Castellaroite is a rare manganese arsenate mineral found as small, vibrant yellow tabular crystals. It is primarily known from the Molinello mine in Italy, where it occurs in manganese-rich mineral assemblages.

Hardness

2.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrarenull family

Castellaroite

Hardness: 2.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this castellaroite?

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 castellaroite with a known reference. Castellaroite sits at Mohs 2.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Castellaroite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Castellaroite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: yellow, yellow-orange.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: tabular crystals.

Often confused with

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

How to tell apart: Tilasite is the harder of the two (Mohs 5 vs. 2.5).

Compare →

Often found alongside castellaroite

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

All properties

Chemical formula: Mn²⁺₅(AsO₄)₂ (AsO₃OH)₂·10H₂O
Mohs hardness: 2.5
Density: 2.98 g/cm³
Colors: Yellow Yellow-orange
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Tabular Crystals
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Manganese-rich Hydrothermal Veins
Typical price: $100-500 thumbnail

Where rockhounds find castellaroite

Classic worldwide localities

Molinello mine, Italy

Field-hunting tip

Look in manganese-rich hydrothermal veins country — that is the host setting where castellaroite typically forms. If you start seeing barite, pyrolusite, manganite 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

How do you identify castellaroite?+

Mohs hardness is 2.5. It typically shows a vitreous luster. The streak is white. Common colors include yellow, yellow-orange.

Where is castellaroite found?+

Notable localities include Molinello mine, Italy.

How much is castellaroite worth?+

Typical asking prices fall in the range of $100-500 thumbnail. Quality, size, and provenance can move individual specimens well outside that range.

Is castellaroite safe to handle?+

It contains toxic constituents. Contains arsenic, which is toxic if ingested or inhaled; wash hands thoroughly after handling and avoid dust generation. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like castellaroite?+

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

What minerals are found with castellaroite?+

Castellaroite commonly co-occurs with barite, pyrolusite, manganite, hausmannite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does castellaroite form in?+

Castellaroite typically forms in manganese-rich hydrothermal veins. Working float back to the host body is the standard way to chase a fresh occurrence.

What is castellaroite used for?+

Castellaroite is used in collector.

Find castellaroite on the map

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