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

The new mineral species pyracmonite , ideally (NH 4 ) 3 Fe(SO 4 ) 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 R 3 c (no. 161) with a 15.2171(14), c 8.9323(8) A, V 1791.3(3) A 3 and Z = 6. The strongest six reflections in the X-ray powder-diffraction pattern [ d obs in A( 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 [(NH 4 ) 2.74 K 0.23 ] ∑2.97 (Fe 0.94 Al 0.04 ) ∑0.98 S 3.02 O 12 , and the simplified formula (NH 4 ,K) 3 (Fe,Al)(SO 4 ) 3 . The measured density is 2.22(1) g/cm 3 , and D calc is 2.228 g/cm 3 . The mineral is uniaxial, and the average index of refraction index is 1.562(3) (λ= 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 > 2σ( I) ]. The mineral is identical to the synthetic compound (NH 4 ) 3 Fe(SO 4 ) 3 and is isostructural to the high-temperature phase of the synthetic compound (NH 4 ) 3 In(SO 4 ) 3 . Building blocks of the structure are (Fe,Al)O 6 octahedra and SO 4 tetrahedra, which are linked together to form infinite columns [(Fe,Al)(SO 4 ) 3 ] ∞ along [001]. The two independent (Fe,Al)–O distances are 1.959(1) and 1.998(1) A; in the sulfate anion, the S–O distances range from 1.415(2) to 1.485(1) A. The ammonium ions, only partly replaced by K + ions, interact with the oxygen atoms of the sulfate groups via hydrogen bonds.