Sadanagaite

Chemical formula: NaCa₂(Mg₃Al₂)(Si₅Al₃)O₂₂(OH)₂

Sadanagaite is a rare, dark green or black amphibole with a complex chemical composition, found in metamorphic rocks.

## Characteristics Sadanagaite is a complex silicate belonging to the amphibole supergroup. It typically forms granular aggregates or poorly developed, prismatic crystals, often embedded in the rock mass. Its color is usually dark green, greenish-brown to almost black. Due to its complicated chemical composition, in which many elements can substitute for one another, its properties can be variable. ## Physical Properties This mineral is characterized by a hardness ranging from 5-6 on the Mohs scale. It has a vitreous luster and is usually opaque, sometimes translucent on thin edges. The density is approximately 3.2-3.4 g/cm³, depending on the specific chemical composition of the specimen. ## Colors and Varieties The dominant colors of sadanagaite are dark shades of green, brown, and black. There are no named commercial or color varieties. The variability in color is directly related to fluctuations in the iron and magnesium content within the crystal structure. ## History and Name The mineral's name comes from its first discovery locality – Yuge Island in Ehime Prefecture, Japan, which was part of the ancient Sadanaga region. It was first described in 1980 by Japanese mineralogists and approved by the International Mineralogical Association (IMA). ## Applications Sadanagaite has no industrial application. It is a mineral primarily of interest to scientists studying metamorphic processes and to advanced collectors of rare minerals and micromounts.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Greenish white
Density: 3.2-3.4
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Sadanagaite is difficult to identify based on visual characteristics. Its dark color, prismatic form (when visible), and occurrence in metamorphic rocks are clues, but not definitive features. The characteristic amphibole cleavage angle (approximately 120°) is a key feature, but often difficult to observe in granular aggregates. Positive identification requires advanced analytical methods, such as chemical analysis (EDS/WDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Sadanagaite is almost impossible to distinguish with the naked eye from other dark amphiboles, such as hornblende, pargasite, or ferro-hornblende. All these minerals have a similar appearance, hardness, and occur in similar environments. It can also be confused with dark pyroxenes (e.g., augite), from which amphiboles differ in cleavage angle (approx. 120° for amphiboles, approx. 90° for pyroxenes). ## Crystal Forms Sadanagaite crystals are usually poorly developed, with a short-prismatic or tabular habit. Most often, it occurs as chaotic granular aggregates or as individual grains embedded in the host rock.

Geological environment

## Genesis Sadanagaite is a mineral characteristic of metamorphic rocks that have undergone transformations under conditions of high temperatures and relatively low to medium pressures. It forms in the amphibolite and granulite facies, often in skarns, which are rocks formed at the contact of magmatic intrusions with carbonate sedimentary rocks. ## Mineral Associations This mineral often co-occurs with other minerals typical of its formation environment, such as diopside, garnets (especially from the grossular-andradite group), scapolite, vesuvianite, titanite, spinel, and calcite. ## Localities The most important and classic locality, from which the holotype originates, is the skarns on Yuge Island in Japan. It is also known from several other locations worldwide, including the Adirondack metamorphic complex in New York State (USA), some localities in Norway, Sweden, and Antarctica. Each of these localities provides specimens primarily of scientific importance.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a sadanagaite specimen is assessed primarily from a scientific and micromount perspective. The most desirable specimens are those with well-formed, even if small, crystals with distinct luster and clean form. Association with other, color-contrasting minerals is also important, as it enhances the aesthetics of the microscopic specimen. Purity and size are less significant, as large and pure crystals practically do not occur. ## Popular Localities Specimens from the type locality – Yuge Island in Japan – are most valued by micromount collectors due to their historical and scientific significance. Specimens from other confirmed localities, such as the USA or Scandinavia, are also sought after by specialized collectors of rare mineral species.

Care and storage

## Cleaning Sadanagaite specimens should be cleaned very carefully, preferably using a soft brush to remove dust. If necessary, compressed air can be used. Due to the possible presence of fractures, washing in water, especially in ultrasonic cleaners, is not recommended. ## What to Avoid Avoid contact with acids and other aggressive chemicals that can damage the mineral's surface. It should not be heated or exposed to sudden temperature changes. Although not particularly light-sensitive, prolonged exposure to strong sunlight is not advisable. ## Storage Collector's specimens of sadanagaite, especially those in the form of micromounts, are best stored in closed, padded boxes to protect them from dust and mechanical damage. Avoid storing in humid environments.

Sources

IMA Mineral List (CNMNC)