Zinc carbonate (ZnCO3), a mineral that was the main source of zinc until the 1880s, when it was substituted by sphalerite, is Smithsonite, also known as turkey fat or zinc spar. It is a secondary mineral mainly formed in the presence of CO2 by the weathering and alteration of sphalerite. The oxidized zone of hydrothermal zinc deposits is typical of this. Historically, before it was known that they were two distinct minerals, smithsonite was associated with hemimorphite. In appearance, the two minerals are very similar and the name calamine for both has been used, leading to some misunderstanding. Today it is a small zinc ore, but it was one of the most important ores in the early days of metallurgy. Brown, gray, white, green, and yellow is the standard Smithsonite colors. Translucent specimens are favorites of mineral collectors in bright blue, green, pink, and yellow colors. Smithsonite is also cut into gems for collection and used as an ornamental stone.
In its crystallized form, which is rhombohedral shaped, it is very difficult to find this mineral. In the Upper Mississippi Valley zinc-lead area located in the southwestern part of the state, the mineral is common. In honor of the English chemist and mineralogist James Smithson (c.1765-1829), whose legacy founded the Smithsonian Institution and who first described the mineral in 1802, François Sulpice Beudant named the distinct mineral smithsonite in 1832. This is the same James Smithson who was the Smithsonian Institution’s founding donor. In Greece, Australia, South Africa, Mexico, and the USA, deposits of this unique zinc mineral have been identified.
Smithsonite is a secondary mineral present in many large zinc deposits in the rocks above and around them. Such occurrences of smithsonite are sometimes seen on the surface or at shallow depths. It is usually found as a secondary mineral or alteration product of primary zinc minerals in the oxidized zone of ore deposits. There are large deposits of Laurium, Greece; Bytom and Tarnowskie Góry, Pol.; Sardinia, Italy; and Leadville, Colo., USA. One of the earliest zinc minerals to be found and mined by pioneering metallurgists was Smithsonite. It is a trigonal mineral of variable color that is only rarely present in well-formed crystals. Like earthy botryoidal masses, the usual habit is It has a hardness of Mohs 4.5 and a particular gravity of 4.4 to 4.5.
In a number of habits, the smithsonite of the Upper Mississippi Valley zinc-lead district occurs. These include strong white masses known as “dry bone ore,” large rather cellular masses and rock coatings known as “sheet ore,” and yellowish plates and fragments known as “skull ore.” Many smithsonite originates from the primary mineral of a deposit, which is also sphalerite, when weathering releases zinc. Zinc ore oxidized in the presence of carbon dioxide will lead to smithsonite being formed. This smithsonite is a secondary mineral that is mostly present on land rock as crack fillings and botryoidal coatings. An excellent example of a secondary mineral is Smithsonite, formed from redeposited zinc.
Smithsonite is a good stone to soothe our minds and relax our feelings. When feelings can feel overwhelming, it drives one’s mind into a state of clarity. In order to move on from any and all negative circumstances, this crystal drives one to find resolution. Like pleasant crystals, Smithsonite is almost never found. Botryoidal coatings are a typical mode of occurrence on rocks and fractures. Another mode of occurrence with a honeycomb-like structure is known as dry-bone ore, an impure and friable variety of smithsonite. Smithsonite occurs often in carbonate rocks as replacement bodies and can constitute zinc ore as well. Hemimorphite, willemite, hydrozincite, cerussite, malachite, azurite, aurichalcite and anglesite are commonly found in association.
Smithsonite is often cut into faceted gems and cabochons with excellent color and clarity. It is rarely used in jewelry because it has a hardness of Mohs of just 4 to 4.5 and even with diligent wear it can be easily scratched and abraded. To help one conquer anxiety, depression, low energy, and shyness, this stone is a great ally. Smithsonite brings an inward reflection to one and helps to bring out a profound intensity that one may not know existed. Showing one the strength they have inside will help to find the confidence and self-esteem to step on through the most daunting challenges in life. Smithsonite forms two small solid solution sequences, with the replacement of rhodochrosite-conducing manganese and siderite-conducing iron. Owing to the intergrowth of iron oxides from the oxidation of abundant pyrite and marcasite in the soils, the content is sometimes ocherous. Tiny colorless to white rhombohedral crystals, stalactitic, fibrous coatings, and green to yellow pearly reniform coatings are also known in small vugs. After calcite, the pseudomorphs may be in either rhombohedral or scalenohedral forms and may be partly hollow where the remaining calcite has been leached away.