Ferroselite is a rare iron selenide mineral typically found in hydrothermal uranium-rich deposits. It is most easily identified by its distinct metallic luster and association with other selenium minerals like clausthalite, often occurring as fine-grained, radiating, or acicular crystals.

Hardness

6-6.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Ferroselite

Hardness: 6-6.5
Luster: Metallic
Streak: Black
Transparency: Opaque

Is this ferroselite?

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 ferroselite with a known reference. Ferroselite sits at Mohs 6-6.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: black.
3
Read the luster
Hold the specimen under a strong light. Luster: metallic.
4
Match the color range
Compare against the expected color range: bronze, steel gray, silver white.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: acicular, striated, twinning common.

Often confused with

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

mineral

Marcasite

Mohs 6-6.5

How to tell apart: Streak differs, so Ferroselite leaves black, Marcasite leaves grayish or brownish black.

How to tell apart: Streak differs, so Ferroselite leaves black, Pyrite leaves greenish black to brownish black.

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Often found alongside ferroselite

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

All properties

Chemical formula: FeSe₂
Mohs hardness: 6-6.5
Density: 7.20 g/cm³
Colors: Bronze Steel gray Silver white
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Orthorhombic
Crystal habit: Acicular, striated, twinning common
Cleavage: Perfect
Rarity: Rare
Uses: Collector
Host rock: In red-bed deposits of the Colorado Plateau type, peripheral to pyrite and uranium concentrations (Utah, USA); in U–V ores in sandstone, with coalified wood (Colorado, USA)

Where rockhounds find ferroselite

Classic worldwide localities

Sandstones of the Powder River basin, Wyoming, USA
Kermac Sec. 10 mine, McKinley Co., New Mexico, USA
Mitten #2 mine, Navajo Co., Arizona, USA
A number of mines in the Uravan district, Montrose Co., Colorado, USA
Ust’-Uyuk V–Se–U deposit, Tuva, Siberia (type locality), Russia
Zapadno-Ozernogo Cu–Zn deposit, Southern Ural Mountains, Russia

Field-hunting tip

Where it forms: In red-bed deposits of the Colorado Plateau type, peripheral to pyrite and uranium concentrations (Utah, USA); in U–V ores in sandstone, with coalified wood (Colorado, USA). If you start seeing uraninite, pyrite, marcasite in float, you are in the right ground. Typical habit: acicular, striated, twinning common. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ferroselite?+

Mohs hardness is 6-6.5. Luster: metallic. The streak is black. Common colors include bronze, steel gray, silver white.

Where is ferroselite found?+

Notable localities include Sandstones of the Powder River basin, Wyoming, USA; Kermac Sec. 10 mine, McKinley Co., New Mexico, USA; Mitten #2 mine, Navajo Co., Arizona, USA; A number of mines in the Uravan district, Montrose Co., Colorado, USA; Ust’-Uyuk V–Se–U deposit, Tuva, Siberia (type locality), Russia.

Is ferroselite safe to handle?+

Ferroselite contains selenium. Contains selenium, which is toxic if inhaled or ingested. Wash hands thoroughly after handling and avoid creating dust or powder when breaking or sawing specimens.

What rocks look like ferroselite?+

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

What minerals are found with ferroselite?+

Ferroselite commonly co-occurs with Uraninite, Pyrite, Marcasite, Chalcopyrite, Sphalerite, Selenium. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ferroselite form in?+

Where it forms: In red-bed deposits of the Colorado Plateau type, peripheral to pyrite and uranium concentrations (Utah, USA); in U–V ores in sandstone, with coalified wood (Colorado, USA). Working float back to the host body is the standard way to chase a fresh occurrence.

What is ferroselite used for?+

Ferroselite is used in collector.

Find ferroselite on the map

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