Pyrrhotite Identification Guide

What Pyrrhotite Looks Like

Pyrrhotite (Fe(1-x)S, an iron-deficient iron sulfide) is bronze-yellow to brownish-bronze with a metallic luster, commonly developing a brown to iridescent tarnish. It is opaque. Crystals are hexagonal/monoclinic tabular or platy, but it most often occurs massive, granular, or as compact grains disseminated in rock. Its standout property is being the most strongly magnetic of the common sulfides — hence the nickname 'magnetic pyrite.'

Step-by-Step Field ID Checklist

1. Note the color: bronze to brownish-bronze, often with iridescent or dull-brown tarnish — warmer/browner than pyrite's pale brass.
2. Test with a magnet: pyrrhotite is attracted to a magnet (variably, often weakly to moderately) — the single best field test.
3. Test hardness: 3.5–4.5; a knife scratches it (pyrite does not).
4. Do the streak: dark gray-black to brownish-black.
5. Heft it: heavy (SG ~4.6).
6. Note associations: with pyrite, pentlandite, chalcopyrite (nickel-copper sulfide ores), and in mafic intrusions.

Key Diagnostic Tests

• Magnetism: attracted to a hand magnet — diagnostic among bronze/brass sulfides.
• Hardness: 3.5–4.5 (Mohs); softer than pyrite (6–6.5), and a knife scratches it.
• Streak: dark gray-black to brownish-black.
• Specific gravity: ~4.6.
• Color/luster: bronze metallic, tarnishing brown.
• No effervescence in HCl (may give off a faint H2S smell when reacting with acid).

Common Look-Alikes and How to Tell Them Apart

• Pyrite: paler brass, much harder (6–6.5, resists a knife), and not magnetic (unless altered). Pyrrhotite is browner, softer, and magnetic — the magnet and hardness settle it.
• Pentlandite: very similar bronze color and often intergrown with pyrrhotite, but pentlandite is not magnetic and has octahedral parting; it is the nickel ore. Magnet test separates them.
• Chalcopyrite: more golden-yellow, softer (3.5–4) with a greenish-black streak, and non-magnetic.
• Niccolite: pale copper-red rather than bronze, and very heavy; non-magnetic.
• Magnetite: strongly magnetic too, but black (not bronze) with a black streak and harder (5.5–6.5).

Where Pyrrhotite Is Found

Pyrrhotite occurs in mafic and ultramafic igneous intrusions (often with nickel-copper sulfides), in contact metamorphic deposits, pegmatites, and high-temperature hydrothermal veins. Major occurrences include Sudbury (Ontario, Canada), Norilsk (Russia), and many Scandinavian and South African deposits. It is important as a host for nickel and platinum-group elements.