Rockbridgeite is an attractive phosphate mineral typically found as dark, radial fibrous clusters or botryoidal crusts. It is most often identified by its distinct greenish-brown streak and its tendency to form in the oxidation zones of phosphate-rich pegmatites or hydrothermal iron deposits.
Is this rockbridgeite?
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 rockbridgeite with a known reference. Rockbridgeite 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. Rockbridgeite leaves a greenish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Rockbridgeite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark green, brown, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: botryoidal, radial fibrous, crusts, or compact masses.
Often confused with
Rockbridgeite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Rockbridgeite leaves greenish-brown, Frondelite leaves yellowish-brown.
How to tell apart: Rockbridgeite is noticeably harder (Mohs 4.5 vs. 1.5-2); streak differs — Rockbridgeite leaves greenish-brown, Vivianite leaves white to light blue.
Often found alongside rockbridgeite
Minerals reported to co-occur with rockbridgeite. 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
- 3.4-3.5 g/cm³
- Colors
- Streak
- Greenish-brown
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Botryoidal, Radial Fibrous, Crusts, Or Compact Masses
- Cleavage
- Poor
- Rarity
- Uncommon
- Uses
- Collector, Mineralogical Study
- Host rock
- Phosphate-rich Pegmatites and Iron Ore Deposits
- Typical price
- $10-60 for small specimens
Where rockhounds find rockbridgeite
Classic worldwide localities
- Rockbridge County, Virginia, USA
- Hagendorf, Bavaria, Germany
- Linopolis, Minas Gerais, Brazil
- Tip Top Mine, South Dakota, USA
Field-hunting tip
Look in phosphate-rich pegmatites and iron ore deposits country — that is the host setting where rockbridgeite typically forms. If you start seeing strengite, cacoxenite, leucophosphite in float, you are in the right ground. Field specimens usually show a botryoidal, radial fibrous, crusts, or compact masses habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find rockbridgeite on the map
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