Hemimorphite

Hemimorphite
General
Category	Sorosilicates
Formula (repeating unit)	Zn4Si2O7(OH)2·H2O
IMA symbol	Hmp
Strunz classification	9.BD.10
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Imm2
Unit cell	a = 8.367(5), b = 10.73 c = 5.155(3) [Å]; Z = 2
Identification
Color	White, grey
Crystal habit	Polar crystals, with different or hemimorphic ends. Also coxcomb masses, mammillary, stalactitic, or massive
Twinning	Rare on {001}
Cleavage	Perfect on {110}, poor on {101}, {001} rare
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5–5
Luster	Vitreous, adamantine, rarely silky
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.516–3.525
Optical properties	Biaxial (+)
Refractive index	nα = 1.614 nβ = 1.617 nγ = 1.636
Birefringence	δ = 0.022
2V angle	Measured: 46°, calculated: 44°
Solubility	Soluble in acid
References

Hemimorphite is the chemical compound Zn₄(Si₂O₇)(OH)₂·H₂O, a component of mineral calamine. It is a silicate mineral which, together with smithsonite (ZnCO₃), has been historically mined from the upper parts of zinc and lead ores. Both compounds were originally believed to be the same mineral and classified as calamine. In the second half of the 18th century, it was discovered that these two different compounds were both present in calamine. They closely resemble one another.

The silicate was the rarer 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.

Occurrence

Hemimorphite "spray" of crystals from Durango, Mexico (size: 2.9 × 2.1 × 2.0 cm)

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, together with silicon, oxygen and hydrogen. The crystals are blunt at one end and sharp at the other.

Blue vug-filling hemimorphite from Wenshan, Yunnan Province, China (size: 9.2 × 4.8 × 3.1 cm)

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 in Tarnowskie Góry area 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.

Further reading

• 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.
• Mineral galleries