File:Molybdenite vein in quartz monzonite (latest Cretaceous to earliest Tertiary, 62-66 Ma; Continental Pit, Butte Mining District, Montana, USA) 1 (19215397436).jpg

Molybdenite vein in quartz monzonite from Montana, USA.

Silvery-gray = molybdenite Light-colored, speckled material = quartz monzonite 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 4900 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. Molybdenite is especially distinctive in being flexible - thin scales or plates of molybdenite will easily bend but won't snap back into shape like 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 specimen shown above is quartz monzonite rock with a molybdenite hydrothermal vein from Montana's Butte Mining District. Butte was the top copper producer on Earth for many decades. In that area, hydrothermal veins have intruded and altered the Butte Quartz Monzonite (Butte Pluton, mid-Campanian Stage, late Late Cretaceous, 76 million years), a large intrusive igneous mass forming part of the Boulder Batholith. Copper sulfide-rich hydrothermal vein intrusion occurred at about 62 to 66 million years, around the time of the Cretaceous-Tertiary transition.

This specimen is from the only currently active mine in Butte - the Continental Pit. The molybdenite in the sample is one of two ore minerals that are the mining targets at Continental Pit. The molybdenite is processed (overseas) for its molybdenum. The other important ore mineral at this mine is chalcopyrite, which is processed (also overseas) for its copper.

Locality: 5640 Bench of the Continental Pit, Butte Mining District, northeastern Silver Bow County, southwestern Montana, USA (46° 00' 37.39" North, 112° 28' 42.36" West)

Photo gallery of molybdenite: