Hulsite is a rare tin-bearing borate mineral typically found in complex skarn deposits formed by contact metamorphism. It is characterized by its dark, submetallic appearance and is often difficult to distinguish from other minerals in the ludwigite-vonsenite series without chemical analysis.
Is this hulsite?
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 hulsite with a known reference. Hulsite sits at Mohs ~3, 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: submetallic to vitreous.
- 4Match the color rangeCompare against the expected color range: black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crystalline, tabular.
Often confused with
Hulsite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Ludwigite is the harder of the two (Mohs 5-5.5 vs. ~3).
How to tell apart: Vonsenite is the harder of the two (Mohs 5 vs. ~3); streak differs, so Hulsite leaves black, Vonsenite leaves brownish black; luster reads submetallic to vitreous on Hulsite and metallic to submetallic, silky if fibrous on Vonsenite.
Often found alongside hulsite
Minerals reported to co-occur with hulsite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₂FeO₂(BO₃)
- Mohs hardness
- ~3
- Density
- 4.28-4.57 g/cm³
- Colors
- Streak
- Black
- Luster
- Submetallic to vitreous
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Crystalline, tabular
- Cleavage
- On {110}, good
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Very rare, in a contact-metamorphosed limestone, close by an intrusive granite
Where rockhounds find hulsite
Classic worldwide localities
- Brooks mountain, Seward peninsula, Alaska (type locality)
Field-hunting tip
Where it forms: Very rare, in a contact-metamorphosed limestone, close by an intrusive granite. If you start seeing calcite, vesuvianite, magnetite in float, you are in the right ground. Typical habit: crystalline, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hulsite?+
Where is hulsite found?+
What rocks look like hulsite?+
What minerals are found with hulsite?+
What kind of rock does hulsite form in?+
What is hulsite used for?+
Find hulsite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
