April 2010 Mineral of the Month

CHRYSOPRASE

Chrysoprase is a gemstone variety of chalcedony (a cryptocrystalline form of silica) that contains small quantities of nickel which provide it with its apple green to deep green color. The darker varieties of chrysoprase are also referred to as prase. The word chrysoprase comes from the Greek chrysos meaning 'gold' and prason, meaning 'leek'. As with all forms of chalcedony, chrysoprase has a hardness of 6 - 7 on the Mohs hardness scale and a conchoidal fracture like flint. The best known sources of chrysoprase are Queensland, Western Australia, Germany, Poland, Russia, Arizona, California, and Brazil. The chrysoprase and Ni silicate ore deposit in Szklary, Lower Silesia, Poland, was probably the biggest European chrysoprase occurrence and possibly also the biggest in the world. Chrysoprase was used by the Greeks, Romans, and the Egyptians in jewelry and other ornamental objects and because of its semi-opaque green color; it is often mistaken for Imperial jadeite. One of the most valuable chalcedony gem stones, chrysoprase is prized for its color and rarity. The stone occurs in serpentine rocks and in weathered materials of nickel ore deposits as nodules or in veins within the host rock

March 2010 Mineral of the Month

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 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.