Zwieselite
Zwieselite is a rare phosphate mineral that is a member of the Triplite Group of minerals that also includes Triplite. The chemical formula of Zwieselite is (Fe2+,Mn2+)2(PO4)F [Iron Manganese Phosphate Fluoride] and its molecular weight is 225.21 gm. In the Triplite-Zwieselite Series, Zwieselite is the iron rich member and Triplite is the manganese rich member. Zwieselite has a specific gravity of 3.89 – 3.97 (g/cm3) and the refractive index of 1.686 – 1.703 Biaxial ( + ). Moh’s Hardness of Zwieselite is 5.0 – 5.5. Zwieselite is most often found as translucent to opaque brown massive material frequently associated with greenish or bluish-gray Triphylite. Zwieselite is rarely found as gemmy crystals suitable for faceting.
Localities for Zwieselite: In Germany, in Bavaria, from Hühnerkobel and in the Birkhöhe pegmatite, near Zwiesel; at Hörlberg; from Döfering; and at Hagendorf. From Horní Slavkov (Schlaggenwald), Czech Republic. In the Mangualde pegmatite, near Mesquitela, Portugal. At the Tremearne pegmatite, Breage, Cornwall, England. From the Storm Mountain pegmatite, Larimer County, Colorado, USA.
| Chemical Formula: | (Fe2+,Mn2+)2(PO4)F |
| Iron Manganese Phosphate Fluoride | |
| Molecular Weight: | 225.21 gm |
| Composition: | Manganese | 12.20 % | Mn | 15.75 % | MnO |
| Iron | 37.20 % | Fe | 47.85 % | FeO | |
| Phosphorus | 13.75 % | P | 31.51 % | P2O5 | |
| Oxygen | 28.42 % | O | |||
| Fluorine | 8.44 % | F | 8.44 % | F | |
| — | — % | F | -3.55 % | -O=F2 | |
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Monoclinic – Prismatic |
| Crystal Habit: | Cleavable, massive, to several cm |
| Twinning: | None |
| Cleavage: | Good on {001}; fair on {010}; poor on {100} (by analogy to triplite) |
| Fracture: | Irregular/Uneven to Sub-conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.0 – 5.5 |
| Density: | 3.89 – 3.97 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Dark brown, clove-brown, yellowish brown; brownish black to black if altered |
| Transparency: | Translucent to Opaque |
| Luster: | Vitreous to Resinous |
| Refractive Index: | 1.686 – 1.703 Biaxial ( + ) |
| Birefringence: | 0.017 |
| Dispersion: | Moderate to strong; r > v |
| Pleochroism: | Distinct; yellow-brown to reddish brown |
