Get to know a Mineral: Molybdenite

About a week ago, I happened upon a new mineral on the property I am working on. We were cutting channel samples when a purplish liquid ran out of the rock cut and upon further inspection, I found silverish goo in the rock. I did not know it at the time, but we had found Molybdenite. So I decided to make it the topic of today's entry.

Molybdenite or "Molly" (as it is affectionately called by geologists) is a soft metallic mineral that is often confused with graphite. In fact, the two minerals can appear so similar that their streaks are often the only way to tell the difference. And if that fails, then it falls to the comparison of density, molybdenite being the denser of the two.

The chemical formula of "Molly" is MoS, and because of its layered structure it has a lubricating effect. Consisting of a sheet of molybdenum atoms squeezed between sheets of sulfur atoms, the atomic structure of molybdenite balances strong Mo-S bonds with weak sulfur bonds between layers. This results in effortless slippage and cleavage planes.

Occurring in high temperature hydrothermal ore deposits, molybdenite is associated with pyrite, quartz and fluorite (to name a few). The element rhenium readily substitutes for molybdenum which can result in a varying structure. Tracing the radioactive isotope rhenium-187 and the daughter isotope osmium-187 allows for geo-chronolgic dating, which is useful for determining dates of deposition and origin.

The main element in "molly" is Molybdenum (Mo) and it has a rather interesting history. It previously held the name molybdena and was often confused with both graphite and galena. It was thought that it contained lead and that is where it gets its name, molybdos (ancient Greek for lead). It was only in 1778 that it was discovered to be its own element and three years later it was successfully isolated using carbon and linseed oil. Reportedly, the alloying of molybdenum with steel was found to occur in one 14th century Japanese sword. Unfortunately, this art was never used widely and was subsequently lost (hence the lack of molybdenum-sword wielding ninjas).

Obviously not Molly...

In World War I, the British replaced the manganese steel plating on their tanks with molybdenum steel plating, which allowed for higher speed, increased maneuverability and greater protection (despite being thinner). Current uses for molybdenum include being used as an adhesive between enamels and metals, as a fertilizer for some plants (including cauliflower) and in biological staining procedures.

Listed on the London Metal Exchange, molybdenum has a value of about $30,000/tonne (as of August 2009). Due to increased demand, it reached a peak of $103,000/tonne in June of 2005! It is the 54th most abundant element in the crust of the Earth and 42nd in the Universe. It was even discovered on the Moon by a Russian space mission.

Crystal System: Hexagonal
Cleavage: Perfect [0001]
Streak: bluish grey
Hardness:1-1.5
Luster: Metallic

Also Get to Know:
Molybdenite
Cummingtonite
Garnet

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