RELATIVELY UNOXIDIZED VIVIANITE IN LIMNIC COAL FROM CAPENI, BARAOLT BASIN, ROMANIA

Vivianite in limnic coal from Capeni, Baraolt Basin, Romania, is parti ally oxidized, despite the strongly reducing environment. The main ref lections of its X-ray powder pattern may be indexed on a monoclinic ce ll 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 an alyses, taken in air, show effects attributable to the oxidation of Fe 2+ to Fe3+ ions, i.e., the splitting of the first endothermic effect a t 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 T GA curve. The Mossbauer spectrum consists, however, of four quadrupole doublets associated with two sites occupied by ferrous iron and two o ccupied by ferric iron. Approximately 13% of Fe(2) and 15% of Fe(1) ar e 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 t he absence of an (OH) band at about 3370 cm(-1), confirming a slight o xidation of the sample analyzed. Chemical analyses show that only 18 t o 22% of the iron is oxidized to Fe3+ and that less than 7.2% of the o ctahedra are occupied by cations other than iron. On the basis of the geological setting and trace-element chemistry, diagenetic formation i n anoxic low-sulfide sediments is indicated. Partial oxidation is due to exposure to air following collection.