smithsonite

[After James SMITHSON.]

A naturally occurring rhombohedral zinc carbonate (ZnCO₃), with a crystal structure similar to that of calcite (CaCO₃). Smithsonite has a hardness on the Mohs scale of 4½, has a specific gravity of 4.30–4.45, and exhibits perfect rhombohedral cleavage. See also Calcite; Crystal structure; Hardness scales.

Smithsonite most commonly forms as an alteration product of the mineral sphalerite (ZnS) during supergene enrichment of zinc ores in arid or semiarid environments. It is seldom pure zinc carbonate, commonly containing other divalent metal ions such as manganese (Mn²⁺), ferrous iron (Fe²⁺), magnesium (Mg²⁺), calcium (Ca²⁷⁺), cadmium (Cd²⁺), cobalt (Co²⁺), or lead (Pb²⁺) substituting for zinc ion. Substitution of other elements for zinc may result in different colors such as blue-green (copper), yellow (cadmium), and pink (cobalt). Smithsonite has been mined as an ore of zinc and has also been used as an ornamental stone. See also Carbonate minerals; Zinc.

Environment

Secondary (weathered) zone of zinc ore deposits.

Crystals normally indistinct and rounded, usually dull, rounded rhombohedrons, sometimes rounded scalenohedrons. Also thick radiating botryoidal and mammillary crusts, with a crystalline surface (usually blue), brown dull crusts, and earthy masses ("dry-bone ore"). A few larger, steep rhombohedral, almost gemmy crystals have been found in Zambia.

White, yellow (from cadmium), greenish or bluish (from copper), or pink (from cobalt). Luster subadamantine to vitreous; hardness 5; specific gravity 4.3-4.4; fracture conchoidal; cleavage rhombohedral, often curving spherically. Brittle; translucent to transparent.

Zinc carbonate (64.8% ZnO, 35.2% CO ₂ ), usually with some of the Zn replaced by Fe, Mg, and Ca; Cd, Cu, and Co may be present.

Only good test is in closed tube, when the white material is coated with film that is yellow when hot and white when cold. A grain heated on charcoal, then touched with cobalt nitrate and reheated, gives good green (zinc) color. Cleavages and crystal shape show it to be a carbonate, as does its bubbling in hot hydrochloric acid.

The hardness is unusual for a carbonate, and the density is high. Crusts sometimes resemble prehnite, which is harder and does not dissolve in acid. Might also be confused with chrysocolla-stained (but much harder) quartz, or with some hemimorphite crusts (Sardinia and Mexico).

In its best development, a mineral of dry climates formed in limestone regions from primary zinc sulfides by weathering. Sometimes an important ore of zinc, as at Leadville, Colorado, where its ore value was overlooked for years.

The most beautiful solid specimens are the thick blue-green crusts from the Kelly Mine, Magdalena, New Mexico. This material was once marketed for a jewelry stone by the Goodfriend Brothers under the name bonamite. Similar but thinner crusts were found in Lavrion, Greece, and in the Barranca de Cobre, Mexico, where some are pink. Large, translucent, frosty-surfaced, somewhat rounded pale pink, green, and blue crystals were common at Tsumeb, Namibia, but only white ones had flat faces. The largest single crystals are some from the Broken Hill Mine in Zambia, steep, to 2-in. (3-5 cm), clear straw yellow rhombohedrons. Yellow, greenockite-stained "turkey-fat ore" crusts and stalactites come from Arkansas and Sardinia. The most common and easily overlooked type is the hard, porous, dull, bonelike mass known as "dry-bone ore."

The mineral was named for James Smithson, an Englishman who willed money for the establishment of a scientific institution in America, now known as the Smithsonian Institution. Smithsonite is often known as calamine in Europe, a name Americans once applied to the zinc silicate but have now dropped in favor of hemimorphite.

Smithsonite
A specimen of smithsonite from Tsumeb, Namibia, in the Smithsonian Institution.
General
Category	Carbonate Mineral
Chemical formula	ZnCO3
Identification
Color	White, yellow, green, blue, purple
Crystal habit	Massive, botryoidal to reniform
Crystal system	Trigonal
Twinning	None observed
Cleavage	Perfect on [1011]
Fracture	Uneven, sub-conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5
Luster	Vitreous
Streak	White
Diaphaneity	Translucent
Specific gravity	4.4 - 4.5
Optical properties	Uniaxial (-)
Refractive index	nω = 1.842 - 1.850 nε = 1.619 - 1.623
Birefringence	δ = 0.223 - 0.227
Ultraviolet fluorescence	May fluoresce pale green or pale blue under UV
References	[1][2][3]

Smithsonite, or zinc spar, is zinc carbonate ZnCO₃, a mineral ore of zinc. Historically, smithsonite was identified with hemimorphite before it was realised that they were two distinct minerals. The two minerals are very similar in appearance and the term calamine has been used for both, leading to some confusion. The distinct mineral smithsonite was first described in 1832 and named for English chemist and mineralogist, James Smithson (1754-1829), whose estate financed the Smithsonian Institution.^[2]

Smithsonite

Smithsonite from Tsumeb, Namibia

Pink (manganoan) smithsonite

Smithsonite is a variably colored trigonal mineral which only rarely is found in well formed crystals. The typical habit is as earthy botryoidal masses. It has a Mohs hardness of 4.5 and a specific gravity of 4.4 to 4.5.

Smithsonite occurs as a secondary mineral in the weathering or oxidation zone of zinc-bearing ore deposits. It sometimes occurs as replacement bodies in carbonate rocks and as such may constitute zinc ore. It commonly occurs in association with hemimorphite, willemite, hydrozincite, cerussite, malachite, azurite, aurichalcite and anglesite. It forms two limited solid solution series, with substitution of manganese leading to rhodochrosite, and with iron, leading to siderite.^[3]

Uses

Zinc carbonate is used as an astringent and excipient in shampoo. It is also used as a fireproofing filler for rubber and plastics, as a feed additive, as a pigment, in cosmetics and lotions, and in the manufacturing of porcelain, pottery, and rubber.^[4]

See also

• List of minerals
• List of minerals named after people

References

1. ^ http://www.mindat.org/min-3688.html Smithsonite: Smithsonite mineral information and data from Mindat
2. ^ ^{a b} http://www.webmineral.com/data/Smithsonite.shtml Smithsonite mineral data from Webmineral
3. ^ ^{a b} http://rruff.geo.arizona.edu/doclib/hom/smithsonite.pdf Handbook of mineralogy
4. ^ "Zinc and compounds" Australian Government

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