Greigite is a rare magnetic iron sulfide that is the sulfur analog of magnetite. It is primarily found in sedimentary environments or as a product of low-temperature hydrothermal processes, often forming tiny octahedral crystals or massive crusts.
Is this greigite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch greigite with a known reference. Greigite sits at Mohs 4-4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: blue, blue black, bronze, pink.
- 5Look at form & habitCrystal system: cubic. Typical habit: microscopic crystals.
Often confused with
Greigite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Pyrite is the harder of the two (Mohs 6-6.5 vs. 4-4.5); streak differs, so Greigite leaves black, Pyrite leaves greenish black to brownish black.
How to tell apart: Magnetite is the harder of the two (Mohs 5.5-6.5 vs. 4-4.5).
How to tell apart: Streak differs, so Greigite leaves black, Pyrrhotite leaves dark grayish black.
Often found alongside greigite
Minerals reported to co-occur with greigite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₃S₄
- Mohs hardness
- 4-4.5
- Density
- 4.049 g/cm³
- Colors
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Cubic
- Crystal habit
- Microscopic crystals
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Formed in lacustrine beds consisting of interbedded calcareous clays, silts, and fine- to medium-grained arkosic sands; in varvelike laminae of grayish black sulfide-bearing clays and fine silts
Where rockhounds find greigite
Classic worldwide localities
- Kramer-Four Corners area, San Bernardino Co. (type locality), California, USA
- Coyote Peak, near Orick, Humboldt Co., California, USA
- Zacatecas, Mexico
- Bed of the Black Sea, and in the Gal-Khaya deposit, northeast Sakha, Russia
- Montemesola, Taranto, Italy
- Lojane chromium deposit, 40 km northeast of Skopje, Macedonia
Field-hunting tip
Where it forms: Formed in lacustrine beds consisting of interbedded calcareous clays, silts, and fine- to medium-grained arkosic sands; in varvelike laminae of grayish black sulfide-bearing clays and fine silts. If you start seeing montmorillonite, chlorite, calcite in float, you are in the right ground. Typical habit: microscopic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify greigite?+
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Find greigite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
