Marumoite

Chemical formula: Pb₂+₈As₃+₁₀S₂-₂₃

Marumoite is an extremely rare sulfosalt of lead and arsenic, forming microscopic, acicular crystals with a metallic luster.

## Characteristics Marumoite is a very rare mineral from the sulfosalt group, chemically a complex sulfide of lead and arsenic. It occurs as very small, acicular or fibrous crystals, which usually form tangled or radial aggregates. Due to the microscopic size of the crystals, it is a typical micromount mineral. It has a lead-gray to black color and a strong, metallic luster. ## Physical Properties This mineral is opaque. It is characterized by a metallic luster and a black streak. Its density, calculated based on chemical composition and crystal structure, is 5.65 g/cm³. Hardness has not been precisely measured, but like other sulfosalts, it is a soft mineral. ## Colors and Varieties Marumoite is monochromatic – it occurs in shades from lead-gray to black. No color varieties or commercial varieties are known. ## History and Name The mineral's name honors Prof. Fumiyuki Marumo (born 1931), a Japanese crystallographer and mineralogist from the Kanagawa Institute of Technology, for his contributions to sulfosalt research. The mineral was first described in 1999 by M. Shimizu, F. Matsuyama, and Y. Iida based on material from the Kajika mine in Japan. ## Applications Marumoite has no industrial application. Its significance is purely scientific and collectible, being an object of interest for specialists in rare minerals and sulfosalt systematics.

Properties

Luster: Metallic
Streak: Black
Density: 5.65
Cleavage: Good on one direction
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying marumoite based on visual characteristics is practically impossible. It is one of many gray, metallic acicular minerals. Certain identification requires advanced analytical techniques, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). A key clue is its occurrence in a specific geological environment and co-occurrence with other rare sulfosalts in known localities. ## Distinguishing from Similar Minerals Marumoite can be confused with many other acicular sulfosalts, such as jamesonite, boulangerite, zinkenite, or even stibnite. All these minerals form aggregates similar in form and color. Without chemical analysis, distinguishing them is impossible. ## Crystal Forms Marumoite forms very fine, elongated crystals with an acicular or hair-like habit. These crystals often occur as tangled, felt-like aggregates or in radial aggregates.

Geological environment

## Genesis Marumoite forms under low-temperature hydrothermal conditions. It is a component of ore veins, particularly quartz veins containing gold and silver. ## Mineral Associations This mineral co-occurs with other sulfides and sulfosalts. In the type locality (Kajika mine), it was found in association with sphalerite, galena, pyrite, chalcopyrite, as well as rarer minerals like hokkaidoite and isoklakeite. ## Localities Marumoite is an extremely rare mineral, known only from a few locations worldwide, all located in Japan. The most important of these are: - Kajika mine, Shimokita District, Aomori Prefecture – type locality. - Nakase mine, Hyōgo Prefecture. - Tsumo mine (T.L. Mine), Shimane Prefecture.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of marumoite specimens, which are exclusively micromounts, is assessed based on several criteria. The most important is the abundance of the mineral on the rock matrix – the richer the aggregate, the more valuable the specimen. The presence of well-formed, albeit microscopic, crystalline forms is also important. Specimens where marumoite co-occurs with other rare minerals are highly valued, as this increases the scientific and aesthetic value of the sample. ## Popular Localities The only source of collectible specimens is Japan. Specimens from the Kajika mine, as the type locality, are most desired by specialized collectors of systematic minerals.

Care and storage

## Cleaning Marumoite specimens are extremely delicate and usually microscopic in size. Cleaning should be kept to an absolute minimum. The safest method is to use a photographic blower bulb or a can of compressed air (from a safe distance) to blow off dust. Avoid any contact with water and chemical agents. ## What to Avoid As a sulfide, marumoite is sensitive to moisture and oxidation. Contact with water, acids, and other chemicals must be strictly avoided. The crystals are very brittle and soft, so they should be protected from impacts and abrasions. ## Storage The best way to store specimens is in a sealed micromount box, which protects against dust, moisture, and mechanical damage. Store in a dry, temperature-stable place, away from direct sunlight.

Sources

IMA Mineral List (CNMNC)