Fetiasite is an extremely rare arsenic-bearing mineral known primarily from the type locality in the Aosta Valley, Italy. It typically forms small, dark, submetallic tabular crystals embedded within manganese-rich metamorphic rocks.

Hardness

Mohs

Luster

Metallic to semimetallic

Streak

Ochre brown

Transparency

Opaque

mineralrare

Fetiasite

Hardness: ~5
Luster: Metallic to semimetallic
Streak: Ochre brown
Transparency: Opaque

Is this fetiasite?

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 fetiasite with a known reference. Fetiasite sits at Mohs ~5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: ochre brown.
3
Read the luster
Hold the specimen under a strong light. Luster: metallic to semimetallic.
4
Match the color range
Compare against the expected color range: brown, black.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: crystalline, globular, tabular.

Often confused with

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

mineral

Ilmenite

Mohs 5-6

How to tell apart: Streak differs, so Fetiasite leaves ochre brown, Ilmenite leaves black to reddish brown.

How to tell apart: Streak differs, so Fetiasite leaves ochre brown, Hematite leaves cherry-red or reddish brown.

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Often found alongside fetiasite

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

All properties

Chemical formula: (Fe²⁺,Fe³⁺,Ti)₃(As³⁺O₃)₄
Mohs hardness: ~5
Density: 4.6 g/cm³
Colors: Brown Black
Streak: Ochre brown
Luster: Metallic to semimetallic
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Crystalline, globular, tabular
Cleavage: {100}, perfect
Rarity: Rare
Uses: Collector
Host rock: Deposited from arsenic-bearing solutions in Alpine fissures in gneisses of the upper greenschist to lower amphibolite facies

Where rockhounds find fetiasite

Classic worldwide localities

East flank of Pizzo Cervandone, Alpe Devero, Val d’Aosta, Piedmont, Italy
Gorb, Binntal, Valais, Switzerland

Field-hunting tip

Where it forms: Deposited from arsenic-bearing solutions in Alpine fissures in gneisses of the upper greenschist to lower amphibolite facies. If you start seeing asbecasite, cafarsite, cervandonite in float, you are in the right ground. Typical habit: crystalline, globular, tabular. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify fetiasite?+

Mohs hardness is ~5. Luster: metallic to semimetallic. The streak is ochre brown. Common colors include brown, black.

Where is fetiasite found?+

Notable localities include East flank of Pizzo Cervandone, Alpe Devero, Val d’Aosta, Piedmont, Italy; Gorb, Binntal, Valais, Switzerland.

Is fetiasite safe to handle?+

Fetiasite contains arsenic and calls for real care. Contains arsenic, which is toxic; wash hands thoroughly after handling and do not inhale dust.

What rocks look like fetiasite?+

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

What minerals are found with fetiasite?+

Fetiasite commonly co-occurs with Asbecasite, Cafarsite, Cervandonite, Anatase, Chlorite, Feldspar. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does fetiasite form in?+

Where it forms: Deposited from arsenic-bearing solutions in Alpine fissures in gneisses of the upper greenschist to lower amphibolite facies. Working float back to the host body is the standard way to chase a fresh occurrence.

What is fetiasite used for?+

Fetiasite is used in collector.

Find fetiasite on the map

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