Potassic-ferro-sadanagaite
Chemical formula: KCa₂(Fe²⁺₃Al₂)(Si₅Al₃)O₂₂(OH)₂
Potassic-ferro-sadanagaite is a rare, dark green to black amphibole, occurring as prismatic crystals in metamorphic rocks.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 3.45
- Cleavage
- Perfect on {110}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification This mineral is identified by its dark green to black color, vitreous luster, and crystal form – elongated prisms. A key diagnostic feature, visible under a microscope in cross-sections, is the amphibole-typical cleavage angle (approx. 56° and 124°). However, final identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Potassic-ferro-sadanagaite can easily be confused with other black amphiboles, such as hornblende, arfvedsonite, or riebeckite, as well as some pyroxenes (e.g., augite). Distinguishing it from hornblende "by hand" is practically impossible without knowledge of the exact locality and chemical analysis. Pyroxenes have a cleavage angle close to 90°, which is the main distinguishing feature from amphiboles. ## Crystal Forms Crystals are prismatic, often columnar or acicular in habit. They usually occur as single crystals embedded in the rock, or as radial or anhedral aggregates. Well-formed, freestanding crystals are rare.
Geological environment
## Genesis Potassic-ferro-sadanagaite is a mineral of metamorphic rocks. It forms under conditions of contact metamorphism, in zones where hot magma interacts with calcium- and iron-rich sedimentary rocks. It forms in skarns and amphibole hornfelses. ## Mineral Associations This mineral co-occurs with other minerals typical of skarns and metamorphic rocks, such as diopside, hedenbergite, garnets (andradite, grossular), epidote, vesuvianite, wollastonite, scapolite, titanite, and calcite. ## Localities The most important and typical occurrence (località typica) of this mineral is Yuge Island in Ehime Prefecture, Japan. This is the primary and best-documented locality for this rare amphibole.
Rarity
Very rare
For collectors
## Quality Criteria Most valued by collectors are specimens with well-formed, sharply terminated crystals with distinct luster, set on a contrasting rock matrix. Rich aggregates of prismatic crystals are also highly prized. Crystal size is a key factor – specimens with crystals exceeding several millimeters are considered exceptional. ## Popular Localities The only source of specimens of collector's significance is the type locality – Yuge Island, Japan. Specimens from this location are rarely available on the market and represent a great find for collectors specializing in mineral systematics or rare species.
Care and storage
## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Compressed air can be used from a safe distance. Due to its cleavage, ultrasonic cleaners should be avoided. ## What to Avoid Avoid contact with all chemicals, especially acids, which can damage the mineral's surface. The mineral is sensitive to sudden temperature changes. It should not be heated or exposed to prolonged strong sunlight. ## Storage Specimens of potassic-ferro-sadanagaite, especially those with well-formed crystals, should be stored in separate, padded collector's boxes to prevent abrasions and mechanical damage. Its brittleness and cleavage make it susceptible to fracturing upon impact.