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 4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: olive-green.
- 3Read the lusterHold the specimen under a strong light. Luster: subadamantine, may be bronzy.
- 4Match the color rangeCompare against the expected color range: brown, reddish brown, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic.
Often confused with
Olmsteadite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Triplite is the harder of the two (Mohs 5-5.5 vs. 4); streak differs, so Olmsteadite leaves olive-green, Triplite leaves white to brown; luster reads subadamantine, may be bronzy on Olmsteadite and vitreous to resinous on Triplite.
How to tell apart: Streak differs, so Olmsteadite leaves olive-green, Triphylite leaves white to grayish white; luster reads subadamantine, may be bronzy on Olmsteadite and vitreous to subresinous on Triphylite.
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
- KFe₂NbO₂(PO₄)₂(H₂O)₂
- Mohs hardness
- 4
- Density
- 3.31-3.36 g/cm³
- Colors
- Streak
- Olive-green
- Luster
- Subadamantine, may be bronzy
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic
- Cleavage
- Good on {100}; good on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare mineral, presumably formed by hydrothermal leaching of primary phosphates and columbite-tantalite in complex granite pegmatites
Where rockhounds find olmsteadite
Classic worldwide localities
- Big Chief mine, one km south of Glendale, USA
- White Cap mine, three km south of Keystone, USA
- Hesnard mine, three km southwest of Keystone, Pennington Co., South Dakota, USA
Field-hunting tip
Where it forms: A rare mineral, presumably formed by hydrothermal leaching of primary phosphates and columbite-tantalite in complex granite pegmatites. If you start seeing siderite, quartz, rockbridgeite in float, you are in the right ground. Typical habit: prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify olmsteadite?+
Where is olmsteadite found?+
What rocks look like olmsteadite?+
What minerals are found with olmsteadite?+
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What is olmsteadite used for?+
Find olmsteadite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
