Ludwigite is a magnesium-iron borate typically found in contact metamorphic rocks, specifically skarns formed from limestone intruded by igneous bodies. It is most easily identified by its characteristic black to dark-green fibrous or acicular crystal habit often occurring in radiating sprays.
Is this ludwigite?
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 ludwigite with a known reference. Ludwigite sits at Mohs 5-5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: black to greenish black.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic.
- 4Match the color rangeCompare against the expected color range: black, dark green, black green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, massive, radial.
Often confused with
Ludwigite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Ludwigite leaves black to greenish black, Vonsenite leaves brownish black; luster reads submetallic on Ludwigite and metallic to submetallic, silky if fibrous on Vonsenite.
How to tell apart: Tourmaline is the harder of the two (Mohs 7-7.5 vs. 5-5.5); streak differs, so Ludwigite leaves black to greenish black, Tourmaline leaves white; luster reads submetallic on Ludwigite and vitreous on Tourmaline.
How to tell apart: Streak differs, so Ludwigite leaves black to greenish black, Aegirine leaves pale yellowish gray; luster reads submetallic on Ludwigite and vitreous to slightly resinous on Aegirine.
Often found alongside ludwigite
Minerals reported to co-occur with ludwigite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₂Fe³⁺(BO₃)O₂
- Mohs hardness
- 5-5.5
- Density
- 3.80 g/cm³
- Streak
- Black to greenish black
- Luster
- Submetallic
- Transparency
- Opaque, translucent only in thin fragments and rims of crystals
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, massive, radial
- Cleavage
- Perfect on {001}
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Typically formed in magnesian iron skarn and contact metamorphic deposits
Where rockhounds find ludwigite
1 mapped spotsClassic worldwide localities
- Pomeroy mine, Anaconda, Deer Lodge Co., and at Philipsburg, Granite Co., Montana, USA
- Crestmore quarry, Riverside Co., California, USA
- Mountain Lake mine, Big Cottonwood district, Salt Lake Co., Utah, USA
- Brosso mine, northwest of Ivrea, Torino, Italy
- Broadford, Isle of Skye, Inverness-shire, Scotland
- Nordmark district, Värmland, Sweden
Field-hunting tip
Where it forms: Typically formed in magnesian iron skarn and contact metamorphic deposits. If you start seeing magnetite, forsterite, clinohumite in float, you are in the right ground. Typical habit: fibrous, massive, radial. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
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Find ludwigite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
