Three ammonium-iron-sulfite phases from a burning dump of the Vasas abandoned opencast coal mine (Pécs, Mecsek Mountains, Hungary) and the new mineral kollerite

Abstract

Three ammonium-iron-sulfites (AIS) from a burning coal dump in an abandoned open coal pit at Pécs-Vasas (Mecsek Mountains, South Hungary) were identified: (NH₄)₉Fe³⁺(SO₃)₆ (AIS-1), (NH₄)₂Fe²⁺(SO₃)₂ (AIS-2), and (NH₄)₂Fe³⁺(OH)(SO₃)₂·H₂O (AIS-3). They were formed by the interaction of decomposing iron sulfides and ammonia released from organic matter. AIS-1 and AIS-2 are metastable; they break down in a few weeks (AIS-1) respectively years (AIS-2). AIS-1 forms red, stubby columnar to thick tabular crystals up to 0.2 mm in length. AIS-2 appears as brown tabular to short prismatic crystals up to 0.1 mm, often they create columnar intergrowths. AIS-3 is more stable. It was approved as a new mineral species (mineral name kollerite, IMA-CNMNC 2018–131). Sprays of natural kollerite up to 1.5 mm are composed of yellow, long-prismatic or lath-like crystals up to 0.1 mm in length. AIS-1 is characterized by powder-X-ray diffraction only. The crystal structures of AIS-2 [synthetic material, R\(\overline {3}\)m, a = 5.3879(8), c = 19.980(4) Å] and kollerite [Cmcm, a = 17.803(15), b = 7.395(5), c = 7.096(5) Å] were investigated by single-crystal X-ray diffraction. AIS-2 is topologically equivalent to bütschliite. Isolated Fe²⁺O₆ polyhedra are corner-connected to sulfite anions. 2D nets with composition [Fe²⁺(SO₃)₂]²⁻ are parallel to (0001). Kollerite crystallizes in a new structure type. The FeO₆ octahedra are corner linked to buckled [Fe³⁺(OH)(SO₃)₂]²⁻ chains. In both cases, ammonium cations are intercalated. Connection is verified by hydrogen bonds only; all H atom positions are located experimentally.