Masutomilite

Chemical formula: KLiAlMn₂+(Si₃Al)O₁₀(F,OH)₂

A layered silicate from the mica group, being the manganese analogue of zinnwaldite, characterized by a pink or reddish-brown color.

## Characteristics Masutomilite is a rare mineral from the mica group, belonging to the layered silicates. It is the manganese (Mn²⁺) analogue of zinnwaldite and forms an isomorphic series with it. Chemically, it is also closely related to lepidolite. It most often occurs as platy or scaly aggregates, as well as well-formed, pseudohexagonal, tabular crystals, often grouped into packets called "books." ## Physical Properties Its hardness on the Mohs scale is 2.5-3, which means it is a soft mineral. It has excellent, unidirectional cleavage, allowing it to easily split into thin, flexible lamellae. The luster is vitreous, and pearly on cleavage surfaces. It is transparent to translucent. The mineral's density ranges from 2.9-3.1 g/cm³. ## Colors and Varieties Masutomilite ranges in color from pink, through purplish-pink, to reddish-brown, and can also be colorless. Its characteristic coloration is due to the presence of manganese (Mn²⁺) ions in the crystal structure. There are no named commercial or gemological varieties. ## History and Name The mineral was first described in 1976 by Mitsuo Harada, Katsutoshi Nakao, and Koji Nagashima. Its name honors Professor Kazunosuke Masutomi (1886-1962), a Japanese mineralogist and former curator of collections at the Kyoto Geological Museum, for his contributions to the mineralogy of Japan. The type locality is the Tanokamiyama pegmatite in Shiga Prefecture, Japan. ## Uses Due to its rarity and small accumulations, masutomilite has no industrial application. However, it is a valued object of scientific research and a sought-after collector's stone.

Properties

Mohs hardness: 2.5-3
Luster: Vitreous, Pearly
Streak: White
Density: 2.9-3.1
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification A key diagnostic feature of masutomilite, like other micas, is its excellent cleavage, allowing for the separation of thin, flexible lamellae. A pink or reddish color combined with occurrence in lithium pegmatites is a strong indicator. Pearly luster on cleavage planes is also characteristic. ## Distinguishing from Similar Minerals Masutomilite is visually indistinguishable from pink lepidolite. Definitive identification requires chemical analysis – masutomilite is defined as a lithium mica where manganese (Mn) dominates over aluminum (Al) at a specific structural position, whereas in lepidolite, the opposite is true. Zinnwaldite, with which it forms a series, typically has brown, gray, or yellowish colors, less often purple. Pink muscovite (manganoan muscovite) can also be similar, but occurs in different geological conditions and has a different chemical composition. ## Crystal Forms It forms tabular crystals with a pseudohexagonal outline, often arranged in book-like packets. It also occurs as platy, scaly aggregates, and radial aggregates and rosettes.

Geological environment

## Genesis Masutomilite is a mineral typical of complex granitic pegmatites, rich in lithium, manganese, and fluorine. It forms in the late stages of pegmatitic magma crystallization, often under hydrothermal conditions, within pockets and miaroles. ## Mineral Associations It most often co-occurs with other minerals typical of lithium pegmatites, such as quartz (often smoky), albite (especially its cleavelandite variety), lepidolite, zinnwaldite, elbaite (lithium tourmaline), topaz, cassiterite, and minerals from the columbite-tantalite group. ## Localities The most important occurrences of this mineral are found in pegmatites worldwide. Classic localities include: Tanokamiyama in Japan (type locality), Rožná and Dolní Bory in the Czech Republic, numerous pegmatites in Minas Gerais, Brazil, as well as in the Pikes Peak region in Colorado (USA) and in Afghanistan.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued masutomilite specimens are characterized by an intense, pure pink or purple color. The crystal habit is important – well-formed, sharp, pseudohexagonal crystals ("books") are much more desirable than amorphous scaly aggregates. The size of the crystals and the aesthetic combination with other minerals, e.g., contrasting white albite, smoky quartz, or green elbaite, are also of great importance. ## Popular Localities Specimens from the type locality – Tanokamiyama in Japan – have classic status and are valued as reference material. Very good quality crystals, often in the form of attractive rosettes, come from pegmatites in the Rožná region in the Czech Republic. Brazil and Afghanistan also supply aesthetic specimens of this mineral to the collector's market.

Care and storage

## Cleaning Masutomilite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, it can be rinsed in distilled water. Avoid strong streams of water, which could separate the lamellae. ## What to Avoid The mineral is very soft and susceptible to scratches. Its excellent cleavage makes it brittle and easily damaged by mechanical impact – dropping or striking it can cause the specimen to break into small lamellae. Ultrasonic cleaners and all chemical agents, acids, and detergents should be absolutely avoided. ## Storage It is recommended to store masutomilite specimens in separate, padded boxes or display cases to protect them from scratching by harder minerals. It should be protected from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)