Sadanagaite
Sadanagaite is a rare silicate mineral that is a silica-poor member of the Calcium Amphibole Subgroup of minerals. Technically speaking, Sadanagaite is a subsilicic, alumina-rich, calcic amphibole. Sadanagaite is also barely radioactive due to its potassium (K) content. Sadanagaite is actually considered a group name with over a dozen members each varying slightly in their chemical composition. The main differences between the members are in the presence, or lack of, and the amounts of, chlorine (Cl), flourine (F), iron (Fe) and potassium (K). The result of these variations are names like Ferri-fluoro-sadanagaite and Potassic-ferro-chloro-sadanagaite. Faceted gems are very rare and are opaque, dark brown to black with vitreous luster and Moh’s hardness of about 6.0. The only current source of gem sized crystals is at Nusfjord, Flakstadøya Island, Flakstad, Lofoten, Nordland, Norway.
Sadanagaite was discovered in 1980 at the Kasuga mine, Kasuga-mura, Ibigawa-cho, Gifu Prefecture, Chubu Region, Honshu Island, Japan and approved by the IMA as a new mineral species in 1984. Since then, the name Sadanagaite has been given Group status and the Sadanagaite Group has been redefined twice, in the 1997 and 2012 IMA amphibole reports. Sadanagaite was named to honor Professor Ryoichi Sadanaga of the Mineralogical Institute, University of Tokyo, Tokyo, Japan.
Sadanagaite distribution: on Yuge Island, Ehime Prefecture, and in the Nogo-Hakusan area, Fukui Prefecture, Japan. At Nusfjord, Flakstadøya Island, Flakstad, Lofoten, Nordland, Norway. At the Botallack mine, St. Just, Cornwall, England. In the Oetztal, Austria.
| Chemical Formula: | (K,Na)Ca2(Fe2+,Mg,Al,Ti)5[(Si,Al)8O22](OH)2 |
| Potassium Sodium Calcium Magnesium Aluminum Iron Silicate Hydroxide | |
| Molecular Weight: | 931.99 gm |
| Composition: | Potassium | 3.15 % | K | 3.79 % | K2O |
| Sodium | 0.62 % | Na | 0.83 % | Na2O | |
| Calcium | 8.60 % | Ca | 12.03 % | CaO | |
| Magnesium | 3.91 % | Mg | 6.49 % | MgO | |
| Titanium | 1.28 % | Ti | 2.14 % | TiO2 | |
| Aluminum | 7.96 % | Al | 15.04 % | Al2O3 | |
| Iron | 14.98 % | Fe | 19.27 % | FeO | |
| Silicon | 18.08 % | Si | 38.68 % | SiO2 | |
| Hydrogen | 0.22 % | H | 1.93 % | H2O | |
| Oxygen | 41.20 % | O | |||
| 100.00 % | 100.21 % | = TOTAL OXIDE |
| Crystallography: | Monoclinic |
| Crystal Habit: | Prismatic crystals, to 1 mm; as polycrystalline aggregates. |
| Twinning: | None |
| Cleavage: | Perfect on {110} |
| Fracture: | n/a |
| Tenacity: | Brittle |
| Moh’s Hardness: | ~6.0 |
| Density: | 3.30 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Barely Detectable; GRapi = 44.72 (Gamma Ray American Petroleum Institute Units) |
| Color: | Dark brown to black; in thin section, paler with greater silicon content |
| Transparency: | Semi-transparent to opaque |
| Luster: | Vitreous |
| Refractive Index: | 1.673 – 1.697 Biaxial ( + ) or ( – ) |
| Birefringence: | 0.024 |
| Dispersion: | Perceptible; r > v |
| Pleochroism: | Strong; X = pale brown, grayish, or greenish yellow; Y = yellowish orange to light olive-brown; Z = greenish brown, light olive-brown to dark yellowish orange |
