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, 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.
