rhodochrosite

n.
A mineral consisting mainly of manganese carbonate, MnCO₃, light pink to rose-red in color with a pearly or vitreous luster, used as a manganese ore.

[German Rhodochrosit, from Greek rhodokhrōs, rose-colored : rhodo-, rhodo- + khrōs, color.]

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The mineral form of manganese carbonate. Calcium, iron, magnesium, and zinc have all been reported to replace some of the manganese. The equilibrium replacement of manganese by calcium increases with the temperature of crystallization.

Rhodochrosite occurs more often in massive or columnar form than in distinct crystals. The color ranges from pale pink to brownish pink. Hardness is 3.5–4 on Mohs scale, and specific gravity is 3.70.

Well-known occurrences of rhodochrosite are in Europe, Asia, and South America. In the United States large quantities occur at Butte, Montana. As a source of manganese, rhodochrosite is also important at Chamberlain, South Dakota, and in Aroostook County, Maine. See also Carbonate minerals; Manganese.

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MnCO
Hexagonal -- Hexagonal scalenohedral

Environment

Ore veins and metamorphic manganese deposits.

Crystal description

Most often in rhombohedrons (sometimes very steep), but also in scalenohedrons. Granular, massive, and in rounded spherical and botryoidal crusts.

Physical properties

Deep rose pink to pale pink, gray, or brown. Luster vitreous to pearly; hardness 3Ɖ-4; specific gravity 3.4-3.6; fracture conchoidal; cleavage perfect rhombohedral. Brittle; transparent to translucent.

Composition

Manganese carbonate (61.7% MnO, 38.3% CO ₂ with any or all of the following present: iron, calcium, magnesium, zinc, and cobalt).

Tests

Dissolves slowly in cold and rapidly in warm hydrochloric acid with effervescence. Powder colors borax bead violet in oxidizing flame (test for manganese).

Distinguishing characteristics

The cleavage and hardness (and acid test) show it to be a carbonate of this group. The borax bead test shows it to be a manganese mineral and eliminates about everything else. Pink color is the best guide.

Occurrence

Rhodochrosite is usually a gangue mineral of copper and lead ore veins, but sometimes it occurs (like siderite) in pegmatites. At Butte, Montana, it was an ore of manganese. Commonly alters to black manganese oxides on weathering, and the black stains are very apparent on the containing rocks. Good specimens not common. Fine crystals have come from several mines (American Tunnel, Sweet Home) in Colorado, where it forms deep pink rhombohedral crystals up to several inches (10 cm) across, associated with pyrite, fluorite, quartz, and ore sulfides. Butte produced rhombohedral and scalenohedral crystals to 1 in. (2.5 cm) in crusts as well as solid cleavable and granular masses, always paler than the Colorado material, of a milky pink color. Pegmatite rhodochrosite is often grayish or brownish. Botryoidal masses and scalenohedral crystals--secondary in character, for they incrust limonite--have come from Germany (where it has been called Himbeerspat, or "raspberry spar"). Rosinca is a name applied to an Argentine occurrence of banded onyx-like pink crusts used for decorative purposes. Good but often thinly quartz-coated crystals have come from Cananea, Mexico. Hotazell, South Africa, has produced many gemmy, deep red Ɖ-1 in. (1-2 cm) crystals of an uncommonly transparent nature, which can perhaps be regarded as the most unusual specimens for collectors. Pasta Buena, Peru, has become a source of Colorado-like pink crystals with similar fluorite associations.

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