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 3.5-4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: greenish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to dull.
- 4Match the color rangeCompare against the expected color range: green, dark olive green, black, brownish green, reddish brown.
- 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 gray, Frondelite leaves brownish gray.
How to tell apart: Rockbridgeite is noticeably harder (Mohs 3.5-4.5 vs. 1.5-2); streak differs, so Rockbridgeite leaves greenish gray, Vivianite leaves white, altering to dark blue, brown.
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
- 3.5-4.5
- Density
- 3.45 g/cm³
- Streak
- Greenish gray
- Luster
- Vitreous to dull
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Botryoidal, radial fibrous, crusts, or compact masses
- Cleavage
- {100}, excellent; {010}, good; {001}, fair
- Rarity
- Uncommon
- Uses
- Collector, Mineralogical study
- Host rock
- In “limonite” iron ore deposits; an alteration product of primary phosphates in complex granite pegmatites
Where rockhounds find rockbridgeite
Classic worldwide localities
- South Mountain, southeast of Midvale, Rockbridge Co., Virginia, USA
- Palermo #1 and Fletcher mines, North Groton, Grafton Co., New Hampshire, USA
- Tip Top, Bull Moose, and other mines, near Custer, Custer Co., and at the Big Chief, Etta, and other mines, near Keystone, Pennington Co., South Dakota, USA
- Énio pegmatite mine, northeast of Galiléia, and near São Jose da Safira, Agua Boa, Minas Gerais, Brazil
- El Criollo pegmatite, Cerro Blanco, Tanti district, 45 km west of Córdoba, Córdoba Province, Argentina
- Sabugal, Mangualde, Bendada, and Moreira, Portugal
Field-hunting tip
Where it forms: In “limonite” iron ore deposits; an alteration product of primary phosphates in complex granite pegmatites. If you start seeing triphylite, huréaulite, barbosalite in float, you are in the right ground. Typical habit: botryoidal, radial fibrous, crusts, or compact masses. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rockbridgeite?+
Where is rockbridgeite found?+
What rocks look like rockbridgeite?+
What minerals are found with rockbridgeite?+
What kind of rock does rockbridgeite form in?+
What is rockbridgeite used for?+
Find rockbridgeite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
