Pyrochroite

Chemical formula: Mn<sup>₂+</sup>(OH)<sub>₂</sub>

Pyrochroite is a manganese hydroxide that rapidly darkens in air, changing color from colorless or light to black.

## Characteristics Pyrochroite is a mineral from the hydroxide group, chemically corresponding to manganese(II) hydroxide. Freshly exposed specimens are colorless, whitish, light blue, or greenish, but due to rapid oxidation of manganese in air, their surface quickly darkens, passing through shades of brown to black. It most often forms massive, granular, or fibrous aggregates. Well-formed crystals are rare and appear as thin, platy hexagonal tablets. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of approximately 2.5. It exhibits perfect cleavage in one direction, causing it to easily split into thin lamellae. The luster of fresh surfaces is pearly or vitreous. Its density is about 3.25 g/cm³. It is transparent to translucent, but oxidized specimens become opaque. ## Colors and Varieties Fresh pyrochroite is colorless, white, light blue, greenish, or pale pink. Under the influence of atmospheric oxygen, its color rapidly changes to brown and eventually black, due to the transformation into higher oxidation state manganese oxides, such as pyrolusite. No named varieties are distinguished. ## History and Name The name pyrochroite comes from the Greek words "pyros" (πῦρ) meaning "fire" and "chroa" (χρώσ) meaning "color," referring to the mineral's color change upon heating. The mineral was first described in 1864 by Lars Johan Igelström based on specimens found in the Paisberg and Nordmark mines in Sweden. ## Uses Pyrochroite has no direct industrial application. However, it serves as a subordinate source of manganese ore in some deposits. For collectors, it is an interesting mineral due to its reactivity and rarity in well-formed specimens.

Properties

Mohs hardness: 2.5
Luster: Pearly, Vitreous
Streak: White, darkening on exposure to air
Density: 3.258
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Trigonal

Diagnostic features

## Identification The key diagnostic feature of pyrochroite is its rapid color change in air – a freshly exposed, light surface quickly turns brown or black. Other characteristics include low hardness (around 2.5), perfect cleavage in one direction, forming thin lamellae, and a pearly luster on cleavage planes. ## Distinguishing from Similar Minerals Fresh pyrochroite may resemble brucite (Mg(OH)₂), with which it forms an isomorphous series. However, brucite is stable in air and does not darken. Talc also has similar softness and pearly luster, but it is greasier to the touch and does not react with air. Oxidized specimens can be confused with other manganese oxides, such as pyrolusite, but their genesis and primary structure are different. ## Crystal Forms Pyrochroite crystallizes in the trigonal system. It rarely forms well-developed, hexagonal, platy crystals. It usually occurs as lamellar, scaly, fibrous aggregates, or as massive, granular aggregates.

Geological environment

## Genesis Pyrochroite is a low-temperature hydrothermal mineral. It forms under reducing (anaerobic) conditions in veins cutting through manganese and iron deposits, often of metamorphic origin. It can also form in bottom sediments of lakes and seas with limited oxygen access. ## Mineral Associations This mineral often co-occurs with other manganese minerals formed under reducing conditions, such as rhodochrosite, hausmannite, tephroite, as well as with calcite, barite, and fluorite. As a result of oxidation, it transforms into pyrolusite, manganite, and other manganese oxides. ## Localities The most important historical localities, from which classic specimens originate, are the mines in the Värmland region of Sweden (Paisberg, Nordmark, Långban). It is also known from the Franklin deposit in New Jersey, USA, where it occurs in association with unique zinc and manganese minerals. Other reported occurrences include Mont Saint-Hilaire in Canada and deposits in South Africa (N'Chwaning mines).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp, hexagonal crystals, which are very rare. The preservation of the original, light color (light green, bluish) also enhances its attractiveness, indicating proper specimen storage. Large, rich crystalline aggregates or interesting associations with other rare minerals (e.g., from Franklin) are also highly prized. ## Popular Localities Classic and most sought-after pyrochroite specimens come from the Swedish mines in Långban and Paisberg. Specimens from the Franklin deposit in New Jersey (USA) are also highly valued, often exhibiting interesting morphology and co-occurring with other rare minerals. Specimens from the Kalahari Manganese Field mines in South Africa, although rarer, also appear on the collector's market.

Care and storage

## Cleaning Cleaning pyrochroite is very difficult due to its chemical and physical instability. Contact with water, which accelerates oxidation, should be avoided. The safest method is to use a soft, dry brush to remove dust. ## What to Avoid Moisture, water, and chemicals, especially acids, must be absolutely avoided. The mineral is very sensitive to atmospheric oxygen, and the darkening process is irreversible. It should not be heated or exposed to prolonged strong light. ## Storage Pyrochroite specimens require storage under special conditions. It is best to keep them in sealed, airtight containers, filled with an inert gas (e.g., argon) or in a vacuum, to prevent contact with air. Oxygen absorbers can also be used. Display should be carried out under controlled conditions, away from moisture and heat sources.

Sources

IMA Mineral List (CNMNC)