Chromite

Chromite is an oxide mineral composed of chromium, iron, and oxygen (FeCr2O4). It is dark gray to black in color with a metallic to submetallic luster and a high specific gravity. It occurs in basic and ultrabasic igneous rocks and in the metamorphic and sedimentary rocks that are produced when chromite-bearing rocks are altered by heat or weathering.
Chromite is important because it is the only economic ore of chromium, an essential element for a wide variety of metal, chemical, and manufactured products. Many other minerals contain chromium, but none of them are found in deposits that can be economically mined to produce chromium.


Properties of Chromite

Chromite can be challenging to identify. Several properties must be considered to differentiate it from other metallic ores. Hand specimen identification of chromite requires a consideration of: color, specific gravity, luster, and a characteristic brown streak. The most important clue to identifying chromite is its association with ultrabasic igneous rocks and metamorphic rocks such as serpentinite.


Physical Properties of Chromite

Chemical Classification Oxide
Color Dark gray to black, rarely brownish black
Streak Dark brown
Luster Metallic to submetallic
Diaphaneity Opaque
Cleavage None
Mohs Hardness 5.5 to 6
Specific Gravity 4.0 to 5.1 (variable)
Diagnostic Properties Luster, streak
Chemical Composition FeCr2O4 with magnesium substituting for iron in significant amounts
Crystal System Isometric
Uses An ore of chromium




Chromite is sometimes slightly magnetic. This can cause it to be confused with magnetite. Chromite and ilmenite have very similar properties. Careful observations of hardness, streak, and specific gravity are required to distinguish these minerals in hand specimens.

Uses of Chromite and Chromium

Chromium is a metal used to induce hardness, toughness, and chemical resistance in steel. The alloy produced is known as "stainless steel." When alloyed with iron and nickel, it produces an alloy known as "nichrome" which is resistant to high temperatures and used to make heating units, ovens, and other appliances. Thin coatings of chromium alloys are used as platings on auto parts, appliances, and other products. These are given the name "chrome plated." It is also used to make superalloys that can perform well in the hot, corrosive, and high-stress environment of jet engines.

Chromium's name comes from the Greek word "chroma" which means "color." Chromium is used as a pigment in paint. The familiar yellow lines painted down the center of highways and the yellow paint used on school buses are often "chrome yellow" - a color produced from chromium pigment. Chromium is an important pigment in many types of paint, ink, dye, and cosmetics. Trace amounts of chromium produce the color in many minerals and gemstones. The red color of ruby, the pink of some sapphires, and the green color of emerald are caused by tiny amounts of chromium.


Sources: Geology.com

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