Potassic-sadanagaite

Chemical formula: KCa<sub>2</sub>Mg<sub>3</sub>Al<sub>2</sub>(Si<sub>5</sub>Al<sub>3</sub>)O<sub>22</sub>(OH)<sub>2</sub>

Potassic-sadanagaite is a rare, dark green or black amphibole group mineral, found in high-temperature metamorphic rocks.

## Characteristics Potassic-sadanagaite is a complex silicate belonging to the amphibole supergroup. It typically forms granular aggregates or columnar, prismatic crystals, often with a hexagonal cross-section. Its color is typically dark green to black. It is an opaque mineral, and on crystal surfaces or fractures, it exhibits a vitreous luster. ## Physical Properties This mineral is characterized by a hardness in the range of 5.5-6 on the Mohs scale. Its density is relatively high, measured at approximately 3.24 g/cm³. It exhibits perfect cleavage in two directions, which is typical for amphiboles. ## Colors and Varieties The dominant colors are dark green, brownish green, and black. No distinct color varieties or commercial varieties have been identified. ## History and Name The mineral's name refers to its chemical composition – the dominance of potassium (potassic) and its relation to other minerals in the sadanagaite series. It was recognized by the International Mineralogical Association (IMA) in 1980. The name "sadanagaite" honors Professor Yoshikazu Sadanaga (1920-2003), a Japanese crystallographer from the University of Tokyo, for his contributions to mineralogy.

Properties

Mohs hardness
5.5-6
Luster
Vitreous
Streak
Greenish gray
Density
3.24
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Potassic-sadanagaite is identified by its dark green to black color, vitreous luster, and crystal habit – elongated prisms, often with a hexagonal outline. A key diagnostic feature, typical for amphiboles, is the cleavage angle of approximately 120°. ## Distinguishing from Similar Minerals It can be confused with other dark amphiboles, such as hornblende, or pyroxenes (e.g., augite). It is distinguished from pyroxenes primarily by its cleavage angle (for pyroxenes, it is approximately 90°). Accurate identification and differentiation from other potassic amphiboles (e.g., potassic-ferro-sadanagaite) require advanced chemical analyses (EDS, WDS) or X-ray diffraction (XRD). ## Crystal Forms It most commonly occurs as columnar, prismatic crystals, which can reach several centimeters in length. It also forms granular and fibrous aggregates.

Geological environment

## Genesis This is a characteristic mineral of metamorphic rocks that have undergone transformation under conditions of high temperatures and pressures, in the amphibolite and granulite facies. It also forms in skarns, which are rocks formed at the contact of magmatic intrusions with calcium-rich sedimentary rocks, such as limestones. ## Mineral Associations It co-occurs with minerals such as calcite, diopside, scapolite, phlogopite, titanite, anorthite, and other minerals from the amphibole and pyroxene groups. ## Localities The most important occurrences of this mineral are in Japan, on Honshu Island (Kasuga mine, Gifu Prefecture – type locality; Myojin mine, Tochigi Prefecture). It has also been reported in Sterling Hill, New Jersey (USA), and in the Ilímaussaq igneous complex in Greenland.

Rarity

Rare

For collectors

## Quality Criteria Collector specimens are primarily valued for well-formed, single crystals with distinct luster and a clean, dark color. Crystals exhibiting a hexagonal outline are particularly sought after, especially if they are embedded in a contrasting matrix (e.g., on white calcite). The size of the crystals also significantly increases the value of the specimen. ## Popular Localities The most classic and prized specimens, often serving as a reference for this mineral, come from the Kasuga mine in Japan.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Compressed air may be used from a safe distance. Due to its complex chemical composition, contact with water and any chemical agents should be avoided. ## What to Avoid Ultrasonic cleaners, chemical agents, acids, and detergents, which can react with the mineral and cause permanent damage, must be strictly avoided. Protect from high temperatures and sudden temperature changes. ## Storage Store in stable conditions, away from moisture and direct sunlight. It is best to store in closed display cases or cabinets to limit dust accumulation.

Sources

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