Mn-bearing eleonorite from Hagendorf South pegmatite, Germany: crystal structure and crystal-chemical relationships with other beraunite-type phosphates

Abstract

Abstract Mn2+-bearing eleonorite from the Hagendorf South pegmatite situated south of Waidhaus, Upper Palatinate, Bavaria, Germany was study based on single crystal X-ray analysis, Mn and Fe K-edge XANES spectroscopy, as well as IR spectroscopy. According to spectroscopic data, all Mn is bivalent, and all Fe is trivalent. The empirical formula of the mineral is (Mn2+ 0.58Zn0.13Mg0.04 Fe3+ 5.24)Σ5.98(PO4)4(H2O,OH,O)11. The monoclinic unit-cell parameters are: a=20.832(3) Å, b=5.1569(3) Å, c=19.200(2) Å, β=93.01(1)°; space group C2/c. The structure was refined to R 1=5.20% in anisotropic approximation using 1994 reflections with I>3σ(I). Despite in most beraunite-group minerals M1-site demonstrates selective accumulation of bivalent cations, in Mn-bearing eleonorite Mn2+-cations are disordered between octahedral sites without statistical predominance anywhere; all octahedral M(1–4)-sites are predominantly occupied by Fe3+. M1-site is half-occupied by Fe3+ and contains subordinate Mn2+ and minor Zn2+ and Mg2+. Based on the new data we suppose that Mn-bearing eleonorite was formed as a secondary phase as a result of oxidation of a primary Mn-bearing beraunite-group mineral in with Mn2+ was initially distributed between different M sites.