Smithsonite belongs to the calcite mineral group. It also forms a series as the zinc-dominant (Zn) end member with iron-dominant (Fe) siderite.
Smithsonite is named for James Smithson, the founder of the Smithsonian Institution. The luster of smithsonite sets it apart from other minerals: it has a silky to pearly luster giving natural specimens a certain play of light across its surface that resembles the fine luster of melted wax glowing under a candle flame.
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.
|Superb Smithsonite Crystals. from: Kelly Mine, Magdalena District, Socorro Co., New Mexico, USA. Credit: Anton Watzl
Smithsonite in addition to wonderful luster also has a varied color assortment. The apple green to blue-green color is probably smithsonite's most well known color, but it is its purple to lavender color that is probably its most sought after hue. There also exists attractive yellow, white, tan, brown, blue, orange, peach, colorless, pink and red smithsonite specimens and all of them are a credit to this mineral.
The typical crystal habit of smithsonite is an interesting form called botryoidal. This form has the appearance of grape bunches and is the result of radiating fibrous crystals that form from central attachment points and grow outward and into each other. The result is a rounded, bubbly landscape for which smithsonite is considered the classic example.
Smithsonite is not easy to confuse with many other minerals. Hemimorphite has a similar botryoidal habit and blue-green color, but the fracture edges of smithsonite's specimens have a plastic-like look while hemimorphite reveals minute, radiating crystals. Prehnite has similar color and habit as well, but is much lighter and harder. Both of these minerals lack the melted wax luster of smithsonite. Its high density, good cleavage, crystal habit, luster, its reaction to hot HCl acid and its high hardness for a carbonate are all quite conclusive for smithsonite to be differentiated from all other minerals. With its lovely luster, many beautiful colors and interesting habits, smithsonite specimens are a source of real pleasure for collectors around the world.
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.
The Kelly Mine in Socorro County, New Mexico, USA produces celebrated blue and blue-green material of fine color in massive crusts.
Properties of Smithsonite
- Formula: ZnCO3
- Class: Carbonates
- Group: Calcite
Color is commonly apple green, blue-green, lavender, purple, yellow and white as well as tan, brown, blue, orange, peach, colorless, gray, pink and red.
- Luster is usually pearly to resinous with light play across its surface and sometimes is simply vitreous.
- Transparency: Crystals are transparent to translucent.
- Crystal System is trigonal; bar 3 2/m
- Crystal Habits include the rhombohedrons and scalenohedrons with generally curved faces. But more commonly is botryoidal or globular.
- Cleavage is perfect in three directions forming rhombohedrons.
- Fracture is uneven.
- Hardness is 4 - 4.5.
- Specific Gravity is approximately 4.4 (heavy for nonmetallic minerals)
- Streak is white.
- Associated Minerals are those found in oxidation zones of zinc sulfide deposits such as hemimorphite, cerussite, wulfenite, limonite, mimetite, dolomite, hydrozincite, aurichalcite, calcite and other carbonate minerals.
- Other Characteristics: Effervesces slightly with warm hydrochloric (HCl) acid.
- Noteable Occurrences include Tsumeb, Namibia and the Broken Hill Mine in Zambia; the Kelly Mine, Magdalena, New Mexico; Leadville, Colorado; Utah; Idaho and Arizona, USA; Mexico; Laurion, Greece; Bytom, Poland; Moresnet, Belgium and many other localities.
- Best Field Indicators are luster, typical botryoidal habit, cleavage, hardness, reaction to hot acids and density.