Ferrirockbridgeite is a rare phosphate mineral typically occurring as an alteration product of triphylite in granitic pegmatites. Collectors often find it in radiating or fibrous sprays that exhibit a characteristic dark, greenish-black hue. It is visually indistinguishable from rockbridgeite without advanced chemical analysis or X-ray diffraction.
Is this ferrirockbridgeite?
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 ferrirockbridgeite with a known reference. Ferrirockbridgeite sits at Mohs 4.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ferrirockbridgeite leaves a yellowish brown streak.
- 3Read the lusterHold the specimen under a strong light. Ferrirockbridgeite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark green, blackish green, brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, acicular crystals, radiating sprays, massive.
Often confused with
Ferrirockbridgeite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Ferrirockbridgeite leaves yellowish brown, Rockbridgeite leaves greenish-brown.
How to tell apart: Streak differs — Ferrirockbridgeite leaves yellowish brown, Frondelite leaves yellowish-brown.
How to tell apart: Streak differs — Ferrirockbridgeite leaves yellowish brown, Strengite leaves white.
Often found alongside ferrirockbridgeite
Minerals reported to co-occur with ferrirockbridgeite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe²⁺Fe³⁺₄(PO₄)₃(OH)₅
- Mohs hardness
- 4.5
- Density
- 3.55 g/cm³
- Streak
- Yellowish Brown
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Acicular Crystals, Radiating Sprays, Massive
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate-rich Granite Pegmatites
- Typical price
- $20-150 for thumbnail to small cabinet specimens
Where rockhounds find ferrirockbridgeite
Classic worldwide localities
- Hagendorf, Bavaria, Germany
- Sapucaia pegmatite, Minas Gerais, Brazil
- Big Chief Mine, South Dakota, USA
- Palermo Mine, New Hampshire, USA
Field-hunting tip
Look in phosphate-rich granite pegmatites country — that is the host setting where ferrirockbridgeite typically forms. If you start seeing triphylite, vivianite, leucophosphite in float, you are in the right ground. Field specimens usually show a fibrous, acicular crystals, radiating sprays, massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find ferrirockbridgeite on the map
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