Relatively unoxidized vivianite in limnic coal from Capeni, Baraolt Basin, Romania
1997; Mineralogical Association of Canada; Volume: 35; Issue: 3 Linguagem: Inglês
ISSN
1499-1276
AutoresŞtefan Marincea, Emil M. Constantinescu, Jean Ladrière,
Tópico(s)Geological and Geochemical Analysis
ResumoVivianite in limnic coal from Capeni, Baraolt Basin, Romania, is partially oxidized, despite the strongly reducing environment. The main reflections of its X-ray powder pattern may be indexed on a monoclinic cell with a 10.037(10), b 13.464(9), c 4.723(5) Angstrom and beta 102.55(4)degrees (space group I2/m) or a 10.113(14), b 13.464(9), c 4.723(5) Angstrom and beta 104.38(3)degrees (space group C2/m). The thermal analyses, taken in air, show effects attributable to the oxidation of Fe2+ to Fe3+ ions, i.e., the splitting of the first endothermic effect at about 190 degrees C, the presence of a supplementary exothermic peak at 270 degrees C on the DTA curve, and a gradual dehydration on the TGA curve. The Mossbauer spectrum consists, however, of four quadrupole doublets associated with two sites occupied by ferrous iron and two occupied by ferric iron. Approximately 13% of Fe(2) and 15% of Fe(1) are oxidized to Fe3+. The infrared absorption spectrum shows a splitting of the fundamental H-O-H stretching at 3000-3500 cm(-1), as well as the absence of an (OH) band at about 3370 cm(-1), confirming a slight oxidation of the sample analyzed. Chemical analyses show that only 18 to 22% of the iron is oxidized to Fe3+ and that less than 7.2% of the octahedra are occupied by cations other than iron. On the basis of the geological setting and trace-element chemistry, diagenetic formation in anoxic low-sulfide sediments is indicated. Partial oxidation is due to exposure to air following collection.
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