Natroalunite is a sodium-rich member of the Alunite group, typically found in low-temperature hydrothermal alteration environments. It often occurs as earthy or massive aggregates and is visually indistinguishable from standard alunite without chemical analysis.
Is this natroalunite?
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 natroalunite with a known reference. Natroalunite sits at Mohs 3.5-4, 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 to pearly on {0001}.
- 4Match the color rangeCompare against the expected color range: white, grayish, yellowish, reddish, brownish.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive, granular, or tabular crystals.
Often confused with
Natroalunite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside natroalunite
Minerals reported to co-occur with natroalunite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaAl₃(SO₄)₂(OH)₆
- Mohs hardness
- 3.5-4
- Density
- 2.75-2.83 g/cm³
- Streak
- White
- Luster
- Vitreous to pearly on {0001}
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
- Crystal habit
- Massive, granular, or tabular crystals
- Cleavage
- Distinct on {0001}; traces on {0112}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Less common than alunite, formed by solfataric or hydrothermal sulfate-bearing solutions reacting with clays, rarely with sillimanite; may be in laterites and as an authigenic sedimentary mineral
Where rockhounds find natroalunite
Classic worldwide localities
- National Belle mine, Red Mountain district, Ouray Co., and in the Rosita Hills, Custer Co., Colorado, USA
- Sugarloaf Butte, near Quartzsite, La Paz Co., Arizona, USA
- Carlsbad Cavern and Lechuguilla Cave, Eddy Co., New Mexico, USA
- Desert Strip, Molokai, Hawaiian Islands, New Mexico, USA
- Ugusu mine and at Shirakawa, Shizuoka Prefecture, and from Sonomi, Makurazaki, Kagoshima Prefecture, Japan
- Tatum copper deposit and Chinkuashi Cu–Au deposit, Taiwan
Field-hunting tip
Where it forms: Less common than alunite, formed by solfataric or hydrothermal sulfate-bearing solutions reacting with clays, rarely with sillimanite; may be in laterites and as an authigenic sedimentary mineral. If you start seeing alunite, kaolinite, halloysite in float, you are in the right ground. Typical habit: massive, granular, or tabular crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify natroalunite?+
Where is natroalunite found?+
What rocks look like natroalunite?+
What minerals are found with natroalunite?+
What kind of rock does natroalunite form in?+
What is natroalunite used for?+
Find natroalunite on the map
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