Hemimorphite

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. The crystal structure produces a different termination at each end of the crystal. The crystal base is formed as a pedion (a crystal form having only a single face, without a symmetrical equivalent) while the visible tip is formed like the point of a pyramid. Some specimens show strong green fluorescence in shortwave ultraviolet light (253.7 nm) and weak light pink fluorescence in longwave UV.

Specimens of hemimorphite are found in two very different forms. One form produces glass like, clear or white, thin, bladed crystals, often well-formed showing many crystal faces. Many times these crystals are arranged in fan shaped aggregates. The other form produces a blue to blue-green botryoidal crust that resembles smithsonite or prehnite. For a collector both forms are a must in their mineral collections.

information courtesy of https://en.wikipedia.org/wiki/Hemimorphite