Mackinawite is a rare iron nickel sulfide that usually occurs as microscopic inclusions within other sulfide minerals. It is best known to collectors as an accessory phase in massive sulfide ores and is characterized by its distinct bronze color and metallic luster.
Is this mackinawite?
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 mackinawite with a known reference. Mackinawite sits at Mohs 2.5, 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: bronze, white gray.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: anhedral grains, microscopic crystals.
Often confused with
Mackinawite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Pyrrhotite is the harder of the two (Mohs 3.5-4.5 vs. 2.5); streak differs, so Mackinawite leaves black, Pyrrhotite leaves dark grayish black.
How to tell apart: Pentlandite is the harder of the two (Mohs 3.5-4 vs. 2.5); streak differs, so Mackinawite leaves black, Pentlandite leaves pale bronze-brown.
How to tell apart: Streak differs, so Mackinawite leaves black, Chalcocite leaves blackish lead-gray.
Often found alongside mackinawite
Minerals reported to co-occur with mackinawite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- FeS
- Mohs hardness
- 2.5
- Density
- 4.30 g/cm³
- Colors
- BronzeWhite gray
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Anhedral grains, microscopic crystals
- Cleavage
- Very good on {001}
- Rarity
- Uncommon
- Uses
- Collector, Research
- Host rock
- Formed by hydrothermal activity in mineral deposits, during serpentinization of peridotites, and in the reducing environment of river bottom muds; may be produced by magnetotactic and sulfate-reducing bacteria
Where rockhounds find mackinawite
Classic worldwide localities
- Mackinaw mine, Snohomish Co., Washington (type locality), USA
- Muds from the bottom of the Mystic River, Boston, Suffolk Co., Massachusetts, USA
- Howard Montgomery quarry, Howard Co., Maryland, USA
- Kramer borate deposit, Boron, Kern Co., California, USA
- Muskox intrusion, Northwest Territories, and at the Falconbridge mine, Sudbury, Ontario, Canada
- Outokumpu mine and Hitura, Finland
Field-hunting tip
Where it forms: Formed by hydrothermal activity in mineral deposits, during serpentinization of peridotites, and in the reducing environment of river bottom muds; may be produced by magnetotactic and sulfate-reducing bacteria. If you start seeing chalcopyrite, cubanite, pentlandite in float, you are in the right ground. Typical habit: anhedral grains, microscopic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify mackinawite?+
Where is mackinawite found?+
What rocks look like mackinawite?+
What minerals are found with mackinawite?+
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Find mackinawite on the map
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