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