Fuenzalidaite is an extremely rare sulfate mineral known from arid, evaporitic environments. It typically forms thin, glassy tabular crystals that are difficult to distinguish from associated secondary sulfates without chemical analysis.

Hardness

3.5

Mohs

Luster

Vitreous

Streak

Yellowish

Transparency

Transparent

mineralrareEuclase-Vonsenite Group family

Fuenzalidaite

Hardness: 3.5
Luster: Vitreous
Streak: Yellowish
Transparency: Transparent

Is this fuenzalidaite?

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 fuenzalidaite with a known reference. Fuenzalidaite 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. Fuenzalidaite leaves a yellowish streak.
3
Read the luster
Hold the specimen under a strong light. Fuenzalidaite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: yellow, brown.
5
Look at form & habit
Crystal system: trigonal. Typical habit: tabular hexagonal crystals.

Often confused with

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

mineral

Jarosite

Mohs 2.5-3.5

How to tell apart: Streak differs — Fuenzalidaite leaves yellowish, Jarosite leaves yellow.

How to tell apart: Fuenzalidaite is noticeably harder (Mohs 3.5 vs. 2.5); streak differs — Fuenzalidaite leaves yellowish, Coquimbite leaves white.

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Often found alongside fuenzalidaite

Minerals reported to co-occur with fuenzalidaite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: K₆(Sb,As)₄(SO₄)₄O₂
Mohs hardness: 3.5
Density: 4.55 g/cm³
Colors: Yellow Brown
Streak: Yellowish
Luster: Vitreous
Transparency: Transparent
Crystal system: Trigonal
Crystal habit: Tabular Hexagonal Crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Evaporite Deposits
Typical price: n/a (extremely rare specimen)

Where rockhounds find fuenzalidaite

Classic worldwide localities

Minas del Sol, Chile

Field-hunting tip

Look in evaporite deposits country — that is the host setting where fuenzalidaite typically forms. If you start seeing jarosite, gypsum, halite in float, you are in the right ground. Field specimens usually show a tabular hexagonal crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify fuenzalidaite?+

Mohs hardness is 3.5. It typically shows a vitreous luster. The streak is yellowish. Common colors include yellow, brown.

Where is fuenzalidaite found?+

Notable localities include Minas del Sol, Chile.

How much is fuenzalidaite worth?+

Typical asking prices fall in the range of n/a (extremely rare specimen). Quality, size, and provenance can move individual specimens well outside that range.

Is fuenzalidaite safe to handle?+

It contains toxic constituents. Contains antimony and arsenic, which are toxic. Wash hands thoroughly after handling and do not ingest. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like fuenzalidaite?+

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

What minerals are found with fuenzalidaite?+

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

What kind of rock does fuenzalidaite form in?+

Fuenzalidaite typically forms in evaporite deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is fuenzalidaite used for?+

Fuenzalidaite is used in collector.

Find fuenzalidaite on the map

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