Molybdenite
Molybdenite is hardness 1 to 1.5, density 4.62 to 4.73, in tabular crystals to 15 cm. Graphite looks identical and weighs less than half as much per cubic cm.
Molybdenite is molybdenum disulphide, hardness 1 to 1.5 and measured density 4.62 to 4.73, in tabular or barrel-shaped hexagonal crystals to 15 cm with perfect basal cleavage and a greasy feel. Graphite looks the same and is density 2.09 to 2.23 — less than half as much. Density is the test; colour and habit are not.
In short
- Molybdenite is more than twice as dense as graphite. 4.62 to 4.73 against 2.09 to 2.23. A cubic centimetre of one weighs 4.7 grams and of the other 2.2, and no other property separates them so decisively.
- The streak colours differ and both are easy to see. Molybdenite leaves a bluish-grey mark, graphite a black to steel-grey one. Both are soft enough to mark paper, so this is a test you can run on a specimen in someone else's hand.
- Hardness 1 to 1.5 makes it softer than a fingernail. It has a greasy feel, perfect cleavage on {0001}, and laminae that are flexible but not elastic — unlike mica, a bent flake stays bent.
- Crystals reach 15 cm, commonly tabular or barrel-shaped with poor face development, and often showing trigonal markings on the basal face. Most molybdenite is foliated, massive or in scales rather than crystallised.
- There are two polytypes and a dimorph. The Handbook records the 2H1 and 3R stacking polytypes, and jordisite as the amorphous dimorph of the same composition.
| Property | Molybdenite | Graphite | Does it separate them? |
|---|---|---|---|
| Density | 4.62 to 4.73 | 2.09 to 2.23 | Yes — decisively |
| Streak | Bluish grey | Black to steel-grey | Yes, with care |
| Hardness | 1 to 1.5 | 1 to 2 | No — both mark paper |
| Cleavage | {0001}, perfect | {0001}, perfect | No |
| Feel | Greasy | Greasy | No |
| Laminae | Flexible, not elastic | Flexible, not elastic | No — but this separates both from mica |
| Colour | Lead-grey, metallic | Iron-black to steel-grey | Barely |
The density difference, worked, so it settles the question
Molybdenite and graphite are the two soft, greasy, lead-grey, perfectly basal-cleaving minerals a collection is likely to hold, and in the hand they are very hard to tell apart. The Handbook of Mineralogy gives them densities of 4.62 to 4.73 and 2.09 to 2.23 respectively.
Take a one-centimetre cube of each. The molybdenite weighs about 4.7 g; the graphite about 2.2 g. That is a factor of 2.15, and it is far and away the largest ratio between any two minerals on this site that genuinely look alike. There is nothing subtle about it in the hand: a thumbnail of molybdenite feels wrong for its size, and a thumbnail of graphite feels exactly right.
If you want the number rather than the impression, a hydrostatic weighing gives it. The method and its limits are in our note on measuring specific gravity at home. The gap here is so wide that even a kitchen scale reading to a tenth of a gram will resolve it on a specimen of 30 g or more, which is unusual — most density tests need far better equipment than that to say anything.
Streak, and the one direction this test is worth running
Both minerals are soft enough to mark almost anything, which makes streak a practical field test rather than a laboratory one. The recorded colours are bluish grey for molybdenite and black to steel-grey, shining, for graphite.
The distinction is real but it is not dramatic, and it is easiest to see on unglazed porcelain under good light rather than on paper, where both look simply grey. Rub each along the plate twice and compare the marks side by side if you can — the eye judges a difference between two greys far better than it judges either one alone. Our note on using a streak plate covers why this is one of the cases where the test earns its place.
One caution specific to molybdenite. Because hardness is 1 to 1.5, it will streak on the plate, on your finger, on the inside of a specimen box and on the specimen next to it. Molybdenite is the species most likely to dirty a collection simply by being stored in it, and it is worth keeping in its own box on that account alone.
Polytypes, a dimorph, and what the labels mean
Molybdenite's structure stacks MoS2 layers, and the stacking can repeat in more than one way. The Handbook records two polytypes: 2H1, the common hexagonal one, and 3R, rhombohedral, whose type material is held at the Geological Survey of Canada in Ottawa under number 12112.
Polytypes are not separate species and a label does not need to state one. They are the same composition in different stacking sequences, distinguishable by X-ray and not by eye. A specimen labelled “molybdenite-3R” has had work done on it that most specimens have not, and that is worth noting on a catalogue card even though it changes nothing about how the specimen looks or behaves.
Separately, the Handbook records jordisite as the dimorph — amorphous MoS2, the same composition without a crystal structure. It is not something a collector meets often and it is not identifiable without analysis. We mention it because “dimorphous with jordisite” appears on the sheet and can otherwise look like a nomenclature problem when it is not one; see our note on type localities for how these distinctions are recorded.
Where crystallised molybdenite occurs
Molybdenite is the most abundant molybdenum mineral and is widespread, but almost all of it is disseminated flakes in porphyry copper and porphyry molybdenum ore — economically important and useless as specimens. Face development is recorded as poor even where crystals do form.
The Handbook names, for fine crystals, the Crown Point mine at Lake Chelan, Chelan County, Washington, and the Frankford quarry in Philadelphia; the Temiskaming district and Aldfield Township in Quebec; Raade near Moss and Vennesla near Arendal in Norway; the Adun-Chilon Mountains south of Nerchinsk in Transbaikal, Miass in the Ilmen Mountains and the Slundyanogorsk deposit in the Urals; Altenberg in Saxony; and Azegour in Morocco.
The typical settings are high-temperature hydrothermal veins, contact-metamorphic deposits in limestone with calcium silicates, and granite pegmatites — which is why molybdenite turns up alongside scheelite, wolframite and cassiterite rather than with the oxidised copper suite. In Britain it occurs sparingly in the Cornish granites and in the Shap and Skiddaw granites, in small flakes rather than in specimens.
We hold no catalogue and nothing here is offered for sale. The wanted list sets out what we look for, and for molybdenite the things that matter are a terminated crystal rather than a flake, and a named mine rather than a district. Our species notes cover the other sulphides of the high-temperature suite.