Hemimorphite
Hemimorphite from Mapimi, Durango, Mexico
General
Category	Mineral
Chemical formula	Zn4Si2O7(OH)2·H2O
Identification
Color	White, blue, greenish
Crystal habit	Polar crystals, with different or hemimorphic ends. Also coxcomb masses, mammillary, stalactitic, or massive
Crystal system	Orthorhombic
Cleavage	Perfect
Fracture	Uneven to conchoidal
Mohs scale hardness	4.5-5
Luster	Vitreous, silky
Specific gravity	3.516 - 3.525
Refractive index	Transparent to translucent
Solubility	Soluble in acid

Hemimorphite, is a sorosilicate mineral which has been mined from days of old from the upper parts of zinc and lead ores, chiefly associated with smithsonite. It was often assumed to be the same mineral and both were classed under the same name of calamine. In the second half of the 18th century it was discovered that there were two different minerals under the heading of calamine - a zinc carbonate and a zinc silicate, which often closely resembled each other.

The silicate was the more rare of the two, and was named hemimorphite because of the hemimorph development of its crystals. This unusual form, which is typical of only a few minerals, means that the crystals are terminated by dissimilar faces. Hemimorphite most commonly forms crystalline crusts and layers, also massive, granular, rounded and reniform aggregates, concentrically striated, or finely needle-shaped, fibrous or stalactitic, and rarely fan-shaped clusters of crystals.

Some specimens show strong green fluorescence in shortwave ultraviolet light (253.7 nm) and weak light pink fluorescence in longwave UV.

[edit] Occurrence

Hemimorphite most frequently occurs as the product of the oxidation of the upper parts of sphalerite bearing ore bodies, accompanied by other secondary minerals which form the so-called iron cap or gossan. Hemimorphite is an important ore of zinc and contains up to 54.2% of the metal.

The regions on the Belgian-German border are well known for their deposits of hemimorphite of metasomatic origin, especially Vieille Montagne in Belgium and Aachen in Germany. Other deposits are near Tarnovice in upper Silesia, Poland; near Phoenixville, Pennsylvania; the Missouri lead-zinc district; Elkhorn, Montana; Leadville, Colorado; and Organ Mountains, New Mexico in the United States; and in several localities in North Africa. Further hemimorphite occurrences are the Padaeng deposit near Mae Sod in western Thailand; Sardinia; Nerchinsk, Siberia; Cave del Predil, Italy; Bleiberg, Carinthia, Austria; Matlock, Derbyshire, England.

[edit] Use

Zinc silicate (Zn₂SiO₄, CAS number [68611-47-2], EINECS number 271-896-0), whether prepared from hemimorphite or from quartz and zinc oxide, can be used as a food additive, as an anti-caking agent for dry foods. As such, its E number is E557. [1] However its use is rare and its status is now unpermitted.

Zinc silicate, doped with manganese, sometimes with addition of arsenic or antimony, is often used as a green phosphor for display tubes, plasma displays, and fluorescent lamps.

Zinc silicate can be also used as a corrosion inhibitor pigment for some primers. [2] In such application it may be called Inorganic Zinc Silicate (IZS). The paint is composed of ethyl silicate and zinc powder. [3]

Zinc silicate can be also a component of some glass ionomer dental cements. [4]

[edit] References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
• Boni, M., Gilg, H.A., Aversa, G., and Balassone, G., 2003, The "Calamine" of southwest Sardinia: Geology, mineralogy, and stable isotope geochemistry of supergene Zn mineralization: Economic Geology, v. 98, p. 731-748.
• Reynolds, N.A., Chisnall, T.W., Kaewsang, K., Keesaneyabutr, C., and Taksavasu, T., 2003, The Padaeng supergene nonsulfide zinc deposit, Mae Sod, Thailand: Economic Geology, v. 98, p. 773-785.
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• Webmineral
• Mindat.org