Sanjuanite is a rare phosphate-sulfate mineral typically found as white, earthy, or chalky masses. It is most commonly identified in hydrothermal alteration zones and is frequently associated with other aluminum-bearing sulfates.
Is this sanjuanite?
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 sanjuanite with a known reference. Sanjuanite sits at Mohs 3, 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 silky.
- 4Match the color rangeCompare against the expected color range: white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: massive.
Often confused with
Sanjuanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull to silky on Sanjuanite and vitreous, somewhat pearly on {0001} on Alunite.
How to tell apart: Streak differs, so Sanjuanite leaves white, Jarosite leaves pale yellow; luster reads dull to silky on Sanjuanite and subadamantine to vitreous, resinous on fractures on Jarosite.
Often found alongside sanjuanite
Minerals reported to co-occur with sanjuanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂(PO₄)(SO₄)(OH)·9H₂O
- Mohs hardness
- 3
- Density
- 1.94 g/cm³
- Colors
- Streak
- White
- Luster
- Dull to silky
- Transparency
- Semitransparent
- Crystal system
- Triclinic
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In mineralized joint fractures cutting slate
Where rockhounds find sanjuanite
Classic worldwide localities
- Sierra Chica de Zonda, San Juan Province, Argentina (type locality)
Field-hunting tip
Where it forms: In mineralized joint fractures cutting slate. If you start seeing gypsum, natrojarosite, iron oxides in float, you are in the right ground. Typical habit: massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify sanjuanite?+
Where is sanjuanite found?+
What rocks look like sanjuanite?+
What minerals are found with sanjuanite?+
What kind of rock does sanjuanite form in?+
What is sanjuanite used for?+
Find sanjuanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
