Quick answer: Sulfide minerals are compounds of sulfur with metals such as iron, copper, lead, zinc, and silver, and they are the main ores of most metals people use. The most common are pyrite (iron), chalcopyrite (copper), galena (lead), and sphalerite (zinc). Most sulfides are dense and metallic, and many tarnish or break down in moist air.
The wire in your walls, the zinc on galvanized steel, the lead in a car battery: most of it began as a sulfide mineral. Sulfides are compounds of a metal with sulfur, and they are the ore minerals behind a large share of the world’s copper, zinc, lead, nickel, and molybdenum, as well as much of its silver.
The main players
- Pyrite (FeS2): the most common sulfide, brassy and cubic. Covered in pyrite: fool’s gold.
- Galena (PbS): the chief lead ore, heavy and silver-grey, breaking into cubes. See galena.
- Sphalerite (ZnS): the main zinc ore. Surprisingly variable, from black to honey-yellow to red, with a resinous luster.
- Chalcopyrite (CuFeS2): the most important copper ore, brassy yellow and softer than pyrite, often tarnishing to iridescent colors.
- Cinnabar (HgS): bright red mercury ore, one of the minerals that needs careful handling, as noted in toxic minerals.
- Molybdenite (MoS2): soft, flexible, lead-grey flakes, the source of most molybdenum.
How sulfides form
Most sulfide ore forms from hot, salty water moving through the crust, often heated by a cooling body of magma. That water dissolves metals and carries them along with sulfur. When it cools, reacts with the surrounding rock, or mixes with other water, the metals and sulfur combine and drop out as sulfide minerals, often in cracks alongside quartz and calcite. We describe the process in hydrothermal veins. A calcite and quartz piece with scattered sulfide grains is a small sample of exactly that environment.
How to recognize a sulfide
- Metallic luster in most species, although sphalerite and cinnabar can look non-metallic.
- Heavy for their size, a quick clue explained in specific gravity.
- Dark streak: pyrite, which looks gold, leaves a greenish-black powder. See the streak test.
- A faint sulfur smell when some are scratched or crushed.
What happens when sulfides meet air
Sulfides formed where oxygen was scarce, so at the surface they are chemically unstable. Water and oxygen break them down into sulfuric acid and metal-bearing solutions. At the top of an ore body this creates a rusty cap described in gossan, and it leads to secondary minerals such as smithsonite, azurite, and malachite. On a larger scale, the same chemistry turns rivers red, as in the Rio Tinto district.
Caring for sulfide specimens
The same reaction can happen slowly in a collection. Some pyrite and marcasite, especially fine-grained material, can decay in humid air, producing a white or yellowish powder and a sour smell. To protect sulfides:
- Store them somewhere dry, with stable, moderate humidity.
- Do not wash them or leave them damp.
- Keep an eye out for powder or a sulfurous smell, and isolate any piece that shows it.
More in our guide to cleaning and storing specimens.
