Pyrochroite is a rare manganese hydroxide mineral that typically forms as soft, pearly, tabular crystals or foliated masses. Collectors should be aware that it is highly sensitive to air and will rapidly darken or turn black when exposed to oxygen due to oxidation into hausmannite. It is most famously found in the historic manganese mines of Långban and Franklin, usually occurring within metamorphosed ore bodies.

Hardness

2.5

Mohs

Luster

Pearly on cleavages

Streak

White

Transparency

Transparent in thin flakes, becoming opaque on exposure

mineraluncommon

Pyrochroite

Hardness: 2.5
Luster: Pearly on cleavages
Streak: White
Transparency: Transparent in thin flakes, becoming opaque on exposure

Is this pyrochroite?

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 pyrochroite with a known reference. Pyrochroite 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. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: pearly on cleavages.
4
Match the color range
Compare against the expected color range: colorless, white, pale green, blue, dark brown.
5
Look at form & habit
Crystal system: trigonal. Typical habit: foliated.

Often confused with

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

mineral

Brucite

Mohs 2.5

How to tell apart: Luster reads pearly on cleavages on Pyrochroite and waxy, pearly on cleavage surfaces on Brucite.

Compare →

Often found alongside pyrochroite

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

All properties

Chemical formula: Mn(OH)₂
Mohs hardness: 2.5
Density: 3.25 g/cm³
Colors: Colorless White Pale green Blue Dark brown
Streak: White
Luster: Pearly on cleavages
Transparency: Transparent in thin flakes, becoming opaque on exposure
Crystal system: Trigonal
Crystal habit: Foliated
Cleavage: {0001}, perfect
Rarity: Uncommon
Uses: Collector, Scientific research
Host rock: A primary mineral in some volcanogenic massive sulfide deposits; a hydration product of manganosite, in turn metamorphosed from rhodochrosite

Where rockhounds find pyrochroite

Classic worldwide localities

Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, and in Alum Rock Park, near San Jose, Santa Clara Co., California, USA
Persberg, Långban, and Nordmark, Värmland, Sweden
Sjö mine, near Grythyttan, Örebro, Sweden
Ljubija and Prijedor, Bosnia-Herzegovina
Gonzen, near Sarganz, Graubünden, Switzerland
Almylak, Uzbekistan

Field-hunting tip

Where it forms: A primary mineral in some volcanogenic massive sulfide deposits; a hydration product of manganosite, in turn metamorphosed from rhodochrosite. If you start seeing hausmannite, rhodochrosite, manganosite in float, you are in the right ground. Typical habit: foliated. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify pyrochroite?+

Mohs hardness is 2.5. Luster: pearly on cleavages. The streak is white. Common colors include colorless, white, pale green, blue.

Where is pyrochroite found?+

Notable localities include Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, and in Alum Rock Park, near San Jose, Santa Clara Co., California, USA; Persberg, Långban, and Nordmark, Värmland, Sweden; Sjö mine, near Grythyttan, Örebro, Sweden; Ljubija and Prijedor, Bosnia-Herzegovina; Gonzen, near Sarganz, Graubünden, Switzerland.

What rocks look like pyrochroite?+

Pyrochroite is most often confused with Brucite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with pyrochroite?+

Pyrochroite commonly co-occurs with Hausmannite, Rhodochrosite, Manganosite, Galaxite, Tephroite, Alabandite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does pyrochroite form in?+

Where it forms: A primary mineral in some volcanogenic massive sulfide deposits; a hydration product of manganosite, in turn metamorphosed from rhodochrosite. Working float back to the host body is the standard way to chase a fresh occurrence.

What is pyrochroite used for?+

Pyrochroite is used in collector, scientific research.

Find pyrochroite on the map

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