Möhnite, (NH4)K2Na(SO4)2, a new guano mineral from Pabellón de Pica, Chile

Abstract

The new mineral mohnite, ideally (NH4)K2Na(SO4)2, the ammonium analogue of aphthitalite, is found in a guano deposit on the Pabellon de Pica mountain, near Chanabaya, Iquique Province, Tarapaca Region, Chile. It is associated with salammoniac, halite, joanneumite, natroxalate, nitratine, chanabayaite, and a clay mineral. Mohnite occurs as random aggregates and clusters of brown imperfect bipyramidal or spindle-shaped crystals. The mineral is brittle, with Mohs’ hardness of 3; Dmeas is 2.4(1) g/cm3 and Dcalc is 2.461 g/cm3. The IR spectrum shows the presence of NH4 + cations (the bands at 1431, 3076 and 3240 cm−1). Mohnite is almost isotropic, optically neutral; e = ω = 1.505(2). The chemical composition (electron-microprobe data, N determined by gas chromatography of products of ignition, H calculated by stoichiometry, wt%) is: (NH4)2O 7.99, Na2O 9.49, K2O 32.34, SO3 51.32, total 101.14. The empirical formula is (NH4)0.95Na0.95 K2.14S1.99O8. The crystal structure was solved and refined to R = 0.049 based on 241 unique reflections with I > 2σ(I). Mohnite is trigonal, space group P $$ \overline{3} $$ m1, a = 5.7402(3) A, c = 7.435(1) A, V = 212.16(4) A3, Z = 1. The strongest reflections of the powder X-ray diffraction pattern [d, A (I,%) (hkl)] are: 4.955 (27) (100), 4.122 (37) (101, 011), 3.708 (29) (002), 2.969 (74) (102, 012), 2.861 (100) (110), 2.474 (20) (003), 2.060 (33) (022). The mineral is named in honour of the German amateur mineralogist Gerhard Mohn (born 1959).