Ferrostrunzite is a rare hydrated iron phosphate mineral typically found as a secondary product in oxidized phosphate-rich pegmatites. It is most commonly identified by its delicate, acicular, or needle-like yellow-brown crystal sprays that form within small vugs or cavities.
Is this ferrostrunzite?
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 ferrostrunzite with a known reference. Ferrostrunzite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ferrostrunzite leaves a yellowish-white streak.
- 3Read the lusterHold the specimen under a strong light. Ferrostrunzite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: yellow, brownish-yellow, tan.
- 5Look at form & habitCrystal system: triclinic. Typical habit: acicular crystals, radial aggregates, fibrous bundles.
Often confused with
Ferrostrunzite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Ferrostrunzite leaves yellowish-white, Strunzite leaves white.
How to tell apart: Streak differs — Ferrostrunzite leaves yellowish-white, Ferristrunzite leaves yellow.
How to tell apart: Streak differs — Ferrostrunzite leaves yellowish-white, Laueite leaves white.
Often found alongside ferrostrunzite
Minerals reported to co-occur with ferrostrunzite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe³⁺Fe³⁺₂(PO₄)₂(OH)₃·6H₂O
- Mohs hardness
- 3.5
- Density
- 2.81 g/cm³
- Colors
- Streak
- Yellowish-white
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Triclinic
- Crystal habit
- Acicular Crystals, Radial Aggregates, Fibrous Bundles
- Cleavage
- Good On {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate-rich Granite Pegmatites
- Typical price
- $20-150 for micromounts and thumbnails
Where rockhounds find ferrostrunzite
Classic worldwide localities
- Hagendorf-Sud, Germany
- Tip Top Mine, South Dakota, USA
- Bull Moose Mine, South Dakota, USA
- Mangualde, Portugal
Field-hunting tip
Look in phosphate-rich granite pegmatites country — that is the host setting where ferrostrunzite typically forms. If you start seeing triphylite, phosphosiderite, rockbridgeite in float, you are in the right ground. Field specimens usually show a acicular crystals, radial aggregates, fibrous bundles habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferrostrunzite?+
Where is ferrostrunzite found?+
How much is ferrostrunzite worth?+
What rocks look like ferrostrunzite?+
What minerals are found with ferrostrunzite?+
What kind of rock does ferrostrunzite form in?+
What is ferrostrunzite used for?+
Find ferrostrunzite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
