Olmsteadite is a rare phosphate mineral typically found in phosphate-rich zones of granitic pegmatites. Collectors usually search for its distinctive dark brown, blocky crystals occurring as coatings on other primary phosphates.
Is this olmsteadite?
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 olmsteadite with a known reference. Olmsteadite sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Olmsteadite leaves a light brown streak.
- 3Read the lusterHold the specimen under a strong light. Olmsteadite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark brown, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular to blocky crystals, often as crusts or aggregates.
Often confused with
Olmsteadite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside olmsteadite
Minerals reported to co-occur with olmsteadite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe²⁺₂(Nb,Ta)(PO₄)₂(OH)₂·2H₂O
- Mohs hardness
- 3-4
- Density
- 3.58 g/cm³
- Colors
- Streak
- Light Brown
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular to Blocky Crystals, Often as Crusts or Aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Granite Pegmatites
- Typical price
- $50-300 per specimen
Where rockhounds find olmsteadite
Classic worldwide localities
- Big Chief mine, South Dakota, USA
- Bull Moose mine, South Dakota, USA
- Mangualde, Portugal
Field-hunting tip
Look in granite pegmatites country — that is the host setting where olmsteadite typically forms. If you start seeing apatite, beusite, triphylite in float, you are in the right ground. Field specimens usually show a tabular to blocky crystals, often as crusts or aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify olmsteadite?+
Where is olmsteadite found?+
How much is olmsteadite worth?+
What rocks look like olmsteadite?+
What minerals are found with olmsteadite?+
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Find olmsteadite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
