May 2010 Mineral of the Month

'Spinel'

The name Spinel comes from the Latin word ‘spina’ meaning ‘thorn’ in reference to its sharp sided crystals. Spinel is a member of the oxides mineral group. Spinel crystallizes in the isometric system; common crystal forms are octahedral, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Some spinels are among the most famous gemstones: Among them is the Black Prince's Ruby and the 'Timur Ruby' in the British Crown Jewels, and the 'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats. True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones in Afghanistan and in Mogok, Burma. Recently gem quality spinels were found in the marbles of Luc Yen in Vietnam, the Mahenge and Matombo regions in Tanzania, the Tsavo region in Kenya and in the gravels of Tunduru in Tanzania and Ilakaka in Madagascar. Spinel is found as a metamorphic mineral, and also as a primary mineral in igneous rocks. In these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminum. Aluminum oxide may form as the mineral corundum or may combine with magnesium to form spinel. This is why spinel and ruby are often found together.

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.

February 2010 Mineral of the Month

Mica Group

Zinnwaldite Biotite

The word "mica" is thought to be derived from the Latin word micare, meaning "to glitter", in reference to the brilliant appearance of this mineral especially when in small scales.

The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having highly perfect basal cleavage. The five most common Micas are: Phlogopite, Biotite, Zinnwaldite, Lepidolite and Muscovite. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Phlogopite Lepidolite

Mica is widely distributed and occurs in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from granitic pegmatites. Until the 19th century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, their price dramatically dropped when large reserves were found and mined in Africa and South America during the early 1800s. The largest sheet of mica ever mined in the world came from a mine in Denholm, Quebec, Canada. Scrap and flake mica is produced all over the world. Flake mica comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are pegmatite deposits.

Muscovite

Mica has several industrial uses including “Isinglass Mica” which are sheets of mica used as peepholes in boilers and lanterns because they are less likely to shatter compared to glass when exposed to extreme heat. Mica has a high dielectric strength and excellent chemical stability, making it a favored material for manufacturing capacitors for radio frequency applications. It is also used as an insulator in high voltage electrical equipment.

January 2010 Mineral of the Month

SULFUR

Named from the Middle English word 'sulphur' meaning brimstone. Crystals are usually yellow to yellowish-brown blocky dipyramids with thick tabular and disphenodal crystals being less common. Native sulfur is usually formed from volcanic action as a sublimate from volcanic gasses associated with realgar, cinnabar, and other minerals. It is also found in some vein deposits and as an alteration product of sulfide minerals. It can also be formed biogenically, a major source being salt domes, where it has formed by the bacterial decomposition of calcium sulfate.

Sulfur is a member of the native elements group and has a hardness of 1-1/2 to 2-1/2 on the Mohs Scale. Its specific gravity is 2.0 to 2 .1. Sulfur has a resinous luster and an imperfect cleavage with an uneven to conchoidal fracture. Its chemical composition is S

Sulfur fuses at the relatively low temperature of 113 degrees and gives off choking fumes of sulfur dioxide when burned. There are many industrial uses of sulfur ranging from matches and fireworks to rubber.