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 5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: white, greenish, light gray.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: fibrous.
Often confused with
Millisite vs. its common look-alikes, and how to tell them apart in the field.
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
- NaCaAl₆(PO₄)₄(OH)₉·3H₂O
- Mohs hardness
- 5.5
- Density
- 2.83 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Tetragonal
- Crystal habit
- Fibrous
- Cleavage
- Good on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As finely banded crusts on variscite nodules (Little Green Monster mine, Utah, USA); in phosphatic laterite (Taiba mine, Senegal); as cement in weathered phosphorite (Homeland mine, Florida, USA)
Where rockhounds find millisite
Classic worldwide localities
- Little Green Monster mine, Clay Canyon, about nine kin west of Fairfield, Utah Co., Utah, and between Bartow and Fort Meade, as in the Homeland mine, Peace Valley, Polk Co., Florida, USA
- Taiba mine, Thies district, Senegal
- Iron Monarch quarry, Iron Knob, South Australia, and on Christmas Island, Indian Ocean, Australia
- Kiluli, Rwanda
Field-hunting tip
Where it forms: As finely banded crusts on variscite nodules (Little Green Monster mine, Utah, USA); in phosphatic laterite (Taiba mine, Senegal); as cement in weathered phosphorite (Homeland mine, Florida, USA). If you start seeing wardite, apatite, crandallite in float, you are in the right ground. Typical habit: fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify millisite?+
Where is millisite found?+
What rocks look like millisite?+
What minerals are found with millisite?+
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Find millisite on the map
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