molybdenite

[MOLYBDEN(UM) + -ITE¹.]

A mineral having composition MoS₂. Molybdenite is the chief ore of molybdenum. It crystallizes in the hexagonal system, but crystals are rare and when found are hexagonal plates. It is commonly in scales or foliated masses. The mineral has a greasy feel. The hardness is 1.5 (Mohs scale) and the specific gravity is 4.7. The luster is metallic and the color lead gray. Molybdenite and graphite have long been confused because of their nearly identical physical properties. They can be distinguished by the streak left on glazed paper, black for graphite and green for molybdenite. Molybdenite has been used as a lubricant.

Molybdenite occurs in various places in Norway, Sweden, Australia, England, China, and Mexico. In the United States, molybdenite is found in small amounts at many localities but the most important occurrence is at Climax, Colorado. See also Molybdenum.

Environment

Disseminated as flakes and tiny crystals in igneous rocks and infrequently in larger crystals in simple pegmatites.

Crystals are common, sometimes well developed but usually misshapen, since they are tabular and split or bend easily. Also in small irregularly shaped flakes; rarely finely granular.

Lead gray. Luster metallic; hardness 1-1Ɖ specific gravity 4.7-4.8; streak gray on paper; cleavage perfect micaceous basal. Flakes flexible, but not elastic; greasy feel; sectile.

Molybdenum disulfide (60.0% Mo, 40.0% S).

Under the oxidizing flame gives sulfur (rotten-egg) fumes on charcoal and colorful coatings around the assay: red near the assay, yellow cooling to white farther away. The white coating touched by the reducing flame becomes azure blue.

Soft and flexible flakes can only be confused with graphite, which is blacker. The blowpipe test is extremely easy and characteristic. Splitting the cleavage, a hint of violet is seen between two slightly separated, mirroring cleavage flakes of molybdenite.

Disseminated flakes of molybdenite became an ore of molybdenum at Climax, Colorado. They are separated with the copper mineral in Bingham Canyon, Utah, where small flakes are abundantly scattered through the porphyry. Large plates (1 in., 3 cm) from Deepwater, New South Wales, associated with quartz are cabinet specimens. Good crystals embedded in pegmatites, coarse granitic rock, or quartz are fairly common. Chelan, Washington, is a notable example for these.

Molybdenite, though seldom seen in rocks, may be the primary source of molybdenum, from which secondary molybdenum minerals (molybdates such as powellite) might form as a result of weathering. (See powellite, p. 265.) Although unrecognizable as a mineral, there also appears to be an amorphous, easily oxidized molybdenum sulfide (jordisite, MoS) that decomposes readily to a water-soluble blue dye. On drying, inky blue hues redeposit as the molybdenum oxide called ilsemannite. This oxide tints rocks blue at the Marysvale, Utah, mine and in the ore piled about the wartime Lakeview, Oregon, uranium refinery. Associated with realgar and orpiment, it makes colorful red-yellow-blue hand specimens. Even at Nevada's Getchell Mine, realgar and orpiment have dry-weather blue spots.

Molybdenite
Euhedral molybdenite on quartz, Molly Hill mine, Quebec, Canada. The large crystal is 15mm across
General
Category	Mineral
Chemical formula	molybdenum sulfide:MoS2
Identification
Color	Black, lead-silvery gray
Crystal habit	Thin, platy hexagonal crystals terminated by pinacoidal faces, also as tapering six-sided pyramids that can be truncated by the pinacoids. Also massive, lamellar and in small grains in sulfide ore bodies
Crystal system	Hexagonal; 6/m 2/m 2/m
Cleavage	[0001] Perfect
Fracture	flaky
Mohs scale hardness	1 - 1.5
Luster	Metallic
Streak	bluish - gray
Specific gravity	4.73
Refractive index	Opaque
Fusibility	Infusible
Other characteristics	Thin cleavage sheets and crystals are flexible, but not elastic. It has a greasy feel and leaves marks on fingers

Crystal structure of molybdenite

Molybdenite is a mineral of molybdenum disulfide, MoS₂. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfide atoms. The Mo-S bonds are strong, but the interaction between the sulfur atoms at the top and bottom of separate sandwich-like tri-layers is weak, resulting in easy slippage as well as cleavage planes.

Molybdenite occurs in high temperature hydrothermal ore deposits. Its associated minerals include pyrite, chalcopyrite, quartz, anhydrite, fluorite, and scheelite. Important deposits include the disseminated porphyry molybdenum deposits at Questa, New Mexico and the Henderson and Climax mines in Colorado. Molybdenite also occurs in porphyry copper deposits of Arizona, Utah, and Mexico.

The element rhenium is always present in molybdenite as a substitute for molybdenum, usually in the parts per million (ppm) range, but often up to 1–2%. High rhenium content results in a structural variety detectable by X-ray diffraction techniques. Molybdenite ores are essentially the only source for rhenium. The presence of the radioactive isotope rhenium-187 and its daughter isotope osmium-187 provides a useful geochronologic dating technique.

References and external links

Wikimedia Commons has media related to: Molybdenite

• Dana's Manual of Mineralogy ISBN 0-471-03288-3
• Mineralgalleries.com
• Webmineral
• Mindat.org
• Molybdenite Dating

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