Relatively unoxidized vivianite in limnic coal from Capeni, Baraolt Basin, Romania

Vivianite 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

Apatite-(CaOH) in the fossil bat guano deposit from the "dry" Cioclovina Cave, Şureanu Mountains, Romania

International audienceApatite-(CaOH) is the most abundant phosphate in the deposit of fossil bat guano in the “dry” Cioclovina Cave, Şureanu Mountains, South Carpathians, Romania. Initial deposits, of both biogenic and authigenic origin, were chemically equilibrated during diagenesis. Individual crystals are tabular, roughly hexagonal, platy on (0001) and usually between 3 and 15 µmm across and up to 1 µmm thick. The mean indices of refraction measured for 10 representative samples are &#949 1.645(2) and &#969 1.653(1). The mean measured density [Dm = 3.17(2) g/cm3] is in good agreement with the individual calculated values. The unit-cell parameters calculated as average of 20 sets of values are a 9.436(13), c 6.868(8) Å. The mineral is Ca-deficient, carbonate- and sulfate-bearing. Less than 2.26% of the phosphate groups are protonated, and less than 4.66% are replaced by sulfate. The cumulative incorporation of other cations in the Ca sites accounts for only 0.71 to 4.07% (mean 2.07%). Both unit-cell parameters and thermal behavior are characteristic for a hydrous A-type carbonated apatite-(CaOH), with molecular H2O and carbonate substituting for hydroxyl in the structural channels. The multiplicity of the bands in the infrared absorption spectrum (3&#9573 + 1&#9571 + 3&#9574 + 2&#9572) is consistent with a C6 point symmetry of the phosphate anion. Whitlockite obtained by thermal breakdown at 1000°C is sulfate-bearing. The authigenesis of the Cioclovina apatite-(CaOH) involved a reaction between calcium carbonate from the moonmilk flows or the cave floor and phosphoric solutions derived from guano, with brushite or an X-ray amorphous phase as a precursor.L’apatite-(CaOH) est le phosphate le plus abondant du dépôt fossile de guano de chauve-souris de la grotte «sèche» de Cioclovina, monts Şureanu, Carpates Méridionales, en Roumanie. Les dépôts initiaux, d’origine aussi bien biogène qu’authigène, ont été portés à l’équilibre chimique durant la diagenèse. Les individus cristallins sont tabulaires, vaguement hexagonaux, aplatis selon (0001). En général, ils ont entre 3 et 15 µmm de diamètre et une épaisseur jusqu’à 1 µmm. Les indices de réfraction moyens, mésures sur 10 échantillons représentatifs, sont &#949 1.6451,645(2) et &#969 1,653(1). La densité mesurée moyenne [Dm = 3.17(2) g/cm3] concorde bien avec les valeurs de densité calculées. Les paramètres réticulaires, obtenus comme moyenne de 20 déterminations de valeurs individuelles, sont a 9.436(13) et c 6.868(8) Å. Le minéral est déficitaire en Ca et contient du carbonate et du sulfate. Jusqu’à 2.26% des groupes phosphate sont protonés et jusqu’à 4.66% sont remplacés par des groupes sulfate. Le remplacement du Ca par d’autres cations est mineur, ne comptant que pour 0.71 à 4.07% des sites occupés normalement par Ca (2.07% en moyenne). Les paramètres de la maille, ainsi que le comportement thermique, sont caractéristiques d’une apatite-(CaOH) carbonatée de type A, contenant de l’eau moléculaire et du carbonate en tant que remplaçants de l’hydroxyle dans les tunnels structuraux. La multiplicité des bandes dans le spectre d’absorption en infrarouge (3&#9573 + 1&#9571 + 3&#9574 + 2&#9572) concorde avec une symétrie ponctuelle C6 de l’anion phosphate. La whitlockite obtenue après la décomposition thermique à 1000°C de l’apatite-(CaOH) contient du sulfate. Si authigène, l’apatite-(CaOH) de Cioclovina est issue des réactions entre le carbonate de calcium des coulées de “lait de lune” ou du socle de la grotte et les solutions phosphatées dérivées du guano, ayant la brushite ou des phases amorphes auxrayons X comme précurseurs