Carlfrancisite

Chemical formula: Mn₂+₃(Mn₂+,Mg,Fe₃+,Al)₄₂[As₃+O₃]₂(As₅+O₄)₄[(Si,As₅+)O₄]₆[(As₅+,Si)O₄]₂(OH)₄₂

Carlfrancisite is a very rare, complex manganese arsenate-silicate, forming small, hexagonal crystals of a reddish-brown color.

## Characteristics Carlfrancisite is an extremely rare mineral from the arsenate and silicate groups, with a very complex chemical composition dominated by manganese. It occurs as small, hexagonal, tabular or prismatic crystals, rarely exceeding 1 mm in length. These crystals often form radial aggregates or rosettes. Its color is characteristic, reddish-brown to dark red, which, combined with the crystal habit, gives it a unique appearance. ## Physical Properties This mineral is characterized by a vitreous luster and is translucent. Its Mohs hardness is 3, and its density is close to 3.45 g/cm³. It exhibits perfect cleavage in one direction, which is visible on some crystals. Its streak is light brown. ## Colors and Varieties Carlfrancisite occurs in a uniform color range from reddish-brown to dark red. Due to its extreme rarity, no color or commercial varieties have been distinguished. ## History and Name The mineral was discovered in the dumps of the Sterling Mine in Sterling Hill, Ogdensburg, New Jersey (USA). It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2012. Its name honors Carl A. Francis (born 1948), a long-time curator of the Harvard University Mineralogical Museum, in recognition of his contributions to mineralogy, especially his research on minerals from Franklin and Sterling Hill. ## Uses Due to its extreme rarity and microscopic crystal sizes, carlfrancisite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals (so-called micromounts).

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Light brown
Density: 3.45
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Carlfrancisite is primarily recognizable by its unique appearance: small, reddish-brown, hexagonal crystals forming radial aggregates. Key to identification is its occurrence location (Sterling Hill) and its association with other rare manganese minerals. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Distinguishing from Similar Minerals It can be confused with other rare, reddish arsenates from Sterling Hill, such as johnbaumite or holdenite. Distinguishing it based on visual features is practically impossible and requires specialized studies. However, the characteristic hexagonal crystal habit is an important diagnostic clue. ## Crystal Forms It forms hexagonal, tabular or short prismatic crystals. Most often, they occur as radial or rosette-like aggregates, growing on other minerals.

Geological environment

## Genesis Carlfrancisite forms as a result of metamorphic processes in zinc, iron, and manganese ore deposits. It is a product of late-stage hydrothermal processes or contact metamorphism occurring within mineralized marbles (skarns). ## Mineral Associations This mineral occurs in association with other, often rare, minerals such as franklinite, willemite, zincite, rhodochrosite, and also rare arsenates, e.g., holdenite. ## Localities The only confirmed locality for carlfrancisite in the world is its type locality – the Sterling Mine in Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA. This is one of the most famous mineralogical sites in the world, known for the occurrence of over 350 different minerals, including many unique to this locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a carlfrancisite specimen is primarily assessed based on the size and development of the crystals and the aesthetics of the entire aggregate. Most valued are specimens with well-formed, undamaged crystals forming visually attractive rosettes or radial groups. The contrast with the matrix mineral on which it grows is also important. The purity and intensity of the reddish-brown color also increase its collector's value. ## Popular Localities All known and valued carlfrancisite specimens come from a single locality in the world: the Sterling Mine in Sterling Hill, New Jersey, USA. Specimens from this locality are unique and are classics for collectors specializing in rare minerals or minerals from Franklin and Sterling Hill.

Care and storage

## Cleaning Carlfrancisite specimens, due to their delicacy and small size, should be cleaned with utmost care. The safest method is to use compressed air (from a safe distance) to remove dust. Any contact with water or chemicals is inadvisable. ## What to Avoid Contact with water, acids, and other chemicals should be strictly avoided. The mineral is sensitive to shocks and impacts, which can damage delicate crystals. It should not be subjected to ultrasonic cleaning. It should be protected from high temperatures and sudden changes in temperature. ## Storage The best way to store carlfrancisite specimens is to place them in a specialized micromount box, which protects them from dust, moisture, and mechanical damage. Display should be away from direct sunlight and heat sources.

Sources

IMA Mineral List (CNMNC)