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.

Hardness

3.5

Mohs

Luster

Dull

Streak

White

Transparency

Opaque

mineralrareFairfieldite Group family

Robertsite

Hardness: 3.5
Luster: Dull
Streak: White
Transparency: Opaque

Is this robertsite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch robertsite with a known reference. Robertsite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Robertsite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Robertsite typically shows a dull luster.
4
Match the color range
Compare against the expected color range: pink, pale pink, white.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: massive, crusts, or radiating fibrous aggregates.

Often confused with

Robertsite vs. its common look-alikes — and how to tell them apart in the field.

mineral

Mitridatite

Mohs 3.5

How to tell apart: Streak differs — Robertsite leaves white, Mitridatite leaves yellowish-white.

How to tell apart: Luster reads dull on Robertsite and vitreous on Fairfieldite.

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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: Ca₃Mn₅(PO₄)₄(OH)₆·3H₂O
Mohs hardness: 3.5
Density: 3.2 g/cm³
Colors: Pink Pale Pink White
Streak: White
Luster: Dull
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Massive, Crusts, Or Radiating Fibrous Aggregates
Cleavage: Perfect On {010}
Rarity: Rare
Uses: Collector
Host rock: Phosphate-rich Granite Pegmatites
Typical price: $20-150 thumbnail specimens

Where rockhounds find robertsite

Classic worldwide localities

Tip Top mine (South Dakota, USA)
Mangualde (Portugal)
Hagendorf (Germany)

Field-hunting tip

Look in phosphate-rich granite pegmatites country — that is the host setting where robertsite typically forms. If you start seeing triphylite, ludlamite, hureaulite in float, you are in the right ground. Field specimens usually show a massive, crusts, or radiating fibrous aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify robertsite?+

Mohs hardness is 3.5. It typically shows a dull luster. The streak is white. Common colors include pink, pale pink, white.

Where is robertsite found?+

Notable localities include Tip Top mine (South Dakota, USA); Mangualde (Portugal); Hagendorf (Germany).

How much is robertsite worth?+

Typical asking prices fall in the range of $20-150 thumbnail specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like robertsite?+

Robertsite is most often confused with Mitridatite, Fairfieldite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with robertsite?+

Robertsite commonly co-occurs with Triphylite, Ludlamite, Hureaulite, Phosphosiderite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does robertsite form in?+

Robertsite typically forms in phosphate-rich granite pegmatites. Working float back to the host body is the standard way to chase a fresh occurrence.

What is robertsite used for?+

Robertsite is used in collector.

Find robertsite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.