Zhanghuifenite, Na3Mn42+Mg2Al(PO4)6, a new mineral isostructural with bobfergusonite, from the Santa Ana mine, San Luis province, Argentina

Abstract

Abstract A new mineral species, zhanghuifenite, ideally Na3Mn42+Mg2Al(PO4)6, has been found in the Santa Ana mine, San Luis province, Argentina. It occurs in irregular veinlets or patches, 5 mm thick, in a nodule of beusite interlaminated with lithiophilite. Broken pieces of zhanghuifenite are blocky or tabular. Single crystals are up to 0.8 × 0.5 × 0.5 mm. No twinning or parting is observed macroscopically. The mineral is deep green, transparent with pale green streak and vitreous luster. It is brittle and has a Mohs hardness of ~5 with good cleavage on {010}. The measured and calculated densities are 3.63(2) and 3.62 g/cm3, respectively. Optically, zhanghuifenite is biaxial (+), with α = 1.675(2), β = 1.680(2), γ = 1.690(2) (white light), 2V (meas) = 74(2)°, and 2V (calc) = 71°. The calculated compatibility index based on the empirical formula is 0.020 (excellent). An electron microprobe analysis yields an empirical formula (based on 24 O apfu) (Na2.80Ca0.11)Σ2.91(Mn3.092+Fe0.472+Mg0.36)Σ3.92(Mg1.31Fe0.692+)Σ2.00 (Al0.81Fe0.193+)(PO4)6. Zhanghuifenite is insoluble in water or hydrochloric acid. Zhanghuifenite is isostructural with bobfergusonite, a member of the alluaudite supergroup. It is monoclinic, with space group P21/n, Z = 4, and unit-cell parameters a = 12.8926(3), b = 12.4658(3), c = 10.9178(2) Å, β = 97.9200(10)°, and V = 1737.93(7) Å3. The crystal structure of zhanghuifenite contains six octahedral M (= Mn, Fe, Mg, Al) sites and five X (= Na, Mn, Ca) sites with coordination numbers between 6 and 8. The six M octahedra share edges to form two types of kinked chains extending along [101], with one consisting of M1-M4-M5 and the other M2-M3-M6. These chains are joined by PO4 tetrahedra to form sheets parallel to (010), which are linked together through corner-sharing between PO4 tetrahedra and MO6 octahedra in the adjacent sheets, leaving open channels parallel to a, where the large X cations are situated. Zhanghuifenite differs from bobfergusonite in two major aspects. One is that the M4 and M5 sites in the former are mainly occupied by Mg, but by Fe2+ and Fe3+, respectively, in the latter. The other is that the X2-X5 sites in zhanghuifenite are all nearly or fully filled with Na, resulting in 3 Na apfu in the ideal formula, but X4 and X5 are merely half-occupied in bobfergusonite, giving rise to 2 Na apfu.