Ferriandrosite-(La)

Chemical formula: Mn₂+LaFe₃+AlMn₂+(Si₂O₇)(SiO₄)O(OH)

Ferriandrosite-(La) is an extremely rare, black mineral from the allanite group, known exclusively from a single deposit in Japan.

## Characteristics Ferriandrosite-(La) is a complex silicate from the epidote supergroup, belonging to the allanite group. Its chemical composition is dominated by manganese, lanthanum, ferric iron, and aluminum. It forms small, poorly developed or completely undeveloped grains, not exceeding 1 mm in size, which often occur as aggregates. The mineral is black and opaque. ## Physical Properties The Mohs hardness is 6.5, and the measured density is 4.10 g/cm³. The mineral exhibits a submetallic luster and an uneven fracture. No cleavage has been observed. The streak is grayish-brown. ## Colors and Varieties The mineral occurs exclusively in black. No color varieties or commercial forms are known. ## History and Name The mineral's name refers to its chemical composition – a high content of ferric iron (ferri) – and its kinship with its manganese analog, androsite-(La). It was officially recognized by the International Mineralogical Association (IMA) in 2013. It was described based on material from the Komatsu mine in Hiroshima Prefecture, Japan, which is its only known locality. ## Applications Due to its extreme rarity, ferriandrosite-(La) has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 6.5
Luster: Sub-metallic
Streak: Greyish brown
Density: 4.10
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Amateur identification of ferriandrosite-(La) is practically impossible. Its black color and occurrence as fine grains require advanced analytical methods. Key features include a grayish-brown streak and submetallic luster. Definitive identification is only possible through chemical analysis (e.g., EDS/WDS) to confirm the dominance of lanthanum, manganese, and ferric iron. ## Distinguishing from Similar Minerals This mineral can be confused with other black minerals from the allanite group, such as allanite-(Ce) or androsite-(La), as well as other dark minerals from manganese rocks, e.g., jacobsite. Differentiation is based solely on precise determination of chemical composition. ## Crystal Forms Ferriandrosite-(La) forms poorly developed (subhedral) or xenomorphic (anhedral) grains. Well-formed, euhedral crystals are not observed. These grains most often intergrow into irregular aggregates.

Geological environment

## Genesis Ferriandrosite-(La) forms under contact metamorphic conditions, within metamorphosed manganese and iron ore deposits, referred to as skarns. ## Mineral Associations This mineral co-occurs with other manganese minerals and silicates. At its type locality, it was found in association with celsian, rhodochrosite, tephroite, spessartine, alleghanyite, sonolite, jacobsite, hausmannite, and quartz. ## Localities The only confirmed occurrence of ferriandrosite-(La) in the world is its type locality – the Komatsu mine, near Kitahiroshima-cho, Hiroshima Prefecture, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria For collectors, the value of a ferriandrosite-(La) specimen primarily depends on the analytically confirmed identity of the mineral. Since it occurs as fine grains, specimens are usually fragments of the host rock. The most highly prized are those where ferriandrosite-(La) is as abundant and clearly visible as possible against contrasting associated minerals. The quality of the mineral itself (form, purity) is of secondary importance due to its typical habit. ## Popular Localities The only source of specimens of this mineral is the Komatsu mine in Japan. Collector material is extremely difficult to obtain and appears on the market very rarely, mainly in inter-institutional circulation or among specialized collectors.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to a lack of data on reactivity, contact with water and all chemical agents should be avoided. ## What to Avoid Avoid ultrasonic cleaners, sudden temperature changes, and all acids and bases that could damage the mineral's surface. Store away from moisture. ## Storage It is recommended to store specimens in stable conditions, in a closed, padded display box, to protect them from dust, light, and mechanical damage. Each specimen should be labeled with its name and locality.

Sources

IMA Mineral List (CNMNC)