calcite pseudomorphs of aragonite crystals and formations are common. Cleavage is distinct in one direction (pinacoidal). Fracture is subconchoidal. Hardness is 3.5-4 Specific Gravity is 2.9+ (average for non-metallic minerals) Streak is white. Other Characteristics: aragonite effervesces easily in cold dilute hydrochloric acid, is strongly birefringent, is fluorescent and its refractive index is 1.7 . Associated Minerals include gypsum, barite, smithsonite, malachite, calcite, serpentine, sulfur, celestite, zeolites, quartz, clays, dolomite, limonite, chalcopyrite and wulfenite among many others. Best Field Indicators are crystal habits, single plane of cleavage and reaction to acid Fracture: subconchoidal Hardness: 3 Calcite Luster: Glassy Occurrence: The type location for aragonite is Molina de Aragón (Guadalajara, Spain), 25 km outside Aragon. An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the USA, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. Massive deposits of oolitic aragonite are found on the seabed in the Bahamas. Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite. Ammolite is primarily aragonite with impurities that make it iridescent and valuable as a gemstone. Notable Occurrences include Aragon, Spain (its type locality and from where it gets its name); Morocco; Bastennes, France; Girgenti, Sicily; Alston Moor and Cleator Moor, Cumberland, England; Baja California, Mexico (Mexican Onyx); Tsumeb, Namibia; Carinthia, Austria; Leadhills, Scotland; Harz Mountains, Germany and in several localities in the Southwestern United States. Specific Gravity: 2.9 Streak: white Variety: Aragonite is a common carbonate mineral. It is unfortunately often thought of as the poor cousin to calcite. But aragonite is an interesting and attractive mineral in its own right. It forms interesting habits and can have a soft pretty color. Its modes of formation and relationship to calcite are both curious and intriguing. Aragonite is a polymorph of calcite, which means that it has the same chemistry as calcite but it has a different structure, and more importantly, different symmetry and crystal shapes. Aragonite's more compact structure is composed of triangular carbonate ion groups (CO3), with a carbon at the center of the triangle and the three oxygens at each corner. Unlike in calcite, the carbonate ions do not lie in a single plane pointing in the same direction. Instead they lie in two planes that point in opposite directions; destroying the trigonal symmetry that is characteristic of calcite's structure.

Collection

Suomynona Mineral Collection

Acquisition

Accession

2004.1

Source or Donor

Suomynona Mineral Collection

Acquisition Method

Donation

Dimensions

Width

6-3/8 in

Length

7 in

Weight

3.72 oz

Dimension Notes

Dimension taken at widest points

Location

Location

* Untyped Location

RD-2

Category

Permanent

Moved By

Ian Cunningham

Date

August 27, 2022

Location

* Untyped Location

RD-2

Category

Permanent