New Mexico State Mineral
Smithsonite, sometimes called zinc spar, is a zinc carbonate and mineral ore of zinc, usually with impurities of iron, magnesium, and calcium. It may also contain cobalt or cadmium. As a member of the carbonate group, this mineral effervesces in hydrochloric acid. Smithsonite normally occurs as yellow, white, gray, colorless, or as shades of green or blue due to copper impurities, but can also be orange, pink, purple, red, or brown. The mineral sometimes has combinations of these colors or bands of color, and always leaves a white streak. Smithsonite is well known for occurring as apple green or blue-green, when it may be mistaken for prehnite.
After the silver ore in New Mexico was depleted and the 19th-century silver mining boom turned bust, mining areas suffering from the loss of the industry were revitalized when advances in metallurgy made mining of smithsonite for zinc production profitable. After turquoise, smithsonite is the most immediately recognizable mineral in New Mexico. The famous Kelly mine in Magdalena, New Mexico, produces highly sought-after blue-green specimens. Smithsonite was nominated in the 1980s by the state legislature to become the state mineral because of its historic importance to New Mexico. The bill did not pass, and New Mexico does not have an official state mineral. Nevertheless, smithsonite is a good representative of the state’s history, geology, and economy.
Smithsonite was named after British chemist and mineralogist James Lewis Smithson (1754-1829). In 1802, he recognized that a zinc carbonate and a zinc silicate were both confusingly called calamine. In 1832, zinc carbonate was posthumously named smithsonite in his honor. Smithson’s estate financed the founding of the Smithsonian Institution in Washington, D.C., despite the fact that the scientist had never visited the United States. The Smithsonian’s National Museum of Natural History possesses excellent specimens of this mineral.
FORMATION AND OCCURRENCE
Smithsonite is often found as a secondary mineral in the oxidization zone of zinc ore deposits such as sphalerite. It also replaces adjacent carbonate rocks, where it may constitute zinc ore.
This mineral forms botryoidal crystals, which resemble bunches of grapes. It can also form as masses, grains, hexagonal crystals, or polyhedrons with rounded faces.
Smithsonite commonly occurs with hemimorphite. In Smithson’s time, both of these minerals were incorrectly called calamine before smithsonite and hemimorphite were distinguished as separate minerals and the term "calamine" was dropped. Hemimorphite is lighter in weight than smithsonite, but otherwise the two minerals are difficult to differentiate. Smithsonite’s other associates include willemite, hydrozincite, cerussite, malachite, azurite, aurichalcite, and anglesite.
Smithsonite localities are widespread. The sites listed below produce fine specimens, large crystals, and/or an abundance of the mineral.
Smithsonite was a principal source of zinc until the 1880s. Today, it is still mined for zinc, but as a minor ore. The unique luster and colors of smithsonite make it popular with mineral collectors. The most sought-after colors are shades of purple or lavender. When polished for use as an ornamental stone, this mineral is called bonamite. Sometimes used as a gemstone, smithsonite can be cut as cabochons and beads.
Chemical Formula: ZnCO3
Crystal Structure: Trigonal
Hardness (Mohs): 4.5
Color: White, yellow, green, blue, purple
Cleavage: Perfect on 
Fracture: Uneven, sub-conchoidal
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press