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

Opaque

mineralrare

Sanjuanite

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

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: monoclinic. 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 — 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: 2.47 g/cm³
Colors: White
Streak: White
Luster: Dull
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Massive, Crusts, Earthy
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Hydrothermal Alteration Zones in Volcanic Rocks
Typical price: $20-100 per specimen

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 — that is 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.

How much is sanjuanite worth?+

Typical asking prices fall in the range of $20-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

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.