Karenwebberite: un nuovo fosfato primario dal filone della Malpensata, Colico, Lecco

Karenwebberite, Na(Fe2+,Mn2+)PO4, belongs to the triphylite group and corresponds to the Fe-equivalent of natrophilite or to the Na-equivalent of triphylite. It occurs in the Malpensata pegmatite, Colico, Lecco Province, Italy. Karenwebberite is found as late-magmatic-stage exsolution lamellae up to 100 ?m thick, hosted by graftonite and associated with Na-bearing ferrisicklerite and with a heterosite-like phase. Lamellae are pale green, with very pale greyish-green streak. The lustre is greasy to vitreous, and lamellae are translucent (pale green) to opaque (dark green). Optically, the mineral is anisotropic, biaxial (+). Pleochroism is moderate with X = dark gray, Y = brown, and Z = yel¬low. The mineral is brittle with a Mohs hardness of 4.5; in thin section it displays a perfect cleavage along {001} with an irregular fracture and its density is 3.65 g/cm3. The empirical formula, calculated on the basis of 1 P atom per formula unit is (Na0.817Ca0.003?0.180)?1.000 (Fe2+0.622Mn2+0.228Fe3+0.151Mg0.010Zn0.002)?1.013PO4. Karenwebberite is orthorhombic, space group Pbnm, a = 4.882(1), b = 10.387(2), c = 6.091(1) Å, V = 308.9(1) Å3, and Z = 4. The eight strongest lines in the X-ray powder pattern are [d in Å (intensities) (hkl)]: 5.16 (50) (020), 4.44 (90) (110), 3.93 (80) (021), 3.56 (90) (120), 3.04 (80) (002), 2.817 (100) (130), 2.559 (100) (131), and 1.657 (50) (061).