Aluminite is a soft, earthy hydrous aluminum sulfate that typically forms as white, chalky-looking masses or crusts. It is most commonly found in clay formations associated with the weathering of pyrite or other sulfur-bearing minerals.
Is this aluminite?
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 aluminite with a known reference. Aluminite sits at Mohs 1-2, 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: dull to earthy.
- 4Match the color rangeCompare against the expected color range: white, grayish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, earthy, mammillary.
Often confused with
Aluminite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull to earthy on Aluminite and pearly to dull earthy on Kaolinite.
How to tell apart: Luster reads dull to earthy on Aluminite and vitreous, pearly on cleavage surfaces, dull in aggregates on Gibbsite.
How to tell apart: Luster reads dull to earthy on Aluminite and pearly, waxy, or dull on Halloysite.
Often found alongside aluminite
Minerals reported to co-occur with aluminite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂SO₄(OH)₄·7H₂O
- Mohs hardness
- 1-2
- Density
- 1.66-1.82 g/cm³
- Colors
- Streak
- White
- Luster
- Dull to earthy
- Transparency
- Translucent, opaque if massive
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, earthy, mammillary
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Typically in clays or lignites, formed by the reaction of sulfate-bearing solutions from the decomposition of marcasite or pyrite at moderate temperatures with aluminous silicates; as a volcanic sublimate
Where rockhounds find aluminite
Classic worldwide localities
- Joplin, Jasper Co., Missouri, USA
- Green River Formation, Emery Co., Utah, USA
- Creston, southwest of Rawlins, Carbon Co., Wyoming, USA
- Cottonwood Cave, Guadalupe Mountains, Eddy Co., New Mexico, USA
- Morl, near Halle, Saxony-Anhalt, Germany
- Milevsko (Mühlhausen), near Kralupy, Kuchelbad, Miletic, and Velvary, Czech Republic
Field-hunting tip
Where it forms: Typically in clays or lignites, formed by the reaction of sulfate-bearing solutions from the decomposition of marcasite or pyrite at moderate temperatures with aluminous silicates; as a volcanic sublimate. If you start seeing basaluminite, gibbsite, epsomite in float, you are in the right ground. Typical habit: botryoidal, earthy, mammillary. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify aluminite?+
Where is aluminite found?+
What rocks look like aluminite?+
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Find aluminite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
