PHYSICAL PROPERTIES:
Chemistry: ZnCO3
Composition: Zinc carbonate
Class: Carbonates
Group: Calcite
Crystal system: trigonal
Fracture: conchoidal
Hardness: 4 - 4.5
Specific gravity: 4.4
Luster: pearly to resinous
Streak: white
Cleavage: perfect in three directions
Color: pink, teal, apple to blue-green,
lavender, yellow, white, tan, brown, blue,
orange, peach, colorless, and red
Transparency: transparent to translucent
Associated Minerals: hemimorphite,
cerussite, wulfenite, limonite, mimetite, dolomite, hydrozincite, aurichalcite,
calcite and other carbonate minerals.
COMPOSITION:
Zinc carbonate (64.8% ZnO, 35.2% C02)
usually some of the Zink is replaced by Fe, Mg, Ca; Cd and Cu. Co may
also be present.
DISTINGUISHING CHARACTERISTICS:
Smithsonite belongs to the calcite group, a group of related
carbonates that are isomorphous with one another. Smithsonite has
a silky to pearly luster. This luster along with it's botryoidal structure
gives Smithsonite an interesting play of light. Almost as if it were
lit from beneath. It is essentially zinc carbonate, but the zinc is
usually partially replaced with other elements. These elements are responsible
for the great color variations that this mineral exhibits. Copper is
responsible for green to blue colors. Cobalt yields pink to purple colors.
While cadmium produces yellowish colors and iron makes the brown to
red colors. Many other zinc ore minerals may have originally
been smithsonite before metamorphism or other altering processes,
formed new minerals. But Smithsonite is not easy to confuse with
many other minerals. Certain Hemimorphites have a similar botryoidal
habit and blue-green color, but the fracture edges of smithsonite specimens
are smooth. Where as, hemimorphite, reveals minute radiating crystals.
Some smithsonite crusts resemble prehnite. But Prehnite
is harder and does not dissolve in acid. Smithsonite's reaction to hot
HCl acid and its high hardness for a carbonate also set it apart.
ENVIRONMENT: Smithsonite
is formed in dry climates in limestone regions. It is a weathering product
of primary sulfide zinc ores such as sphalerite.
CRYSTAL DESCRIPTION:
The typical crystal habit of smithsonite is a rounded, bubbly form called
botryoidal. More rarely, small and poorly formed crystals occur, in
rounded rhombohedrons or scalenohedrons. Smithsonite may also form as
mainmillary crusts, with a crystalline surface.
TESTS:
Effervesces slightly with warm hydrochloric (HCl) acid.
Another good test is in a closed tube. Heat the tube to form
a film that is yellow when hot and white when cold. Also a small grain
may be heated on charcoal, touched with cobalt nitrate and reheated.
This gives a good green (Zn) color.
LOCALITIES:
Tsumeb, Namibia and the Broken Hill Mine in Zambia are
two famous
African mines. The Tsumeb specimens are colored by trace amounts of
cobalt and can have some interesting colors. Crystals are common only
at Tsumeb. The Kelly Mine, Magdalena, New Mexico has produced the best
blue-green botryoidal masses of smithsonite. This material was once
marketed for a jewelry stone under the name bonamite. Similar, but thinner,
crusts were found in Laurium, Greece. Crusts and stalactites come from
Arkansas. Other USA locations include Leadville, Colorado; Utah; Idaho
and Arizona. Mexico has also been a good source.
USES:
It is sometimes an important ore of zinc, as at Leadville,
Colorado, where its ore value was overlooked for years. It is used
mostly as mineral specimens.
FACTS & HISTORY:
Smithsonite was named for James Smithson, the Englishman
who left money for the establishment of the Smithsonian Institution.
The United States received $500,000 for this purpose and Smithsons
collection of minerals. Smithson was further immortalized when
the mineral calamine was renamed after him.
2000 years ago, Romans extracted zinc from smithsonite and mixed
it with copper to make brass.
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