Millisite is a scarce hydrated phosphate mineral often found as compact, botryoidal, or massive crusts within phosphate nodules. It is most frequently encountered in the Fairfield vivianite deposits in Utah, where it is often associated with other rare phosphate species like wardite and variscite. Collectors look for its characteristic pale, dull appearance and its occurrence as thin coatings on matrix specimens.
Is this millisite?
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 millisite with a known reference. Millisite sits at Mohs 5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Millisite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Millisite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, light blue, greenish.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: massive, crusts, botryoidal.
Often confused with
Millisite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Millisite is noticeably harder (Mohs 5 vs. 3.5-4); luster reads dull on Millisite and vitreous on Wavellite.
How to tell apart: Luster reads dull on Millisite and vitreous on Wardite.
How to tell apart: Luster reads dull on Millisite and waxy on Variscite.
Often found alongside millisite
Minerals reported to co-occur with millisite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,K)CaAl₆(PO₄)₄(OH)₉·3H₂O
- Mohs hardness
- 5
- Density
- 2.35 g/cm³
- Streak
- White
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Massive, Crusts, Botryoidal
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Phosphate Nodules in Sedimentary Rocks
- Typical price
- $20-150 for micro-mounts or small cabinet specimens
Where rockhounds find millisite
Classic worldwide localities
- Fairfield, Utah, USA
- Cedar Valley, Utah, USA
- Kallmünz, Bavaria, Germany
Field-hunting tip
Look in phosphate nodules in sedimentary rocks country — that is the host setting where millisite typically forms. If you start seeing wardite, variscite, wavellite in float, you are in the right ground. Field specimens usually show a massive, crusts, botryoidal habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find millisite on the map
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