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.

Hardness

3.5

Mohs

Luster

Dull

Streak

White

Transparency

Translucent

mineralrare

Sanjuanite

Hardness: 3.5
Luster: Dull
Streak: White
Transparency: Translucent

Is this sanjuanite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch sanjuanite with a known reference. Sanjuanite sits at Mohs 3.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Sanjuanite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Sanjuanite typically shows a dull luster.
4
Match the color range
Compare against the expected color range: white.
5
Look at form & habit
Crystal system: triclinic. Typical habit: massive, crusts, earthy.

Often confused with

Sanjuanite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Alunite

Mohs 3.5-4

How to tell apart: Luster reads dull on Sanjuanite and vitreous on Alunite.

How to tell apart: Streak differs, so Sanjuanite leaves white, Jarosite leaves yellow; luster reads dull on Sanjuanite and vitreous on Jarosite.

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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: 3.5
Density: 1.94 g/cm³
Colors: White
Streak: White
Luster: Dull
Transparency: Translucent
Crystal system: Triclinic
Crystal habit: Massive, Crusts, Earthy
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Hydrothermal Alteration Zones in Volcanic Rocks

Where rockhounds find sanjuanite

Classic worldwide localities

San Juan, Argentina
Cerro de los Colorados, Argentina

Field-hunting tip

Look in hydrothermal alteration zones in volcanic rocks country, the host setting where sanjuanite typically forms. If you start seeing alunite, jarosite, gypsum in float, you are in the right ground. Field specimens usually show a massive, crusts, earthy habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify sanjuanite?+

Mohs hardness is 3.5. It typically shows a dull luster. The streak is white. Common colors include white.

Where is sanjuanite found?+

Notable localities include San Juan, Argentina; Cerro de los Colorados, Argentina.

What rocks look like sanjuanite?+

Sanjuanite is most often confused with Alunite, Jarosite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with sanjuanite?+

Sanjuanite commonly co-occurs with Alunite, Jarosite, Gypsum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does sanjuanite form in?+

Sanjuanite typically forms in hydrothermal alteration zones in volcanic rocks. Working float back to the host body is the standard way to chase a fresh occurrence.

What is sanjuanite used for?+

Sanjuanite is used in collector.

Find sanjuanite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.