GEMAS DO BRASIL: Rhodochrosite

domingo, 21 de julho de 2013

Rhodochrosite

Rhodochrosite
Rhodochrosite from Mina Capillitas, Province of Catamarca, Argentina
General
Category	Carbonate minerals
Formula (repeating unit)	MnCO₃
Strunz classification	05.AB.05
Crystal symmetry	Trigonal hexagonal scalenohedral H-M symbol: (32/m) Space group: R3c
Unit cell	a = 4.777 Å, c = 15.67 Å; Z=6
Identification
Formula mass	114.95 g/mol
Color	Pink, rose-red, cherry-red, yellow, yellowish gray, cinnamon-brown, may be banded
Crystal habit	Rhombohedral and scalenohedral crystals; also commonly bladed, columnar, stalactitic, botryoidal, granular or massive
Crystal system	Trigonal
Twinning	On {1012} as contact and lamellar
Cleavage	On {1011} perfect; parting on {1012}
Fracture	Uneven, conchoidal
Tenacity	Brittle
Mohs scale hardness	3.5-4
Luster	Vitreous to pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.7
Optical properties	Uniaxial (-)
Refractive index	n_ω = 1.814 - 1.816 n_ε = 1.596 - 1.598
Birefringence	δ = 0.218
Pleochroism	weak
Ultraviolet fluorescence	None
References	^[1]^[2]^[3]

Rhodochrosite is a manganese carbonate mineral with chemical composition MnCO₃. In its (rare) pure form, it is typically a rose-red color, but impure specimens can be shades of pink to pale brown. It streaks white, and its Mohs hardness varies between 3.5 and 4. Its specific gravity is between 3.5 and 3.7. It crystallizes in the trigonal system, and cleaves with rhombohedral carbonate cleavage in three directions. Crystal twinning often is present. It is transparent to translucent with refractive indices of nω=1.814 to 1.816, nε=1.596 to 1.598. It is often confused with the manganese silicate, rhodonite, but is distinctly softer.
Rhodochrosite forms a complete solid solution series with iron carbonate (siderite). Calcium, (as well as magnesium and zinc, to a limited extent) frequently substitutes for manganese in the structure, leading to lighter shades of red and pink, depending on the degree of substitution. It is for this reason that the most common color encountered is pink.

Occurrence and discovery

Rhodochrosite occurs as a hydrothermal vein mineral along with other manganese minerals in low temperature ore deposits as in the silver mines of Romania where it was first found. Banded rhodochrosite is mined in Capillitas, Argentina. Catamarca, Argentina has an old Incan silver mine that has produced fine stalactitic examples of rhodochrosite that are unique and very attractive. Cut cross-sections reveal concentric bands of light and dark rose colored layers. These specimens are carved and used for many ornamental purposes.^[4]
It was first described in 1813 in reference to a sample from Cavnic, Maramureş, present-day Romania. According to Dimitrescu and Radulescu, 1966 and to Papp, 1997, this mineral was described for the first time in Sacaramb, Romania, not in Cavnic, Romania. The name is derived from the Greek word ῥοδόχρως meaning rose-colored.

Use

Its main use is as an ore of manganese which is a key component of low-cost stainless steel formulations and certain aluminium alloys. Quality banded specimens are often used for decorative stones and jewelry. Due to its being relatively soft, and having perfect cleaveage, it is very difficult to cut, and therefore rarely found faceted in jewelry.

Rhodochrosite and silver mining

Manganese carbonate is extremely destructive to the amalgamation process used in the concentration of silver ores, and so until quality mineral specimens became highly sought after by collectors, they were often discarded on the mine dump.

Culture and legend

Colorado officially named rhodochrosite as its state mineral in 2002.^[5] Large specimens have been found in the Sweet Home Mine near Alma, Colorado.

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Small Rhodochrosite specimen featured in a mineral kit, from Wuton mine, Guangxi prov, China.

The Incas believed that rhodochrosite is the blood of their former rulers, turned to stone, therefore it is sometimes called "Rosa del Inca" or "Inca Rose".^[6]

Gallery

Large red rhodochrosite from Alma, Colorado
Pink is the most common color of Rhodochrosite. Specimen mined near Silverton, Colorado
The Alma King is the largest known rhodochrosite crystal; it was found in the Sweet Home Mine near Alma, Colorado. It is on display in the Denver Museum of Nature and Science.
Exhibit in La Plata Museum, La Plata, Argentina