Guam Territorial Mineral
Olivine is a silicate mineral containing iron and magnesium. Also known as chrysolite, olivine is green, brownish-green, or yellow-green in color and is glassy in appearance. Olivine is not scientifically recognized as a mineral separate from the olivine group. It is actually a series with two end-member minerals, forsterite and fayalite. Iron and magnesium are substituted for each other to form the different minerals in the series. Dunite and peridotite, which have the same chemistry as Earth’s magma, are rocks made almost entirely of olivine.
Although Guam does not have an official mineral, olivine is representative of the territory's geology. Olivine is a primary mineral in the lavas that gave birth to most of Guam. Guam even has a green sand beach, in which the sand is made of olivine. Since olivine crystals are heavier than most other types of sand, they remain behind when lighter sand grains are washed away by strong wave activity. Guam does not have an official mineral, but olivine is a good representative of the territory’s geology.
Olivine gets its name from its olive-green color. Traces of nickel are thought to give the mineral its green color, while an abundance of iron can add a reddish tone. The word chrysolite comes from the Greek words for "gold" and "stone."
FORMATION AND OCCURRENCE
Olivine frequently forms at high temperatures and in igneous rocks, normally basalt and rocks containing iron and magnesium with little or no silica. In addition, olivine sometimes forms in certain types of metamorphic rocks. The mineral is therefore common in volcanic regions. Olivine normally occurs in microscopic to small grains or masses, and is a widespread mineral. Despite the mineral’s abundance, however, large specimens are rare and occur in few localities. Larger crystals of olivine are found in meteorites, when the mineral is called olivinoid.
Olivine is common throughout the world. Many localities occur in Australia, Brazil, Canada, China, the Czech Republic, France, Germany, Greenland, Italy, Norway, Russia, Slovakia, South Africa, Spain, Sweden, Switzerland, and the United States.
Olivine, which weathers easily by water, has been found in abundance in volcanic regions, not just on the Hawaiian Islands and Guam, but also on the Moon and Mars. The presence of olivine on Mars implies that the red planet’s chemical erosion by water is low. It also indicates that Mars has been cold and dry throughout most of its geologic history.
Furnaces that are used to bake bricks are lined with olivine. Olivine is used as a refractory liner in kilns and heating furnaces, and is also used as an abrasive. Olivine is an ore of magnesium. It is used as a flux, meaning it is added to liquids in the steel-making process to improve the flow of the liquids.
Peridot is a well-known gem variety of olivine. This bright yellow-green to green gemstone has delighted humans for thousands of years. Some historians even suspect that at least some of the "emeralds" worn by Cleopatra were actually peridot. Because olivine weathers quickly, it rarely grows larger than the size of a pebble; large specimens are highly sought after. And although olivine is quite common, gem-quality peridot is somewhat rare. It remains a relatively affordable gem, however.
When olivine is found in meteorites of iron-nickel composition, the result can make a striking specimen. These are sought after by collectors because of the attractive contrast between the polished gray iron and the gemmy green olivine.
Peridot is the birthstone for those born in August and the stone for the zodiac sign of Libra. Peridot may be given as a gift for a 16th wedding anniversary.
Chemical Formula: (Mg, Fe)2SiO4
Crystal Structure: Orthorhombic
Hardness (Mohs): 6.5–7
Color: Yellow to yellow-green
Transparency: Transparent to translucent
Fracture: Conchoidal - brittle
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press