Tuesday, March 2, 2010

March 2010 Mineral of the Month


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 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 and a weak light pink fluorescence in longwave UV.

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.
Hemimorphite is a member of the silicates group. Its hardness is 4-1/5 to 5 on the Mohs scale. It has an uneven to conchoidal fracture and a perfect cleavage. It can be white, colorless, blue, greenish, gray, yellowish or brown and has a colorless streak. It is transparent to translucent with a vitreous luster. It will give off water when heated in a closed tube and is soluble in acids with gelatinization.