Maryland State Mineral
Chromite is an iron chromium oxide that often contains aluminum in place of chromium or magnesium in place of iron. It occurs as brownish black to greenish black to black, and has a dark brown streak. Chromite is heavy and almost as hard as steel.
In about 1808, Isaac Tyson, Jr. first discovered chromite ore when he found deposits of the mineral on his farm at Bare Hills, just north of Baltimore, Maryland. Significant stores of the ore were in the Soldiers Delight Serpentine Barrens, a terrain named for its sparse vegetation and characterized by specific flora able to tolerate the high magnesium and low calcium content of serpentine soils. Tyson discovered that, in Maryland, chromite is found only in serpentine and began mining the ore. He established the Baltimore Chrome Works, which operated from 1845 to 1985, mining and producing chromite for use in metallurgy. Maryland was then the largest producer of chrome in the world.
Today, visitors to Soldiers Delight Natural Environment Area and the historic Choate Chrome Mine can view and learn about almost 40 rare, threatened, or endangered plant species, as well as rare minerals and rocks. In 2000, a bill designating chromite as the state mineral was introduced in the state legislature, but the bill failed, and Maryland does not have an official state mineral. However, chromite is an excellent representative of the state’s mining history, economy, and geological heritage.
This mineral was named for its chromium content.
FORMATION AND OCCURRENCE
Chromite forms a series with the rare mineral magnesiochromite and another series with hercynite. It is mainly formed in ultramafic igneous rocks, or rocks with large amounts of magnesium and iron but low amounts of silica, such as peridotites. Chromite is also found in serpentinites and other metamorphic rocks. It is common in most meteorites.
Chromite usually occurs massive, grainy, or in nodules, and rarely forms well-defined crystals. Other minerals it occurs with include olivine, talc, serpentine, uvarovite, pyroxenes, biotite, magnetite, enstatite, plagioclase, ilmenite, and anorthite.
Chromite was initially used to make tool steel; at the turn of the century, its importance increased with the development of stainless steel, for which chromite is an alloying agent. Chromite is the only ore of chromium. Chromium has a wide range of uses in metals, chemicals, and refractories. Chromium use in iron, steel, and nonferrous alloys enhances hardenability and resistance to corrosion and oxidation. The use of chromium to produce stainless steel and nonferrous alloys are two of its more important applications. Other applications are in alloy steel, plating of metals, pigments, leather processing, catalysts, surface treatments, and refractories. Chromite is also used in dyes and mordants.
Because the United States has no chromite ore reserves and a limited reserve base, domestic supply has been a concern during every national military emergency since World War I. World chromite resources, mining capacity, and ferrochromium production capacity are concentrated in the Eastern Hemisphere. The U.S. National Defense Stockpile contains chromium in various forms including chromite ore, chromium ferroalloys, and chromium metal in recognition of the vulnerability of long supply routes during a military emergency.
Chromite is popular among mineral collectors, although there are not many occurrences of collectible specimens.
Since chromite is abundant in the Earth’s crust, substitutes are not necessary. Deposits currently known are sufficient to meet the world’s need for chromium for centuries.
Chemical Formula: FeCr2O4
Crystal Structure: Isometric
Hardness (Mohs): 5½
Transparency: Subtranslucent to opaque
Streak: Dark brown
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press