Finnemanite

Chemical formula: Pb₂+₅(As₃+O₃)₃Cl

Finnemanite is a rare lead arsenite with chlorine, forming characteristic, hexagonal crystals with a vitreous or resinous luster.

## Characteristics Finnemanite is a rare mineral from the arsenite group, chemically classified as a basic lead(II) arsenite with chlorine. It occurs as well-formed, hexagonal crystals, which can be prismatic or tabular. Crystals often exhibit vertical striations on their prism faces. This mineral is typically opaque to translucent and has a luster ranging from vitreous to slightly resinous. ## Physical Properties Finnemanite is a relatively soft and dense mineral. Its Mohs hardness is 2-3, meaning it can be scratched by a copper coin. It is very heavy due to its high lead content – its density is approximately 7.25 g/cm³. It does not exhibit cleavage, and its fracture is uneven to conchoidal. ## Colors and Varieties It typically occurs in shades of gray, from light gray to dark gray, sometimes with a greenish or yellowish tint. It can also be colorless. No named varieties of this mineral are distinguished. ## History and Name The mineral was named in honor of K.J. Finneman, the chief metallurgist at the Långban smelter in Sweden, where the mineral was first discovered. It was described by Gustaf Flink in 1923. The type locality is the Långban mine in the Värmland region. ## Uses Finnemanite has no industrial applications. Due to its rarity and attractive, though small, crystals, it is a mineral valued exclusively by collectors specializing in rare species or minerals from Långban.

Properties

Mohs hardness: 2-3
Luster: Vitreous to resinous
Streak: Gray
Density: 7.25
Cleavage: None
Fracture: Uneven to sub-conchoidal
Transparency: Translucent to opaque
Crystal system: Hexagonal

Diagnostic features

## Identification Key diagnostic features of finnemanite include its hexagonal crystals, high density (the specimen feels surprisingly heavy for its size), low hardness, and occurrence in a specific geological environment. The gray color and resinous luster are also characteristic. ## Distinguishing from Similar Minerals Finnemanite can be confused with other rare lead minerals from Långban, such as hedyphane or other arsenites. Definitive differentiation often requires chemical analysis (EDS) or X-ray diffraction (XRD). Its characteristic crystal form and very high density help distinguish it from most common minerals. ## Crystal Forms It forms hexagonal, prismatic, or thick tabular crystals, often terminated by pyramids. Crystals can reach up to 2 cm in length, which is a significant size for this mineral. It also occurs in granular aggregates.

Geological environment

## Genesis Finnemanite is a mineral of metamorphic origin. It forms in skarns, which are rocks formed at the contact of magmatic intrusions with carbonate-rich sedimentary rocks (such as limestones or dolomites), in iron and manganese ore deposits that have undergone metamorphism. ## Mineral Associations It most commonly co-occurs with calcite, native lead, hausmannite, magnetite, pyrochroite, and other rare minerals from the Långban deposit, such as magnussonite or hedyphane. ## Localities This is an extremely rare mineral, known practically only from two locations worldwide. The type locality and main source of the best specimens is the historic Långban mine in Värmland, Sweden. The second confirmed occurrence is the Jakobsberg mine, also in Sweden.

Rarity

Very rare

For collectors

## Quality Criteria The most desirable finnemanite specimens are those with sharply terminated, well-formed, and freestanding crystals of significant size (above 1 cm). Specimens where gray finnemanite crystals contrast with a white calcite matrix are highly valued. Purity and lack of mechanical damage are crucial for collector's value. ## Popular Localities The only source of specimens of collector's significance is the historic Långban mine in Sweden. Material from this locality is considered classic and is the standard for this species.

Care and storage

## Cleaning Due to its high softness and potential toxicity (lead and arsenic content), cleaning should be limited to an absolute minimum. If necessary, a soft brush can be used to remove dust. Avoid water and any chemical agents. ## What to Avoid Absolutely avoid contact with water, acids, and other chemicals that may react with the mineral. It is very soft, so protect it from scratches and impacts. Due to its lead and arsenic content, avoid inhaling dust and wash hands after any contact with the specimen. ## Storage Finnemanite specimens should be stored in stable conditions, in a closed, dry container or display case, away from moisture and sunlight. It is best to store it separately to avoid scratching by harder minerals.

Sources

IMA Mineral List (CNMNC)