English: Molybdenite-quartz hydrothermal veins from the Miocene of Colorado, USA. (public display, Denver Museum of Nature & Science, Denver, Colorado; donated by Amax Exploration)

Dark gray = molybdenite (MoS2) Light gray = quartz Light grayish-brown = host rock

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.

Molybdenite is a molybdenum sulfide mineral (MoS2). It has hexagonal crystals, metallic luster, a bright silvery color, and a dark gray streak. It is fairly soft (H=2) and has one cleavage plane. Molybdenite is especially distinctive in being flexible - thin scales or plates of molybdenite will easily bend but won't snap back into shape as do biotite or muscovite mica.

Molybdenite is nearly identical to graphite (C) in its physical characteristics (see: www.flickr.com/photos/jsjgeology/sets/72157650963514503). Graphite is a principally a metamorphic mineral. Molybdenite is usually an igneous mineral, occurring in hydrothermal veins and pegmatites. It also occurs in some contact metamorphic rocks (skarns - www.flickr.com/photos/jsjgeology/sets/72157646562268189).

The molybdenite-quartz veins seen here are part of the Mount Emmons Stock, a complex of intrusions in the Creteceous-aged Mancos Shale and Mesaverde Sandstone in western Colorado.

Geologic unit: Mount Emmons Molybdenum Deposit, Early Miocene, 17-18 Ma

Locality: Keystone Mine (= Mount Emmons Mine), west of the town of Crested Butte, northern Gunnison County, western Colorado, USA

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Site-specific info. from:

Dowsett et al. (1981) - Geology of the Mount Emmons Molybdenum Deposit, Crested Butte, Colorado. New Mexico Geological Society Guidebook, 32nd Field Conference, Western Slope Colorado, 1981: 325-331.