Ardennite-(As) is a rare silicate mineral primarily found in manganese-rich metamorphic deposits. It typically forms elongated, prismatic crystals that often appear in radiated sprays or fibrous masses with a distinctive yellowish-brown to reddish-brown coloration.

Hardness

6-7

Mohs

Luster

Subadamantine

Streak

Pale brownish yellow

Transparency

Translucent

mineralrareArdennite Group family

Ardennite-(As)

Hardness: 6-7
Luster: Subadamantine
Streak: Pale brownish yellow
Transparency: Translucent

Is this ardennite-(as)?

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 ardennite-(as) with a known reference. Ardennite-(As) sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: pale brownish yellow.
3
Read the luster
Hold the specimen under a strong light. Luster: subadamantine.
4
Match the color range
Compare against the expected color range: yellow brown, yellow, dark brown, orange, black.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: euhedral crystals, prismatic.

Often confused with

Ardennite-(As) vs. its common look-alikes, and how to tell them apart in the field.

mineral

Epidote

Mohs 6-7

How to tell apart: Streak differs, so Ardennite-(As) leaves pale brownish yellow, Epidote leaves grayish white; luster reads subadamantine on Ardennite-(As) and vitreous, pearly, somewhat resinous on Epidote.

Compare →

Often found alongside ardennite-(as)

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

All properties

Chemical formula: Mn₄Al₄(AlMg)(AsO₄)(SiO₄)₂(Si₃O₁₀)(OH)₆
Mohs hardness: 6-7
Density: 3.62-3.75 g/cm³
Colors: Yellow brown Yellow Dark brown Orange Black
Streak: Pale brownish yellow
Luster: Subadamantine
Transparency: Translucent
Crystal system: Orthorhombic
Crystal habit: Euhedral crystals, prismatic
Cleavage: Perfect on {010}; distinct on {110}; parting on {001}
Rarity: Rare
Uses: Collector
Host rock: In pegmatites and quartz veins in schist and highly oxidized Mn, Al-rich metasediments

Where rockhounds find ardennite-(as)

Classic worldwide localities

Salmchateau in the Ardennes Mountains, Belgium (type locality)

Field-hunting tip

Where it forms: In pegmatites and quartz veins in schist and highly oxidized Mn, Al-rich metasediments. If you start seeing quartz, spessartine, braunite in float, you are in the right ground. Typical habit: euhedral crystals, prismatic. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ardennite-(as)?+

Mohs hardness is 6-7. Luster: subadamantine. The streak is pale brownish yellow. Common colors include yellow brown, yellow, dark brown, orange.

Where is ardennite-(as) found?+

Notable localities include Salmchateau in the Ardennes Mountains, Belgium (type locality).

Is ardennite-(as) safe to handle?+

Ardennite-(As) contains arsenic and calls for real care. Contains arsenic. Wash hands thoroughly after handling and avoid creating dust when breaking specimens.

What rocks look like ardennite-(as)?+

Ardennite-(As) is most often confused with Epidote. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with ardennite-(as)?+

Ardennite-(As) commonly co-occurs with Quartz, Spessartine, Braunite, Hematite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ardennite-(as) form in?+

Where it forms: In pegmatites and quartz veins in schist and highly oxidized Mn, Al-rich metasediments. Working float back to the host body is the standard way to chase a fresh occurrence.

What is ardennite-(as) used for?+

Ardennite-(As) is used in collector.

Find ardennite-(as) on the map

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