Smithsonite is a mineral composed of zinc carbonate. It belongs to the carbonate class of minerals and often exhibits a rhombohedral crystal structure, though well-formed crystals are rare. The mineral typically forms as a secondary product in the weathering zone of primary zinc-bearing ore deposits, where original zinc sulfides have been altered. Smithsonite’s presence indicates an oxidized zone rich in zinc, establishing its importance as a source mineral for this metal.
Smithsonite as a Primary Source of Zinc
The primary modern application of smithsonite lies in its role as a significant ore for zinc extraction, particularly in deposits where the main zinc sulfide mineral, sphalerite, has been oxidized. Smithsonite has a high zinc content, with the pure form containing approximately 52 weight percent zinc metal, making it commercially viable for mining operations. The extraction process typically involves crushing the ore followed by hydrometallurgical techniques such as leaching with sulfuric acid, which dissolves the zinc carbonate.
This zinc-rich solution is then purified, and the metallic zinc is recovered through electrolysis. The resulting metallic zinc is widely used in galvanizing, a process that applies a protective zinc coating to steel or iron to prevent rust. Zinc is also a component in various alloys, most notably brass, a mixture of copper and zinc known for its strength and corrosion resistance. Zinc is also a component in the production of certain types of batteries.
Applications in Jewelry and Decorative Arts
Beyond its industrial utility, smithsonite is valued for its aesthetic properties, making it a desirable material in decorative arts and for mineral collectors. The mineral rarely appears as distinct crystals, instead commonly forming in masses with a characteristic botryoidal or “grape-like” habit. This bubbly, rounded appearance is often translucent and displays a lustrous, vitreous sheen.
The appeal of smithsonite is enhanced by its wide range of vibrant colors, which are caused by trace impurities within the zinc carbonate structure. Copper impurities, for instance, create attractive blue and green hues, while cadmium can produce shades of yellow, sometimes known as “turkey fat ore.” Due to its relative softness, measuring 4 to 4.5 on the Mohs scale, smithsonite is generally considered too fragile for use in rings or other everyday jewelry. Instead, it is frequently cut into cabochons, which are polished, rounded shapes, or carved into ornamental objects for display.
Addressing Historical Nomenclature and Uses
For centuries, smithsonite was grouped with another zinc mineral, hemimorphite, under the single, confusing name “calamine.” The historical term “calamine” was used for any zinc-bearing ore that was non-sulfide, leading to a lack of distinction between the zinc carbonate (smithsonite) and the zinc silicate (hemimorphite). It was not until the early 1800s that the British mineralogist James Smithson identified the zinc carbonate as a distinct mineral species.
The mineral was subsequently named in his honor in 1832. The historical term “calamine” has persisted in common language, though, which can lead to a misunderstanding with the modern pharmaceutical product, calamine lotion. Modern calamine lotion is not made from the mineral smithsonite, but rather uses zinc oxide and ferric oxide to create its soothing pink mixture. Historically, the early use of smithsonite was in the production of brass, where the ore was heated with copper metal, allowing the zinc to vaporize and combine with the copper in a process called cementation.