Fillowite is a rare phosphate mineral typically found as granular or massive aggregates in complex granite pegmatites. It is often identified by its yellowish-brown color and vitreous luster, frequently occurring alongside other rare phosphate minerals in pockets. Collectors prize it as a significant, though obscure, member of the phosphate-rich zones in pegmatites.
Is this fillowite?
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 fillowite with a known reference. Fillowite sits at Mohs 4.5, 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: subresinous to greasy.
- 4Match the color rangeCompare against the expected color range: yellow, reddish brown, colorless.
- 5Look at form & habitCrystal system: trigonal. Typical habit: euhedral crystals, granular.
Often confused with
Fillowite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside fillowite
Minerals reported to co-occur with fillowite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₃CaMn₁₁(PO₄)₉
- Mohs hardness
- 4.5
- Density
- 3.43 g/cm³
- Streak
- White
- Luster
- Subresinous to greasy
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
- Crystal habit
- Euhedral crystals, granular
- Cleavage
- {0001}, nearly perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A primary accessory mineral in a zoned granite pegmatite (Branchville, Connecticut, USA)
Where rockhounds find fillowite
Classic worldwide localities
- Branchville, Fairfield Co., Connecticut, USA
- White Picacho district, Maricopa and Yavapai Cos., Arizona, USA
- Animike Red Ace pegmatite, Florence Co., Wisconsin, USA
- Buranga, Kiluli, and Rusororo, Rwanda
- Kabira, Uganda
Field-hunting tip
Where it forms: A primary accessory mineral in a zoned granite pegmatite (Branchville, Connecticut, USA). If you start seeing triploidite, fairfieldite, reddingite in float, you are in the right ground. Typical habit: euhedral crystals, granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fillowite?+
Where is fillowite found?+
What rocks look like fillowite?+
What minerals are found with fillowite?+
What kind of rock does fillowite form in?+
What is fillowite used for?+
Find fillowite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
