Johannsenite

Chemical formula: CaMn²⁺Si₂O₆

Johannsenite is a rare manganese pyroxene, forming prismatic crystals and radial aggregates, valued by collectors for its unique forms.

## Characteristics Johannsenite is a mineral from the clinopyroxene group, being a calcium-manganese silicate. It forms elongated, prismatic crystals, often terminated by steep faces. It most commonly occurs as radial or fibrous aggregates, which can form spherulitic (spherical) structures. Specimens can also be granular or massive. Due to its rarity and attractive forms, it is a sought-after collector's mineral. ## Physical Properties This mineral is characterized by a hardness of 6 on the Mohs scale, which places it on par with orthoclase. Its density is approximately 3.56 g/cm³. Johannsenite's luster is vitreous, and on fracture surfaces, it can be dull. It is a translucent mineral. ## Colors and Varieties Johannsenite exhibits a variety of colors, from dark brown, through pale brown, grayish-white, to bluish-green and pale green hues. Colorless specimens are also found. Some specimens may show weak, red fluorescence under ultraviolet light. No named varieties are distinguished. ## History and Name Johannsenite was first described in 1932. Its name honors Albert Johannsen (1871-1962), an American petrographer and professor at the University of Chicago, author of the multi-volume work "A Descriptive Petrography of the Igneous Rocks". The mineral was identified simultaneously from three different localities, which are considered type localities: Franklin Mine in the USA, Mount Civillina in Italy, and Temperino Mine, also in Italy. ## Uses Johannsenite has no industrial significance. It is solely a mineral of scientific and collecting importance.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: white
Density: 3.56
Cleavage: Good on {110}
Fracture: Irregular/Uneven,Step-Like
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Johannsenite is most easily recognized by its characteristic radial or spherulitic aggregates. Prismatic crystals, if present, are also a diagnostic feature. Its hardness (6) and relatively high density can be helpful in identification. Occurrence in manganese skarns is a key indicator. ## Distinguishing from Similar Minerals Johannsenite can be confused with other pyroxenes, such as diopside or hedenbergite. Differentiation often requires advanced chemical analysis to confirm manganese dominance. Bustamite, another manganese silicate, has a similar appearance but crystallizes in the triclinic system and often has a pinkish hue. Rhodonite is usually more pink or red and has excellent cleavage. ## Crystal Forms Johannsenite crystals are prismatic, often elongated along the crystallographic axis. They usually occur as radial, fibrous aggregates forming fan-shaped or spherical forms (spherulites). Less commonly, it is found as granular aggregates or massive forms.

Geological environment

## Genesis Johannsenite is a mineral typical of metasomatically altered manganese-rich carbonate rocks (manganese skarns). It also forms as a result of hydrothermal processes in ore veins, where it replaces other manganese minerals, such as rhodonite. ## Mineral Associations This mineral often co-occurs with other manganese minerals, such as rhodonite, bustamite, tephroite, galaxite, as well as with calcite, quartz, and various garnets (e.g., spessartine). ## Localities The most important johannsenite localities worldwide include its type localities: Franklin Mine in New Jersey (USA), where it occurred in a zinc deposit, and Temperino and Civillina mines in Italy. Significant specimens also come from Broken Hill in New South Wales (Australia), Wessels Mine in Hotazel (RSA), as well as from Japan (Oppu and Hoei mines) and Mexico (Santa Eulalia, Chihuahua).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those forming well-developed, radial aggregates of spherical shape (spherulites), especially large ones. Intense, unusual color (e.g., bluish-green) and contrasting association with other minerals, such as white calcite, enhance its attractiveness. Well-formed, single prismatic crystals are rarer and also highly prized. ## Popular Localities Specimens from historical localities, such as Franklin in the USA and Broken Hill in Australia, are considered classic and most desirable. In recent years, interesting specimens from Mexico and Japan have also appeared on the market.

Care and storage

## Cleaning Johannsenite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always thoroughly dry the specimen after cleaning. ## What to Avoid This mineral is sensitive to strong acids, which can damage it. Contact with household chemicals and cleaning agents should be avoided. It should not be subjected to sudden temperature changes or cleaned with ultrasonic cleaners. ## Storage Johannsenite is relatively hard but brittle. It is best stored in separate, padded boxes or display cases to prevent scratches and impacts from other minerals. It should be protected from dust and moisture.