PYRACMONITE, (NH4)3Fe(SO4)3, A NEW AMMONIUM IRON SULFATE FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

The new mineral species pyracmonite, ideally (NH4)3Fe(SO4)3, was found in a medium-temperature (~250°C) active intracrater fumarole at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. It occurs on a pyroclastic breccia as colorless to white prismatic crystals up to 0.2 mm in length, in association with salammoniac and kremersite. The mineral is trigonal, space group R3c (no. 161) with a 15.2171(14), c 8.9323(8) Å, V 1791.3(3) Å3 and Z = 6. The strongest six reflections in the X-ray powder-diffraction pattern [dobs in Å(I)(hkl)] are: 7.596(100)(110), 3.320(30)(122), 3.371(26)(131), 4.358(23)(121), 2.829(14)(312), and 2.863(8)(321). The empirical formula, based on 12 anions, is [(NH4)2.74K0.23]S2.97(Fe0.94Al0.04)S0.98S3.02O12, and the simplified formula (NH4,K)3(Fe,Al)(SO4)3. The measured density is 2.22(1) g/cm3, and Dcalc is 2.228 g/cm3. The mineral is uniaxial, and the average index of refraction index is 1.562(3) (l = 589 nm). Using single-crystal X-ray-diffraction data, the structure was refined to a final R(F) = 0.039 for 1010 independent observed reflections [I > 2s(I)]. The mineral is identical to the synthetic compound (NH4)3Fe(SO4)3 and is isostructural to the high-temperature phase of the synthetic compound (NH4)3In(SO4)3. Building blocks of the structure are (Fe,Al)O6 octahedra and SO4 tetrahedra, which are linked together to form infinite columns [(Fe,Al)(SO4)3]∞ along [001]. The two independent (Fe,Al)–O distances are 1.959(1) and 1.998(1) Å; in the sulfate anion, the S–O distances range from 1.415(2) to 1.485(1) Å. The ammonium ions, only partly replaced by K+ ions, interact with the oxygen atoms of the sulfate groups via hydrogen bonds.