Smithsonite occurs in a wide variation of colors. Its typical globular habit and often vibrant colors make this a desirable mineral for collectors. This mineral is composed of zinc carbonate, but the zinc may be partially replaced with other elements. Those elements are responsible for the color variations. For example, copper often causes green or bright blue colors, and cobalt causes a pink to purple color. Cadmium makes it yellow, and iron gives it a brown to reddish-brown color. The many colors of this mineral include blue, green, yellow, yellow-green, orange-yellow, pink, purple, gray, brown, white, and colorless. It may contain multicolored color zoning patterns and banding.
Smithsonite and Hemimorphite
Historically, Smithsonite was identified with Hemimorphite before it was realized that they were two different 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 named in honor of English chemist and mineralogist James Smithson, who first identified the mineral in 1802. Smithson is better known as the founder of the Smithsonian Institution.
Smithsonite and Hemimorphite are two very different minerals. Smithsonite is a carbonate mineral and a member of the trigonal crystal system, while hemimorphite is a silicate mineral and orthorhombic. They also have noticeable differences in specific gravity and cleavage.
Smithsonite, with excellent color and clarity, is often cut into faceted gems and cabochons. It is rarely used in jewelry because it has a Mohs hardness of only 4 to 4.5 and would be quickly scratched if worn as jewelry.
Varieties of Smithsonite
Bonamite – Blue or green globular Smithsonite with a pearly luster. This term is usually used to describe the mineral in the gem trade.
Cadmium Smithsonite – Yellow or yellow-green colored by cadmium impurities.
Copper Smithsonite – Blue to green colored by copper impurities.
Dry Bone Ore – Describes the massive, porous, and dull variety of Smithsonite, which often assumes a honeycomb shape.
Turkey Fat Ore – Describes globular, botryoidal, and stalactitic forms of yellow Smithsonite.
Trigonal Crystal System
Smithsonite rarely occurs in visible crystals. The only two locations to produce large crystals of significance are Tsumeb, Namibia; and the Kabwe Mine (Broken Hill), Zambia. Virtually all other findings of this mineral are in globular or botryoidal-like forms. Many of the rounded forms have a very distinct feathery or sparkling light effect. Crystals are rhombohedral and scalenohedral, and usually are rounded with curved faces. Crystals may contain triangular growth patterns. It is also known to form pseudo-morphs of other minerals such as Calcite, Galena, and Fluorite, assuming the crystal shapes of those minerals.
Smithsonite belongs to the Trigonal Crystal System, Hexagonal scalenohedral Class. It has a Mohs hardness of 4 – 4.5 and a specific gravity of 4.4. Smithsonite is a member of the Calcite Group of Minerals. It forms two limited solid solution series, with substitution of manganese leading to Rhodochrosite, and with iron, leading to Siderite. Smithsonite will sometimes fluoresce pink in shortwave ultraviolet light.
Smithsonite occurs as a secondary mineral in the weathering or oxidation zone of zinc-bearing ore deposits. It commonly occurs in association with hemimorphite, willemite, hydrozincite, cerussite, malachite, azurite, aurichalcite and anglesite.
Noteworthy Localities of Smithsonite
Individual Smithsonite crystals and crystal clusters of all colors are well-known from Tsumeb, Namibia. Two other African localities which provided visible crystals of this mineral are Berg Aukas, Grootfontein, Namibia; and the Kabwe Mine (Broken Hill), Zambia. The famous Australian locality of Broken Hill, New South Wales, is known for its abundance of minerals including Smithsonite.
Large Smithsonite crusts are found in several areas on the island of Sardinia, Italy, particularly at the Massua and Monteponi Mines, in Iglesias. Blue-green botryoidal masses and crusts are common at the mines at Lavrion, Greece.
Mexico has two outstanding Smithsonite localities which contain beautifully colored Smithsonite, including deep pink and electric green colors. These are the Refugio Mine, Choix, Sinaloa; and the San Antonio Mine, Santa Eulalia District, Chihuahua.
The United States has many fine Smithsonite occurrences; perhaps the most famous being the Kelly Mine, Magdalena, Socorro County, New Mexico. The No. 79 Mine, Hayden, Gila County, Arizona is known for its dark and apple-green Smithsonite. Bright yellow and orange-yellow (Turkey Fat) specimens have come from Rush, near Yellville, Marion County, Arkansas. A large industrial zinc deposit produced Smithsonite in Leadville, Lake County, Colorado. Other localities are Cerro Gordo, Inyo County, California; the Hidden Treasure Mine, Ophir Hill, Tooele County, Utah; and Mineral Point, Iowa County, Wisconsin.
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Article by Bill Jones, Sidewinder Minerals