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, so 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, so 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, 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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