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Hemimorphite

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Title: Hemimorphite  
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Subject: WikiProject Rocks and minerals/Worklist, Crystal habit, Zinc silicate, Gejiu, Sauconite
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Hemimorphite

Hemimorphite
Hemimorphite from Mapimi, Durango, Mexico
General
Category Sorosilicates
Formula
(repeating unit)
Zn4Si2O7(OH)2·H2O
Strunz classification 09.BD.10
Crystal symmetry Orthorhombic mm2
Unit cell a = 8.367(5) Å, b = 10.73Å, c = 5.155(3) Å; Z = 2
Identification
Color White, blue, greenish
Crystal habit Polar crystals, with different or hemimorphic ends. Also coxcomb masses, mammillary, stalactitic, or massive
Crystal system Orthorhombic pyramidal
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 [1][2][3]

Hemimorphite, is Zn4(Si2O7)(OH)2·H2O, a component of calamine. It is a sorosilicate mineral which has been historically mined from the upper parts of zinc and lead ores, chiefly associated with smithsonite, ZnCO3. They were 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 these two different minerals were both present in calamine. They closely resemble each other.

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 x 2.1 x 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 x 4.8 x 3.1 cm)

The regions on the 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.

References

  1. ^ Handbook of Mineralogy
  2. ^ Webmineral
  3. ^ Mindat.org
  • 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
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