Robertsite is a rare phosphate mineral often found as pinkish, botryoidal or fibrous crusts within phosphate-rich pegmatites. Collectors primarily search for it in weathered phosphate zones where it forms as an alteration product of primary minerals like triphylite.
Is this robertsite?
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 robertsite with a known reference. Robertsite 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. Streak: chocolate-brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to resinous or waxy, silky in fibrous material.
- 4Match the color rangeCompare against the expected color range: black, blood red, brownish black, dark red, reddish brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, felted.
Often confused with
Robertsite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Robertsite is noticeably harder (Mohs 3.5 vs. 2.5); streak differs, so Robertsite leaves chocolate-brown, Mitridatite leaves olive-green; luster reads vitreous to resinous or waxy, silky in fibrous material on Robertsite and dull, earthy to resinous on Mitridatite.
How to tell apart: Streak differs, so Robertsite leaves chocolate-brown, Fairfieldite leaves white; luster reads vitreous to resinous or waxy, silky in fibrous material on Robertsite and pearly to subadamantine, vitreous on the {001} cleavage on Fairfieldite.
Often found alongside robertsite
Minerals reported to co-occur with robertsite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂Mn₃O₂(PO₄)₃(H₂O)₂·H₂O
- Mohs hardness
- 3.5
- Density
- 3.13-3.17 g/cm³
- Colors
- Streak
- Chocolate-brown
- Luster
- Vitreous to resinous or waxy, silky in fibrous material
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, felted
- Cleavage
- On {100}, good
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A late secondary mineral in complex zoned granite pegmatites; in a phosphate deposit in limestone, derived from bat guano
Where rockhounds find robertsite
Classic worldwide localities
- Tip Top mine, 8.5 km southwest of Custer, and the White Elephant mine, near Pringle, Custer Co., and the Gap Lode pegmatite, Pennington Co., South Dakota, USA
- White Picacho district, Yavapai Co., Arizona, USA
- Stewart mine, Pala, San Diego Co., California, USA
- Emmons quarry, Greenwood, Oxford Co., Maine, USA
- Hagendorf, Bavaria, Germany
- Jebilet, Morocco
Field-hunting tip
Where it forms: A late secondary mineral in complex zoned granite pegmatites; in a phosphate deposit in limestone, derived from bat guano. If you start seeing rockbridgeite, ferrisicklerite, leucophosphite in float, you are in the right ground. Typical habit: aggregates, felted. Train your eye for that shape before scanning the outcrop.
Common questions
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Find robertsite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
