17 Mart 2013 Pazar

Georgia State Mineral

Georgia State Mineral


Staurolite is a silicate hydroxide mineral that contains iron, magnesium, zinc, and aluminum. It occurs as dark brown, reddish-brown, brownish-yellow, brownish-black, or black. Staurolite is mostly opaque and sometimes transparent, and its crystals are normally prismatic. A unique property of this mineral is that its crystals often form in a cross shape. Staurolite frequently exhibits an effect called penetration twinning, when individual crystals seem to have grown through each other. This type of twinning occurs because the mineral crystallizes at 60-degree angles, or less commonly, at 90-degree angles. In some locations, however, staurolite occurs as tiny, rounded pebbles.
Staurolite is abundant in northern Georgia, notably in Cherokee and Fannin Counties. It is popular as a souvenir, good luck charm, and mineral specimen. Staurolite was designated the official state mineral of Georgia in 1976.
The name staurolite comes from the Greek words stauros, meaning "cross," and lithos, meaning "stone." The name refers to the crossed twins this mineral sometimes displays.
Staurolite is common in clayey schists, gneisses, and other rocks, and is regionally metamorphosed at intermediate temperatures and pressures to a hydrous silicate. This mineral also occurs as small debris material, the result of erosion by water or glacial activity. In relation to other silicate minerals, staurolite weathers very slowly. Georgia’s state mineral is frequently associated with almandine, sillimanite, kyanite, tourmaline, muscovite, quartz, chloritoid, and garnet.
Staurolite is widespread. Examples of localities for fine specimens are listed below.
  • Switzerland: Pizzo Forno and Alpe Piona, Ticino
  • Austria: Mt. Greiner, Zillertal, Tirol
  • Russia: Keivy massif, Kola Peninsula
  • France: Finistère and Morbihan Provinces in Brittany
  • Portugal: Fanzeres
  • United States: Franconia, Grafton County, New Hampshire; Chesterfield, Hampshire County, Massachusetts; near Blue Ridge, Fannin County, Georgia; near Picures, Taos County, New Mexico
  • Brazil: Rubellita and Ardenella, Minas Gerais; Agua Quente, Rio Pardo de Minas; and Fazenda Graniais
Staurolites are popular with rock collectors for their distinctive shapes. Since the 90-degree angle crosses are less common, they are particularly sought after. Even scarcer are staurolites that have both 60- and 90-degree twinning, resembling a star. Staurolite’s cross shape, occasionally even occurring without matrix, make it popular as a symbol of Christianity. Some specimens are said to resemble St. Andrew’s, Maltese, and Roman crosses. The crystals are sometimes called fairy crosses or fairy stones, and are collected as good luck charms. 
Many legends surround staurolite’s origins and formation. It is believed that Pocahontas gave John Smith a necklace made of a "fairy cross," as staurolite specimens are also known. It has been said that President Theodore Roosevelt carried one for good luck.
Staurolite is a metamorphic mineral of medium to high grade and is fairly common worldwide. These qualities make the mineral useful to geologists in estimating the temperature, depth, and pressure at which a rock undergoes metamorphism.
Staurolite is important to the construction industry. The primary use of this mineral is in the production of portland cement, where it substitutes for clay. Staurolite is also used as a sandblasting material.
State Mineral
Staurolite in Matrix
State Mineral
Staurolite Showing "Star" Effect
State Mineral
Cross Example
State Mineral
Staurolite Specimens on Display
Group: Staurolite
Chemical Formula: (Fe2+,Mg,Zn)1.5-2Al9[O6|(OH,O)2|(SiO4)4]
Crystal Structure: Monoclinic
Hardness (Mohs): 7 - 7½
Color: Dark brown, brownish-black, red-brown
Transparency: Translucent
Luster: Sub-Vitreous, resinous
Density: 3.74 - 3.83 g/cm3
Streak: White to grayish
Cleavage: Distinct/Good
Distinct on {010}
Fracture: Sub-Conchoidal
Tenacity: Brittle
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press

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