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.
Is this fetiasite?
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 fetiasite with a known reference. Fetiasite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fetiasite leaves a yellowish brown streak.
- 3Read the lusterHold the specimen under a strong light. Fetiasite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: dark brown, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular crystals.
Often confused with
Fetiasite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Manaccanite is the harder of the two (Mohs 5-6 vs. 3-4); streak differs, so Fetiasite leaves yellowish brown, Manaccanite leaves black.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 3-4); streak differs, so Fetiasite leaves yellowish brown, Iron Ore leaves reddish-brown to black; luster reads submetallic on Fetiasite and metallic to submetallic on Iron Ore.
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
- 3-4
- Density
- 4.2 g/cm³
- Colors
- Streak
- Yellowish Brown
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Tabular Crystals
- Cleavage
- None Observed
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metamorphosed Manganese Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find fetiasite
Classic worldwide localities
- Fetias, Saint-Marcel, Aosta Valley, Italy
Field-hunting tip
Look in metamorphosed manganese deposits country, the host setting where fetiasite typically forms. If you start seeing braunite, quartz, hematite in float, you are in the right ground. Field specimens usually show a tabular crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find fetiasite on the map
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