Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

The temperature and compositional dependence of disordering in Fe-bearing dolomites

Abstract: Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered ( R 3), but transforms with increasing temperature into a disordered higher symmetry structure ( R 3 c ). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300–1350 K and 2.6–4.2 GPa. The long-range order parameter s , quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20–30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe. Complete disorder is attained in dolomite at ~1240 K, 100–220 K lower than previously thought, and in an ankeritic-dolomites.s. with an X Fe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

Characteristics of Authigenic Carbonate Chimneys in Shenhu Area, Northern South China Sea: Recorders of Hydrocarbon-enriched Fluid Activity

Abstract: Authigenic carbonates often occur in continental margin with fluids venting and/or gas hydrate dissociation. Authigenic carbonates found in Shenhu area, northern South China Sea, are in the form of chimney, and mainly comprise ankerite, aragonite and calcite. Carbon and oxygen stable isotope studies show that δ 13 C _ PDB ranges from -40.18‰ to -38.69‰ , and δ 18 O _ PDB varies between 3.75‰ and 4.31‰. The typical isotope ratios suggest that these carbonate chimneys are derived from anaerobic methane oxidation and preticipated during methane-enriched fluid venting.

Carbonate cementation patterns, potential mass transfer, and implications for reservoir heterogeneity in Eocene tight-oil sandstones, Dongying depression, Bohai Bay Basin, China: Evidence from petrology, geochemistry, and numerical modeling

Abstract: This paper investigates transport mechanisms involving carbonate cementation in Eocene, tight-oil sandstones in Bohai Bay Basin, China, to determine potential mass transfer between adjacent mudstones and sandstones. Evidence from petrology, geochemistry, and numerical modeling suggests two generations of carbonate cementation: (1) early nonferroan calcite (formed at 28°C–41°C) and dolomite (formed at 45°C–63°C); and (2) later ferroan calcite (formed at 105°C–124°C) and ankerite (formed at 101°C–137°C). Based on a one-dimensional model for a coupled sandstone–mudstone system under low and high temperatures, different distribution patterns of carbonate cements reflect episodic concentration gradients that led to diffusive transport of bicarbonate species during progressive burial. Firstly, extensive precipitation of early nonferroan calcite followed by dolomite at or near mudstone–sandstone contacts resulted from initial concentration gradients related to different compositions in primary mineral assemblages. Secondly, introduction of aqueous CO2 from adjacent mudstones into sandstones resulted in dissolution of early nonferroan carbonates and led to diffusive transport of bicarbonate species. These bicarbonate species were incorporated with Fe2+ and subsequently reprecipitated as ferroan carbonate minerals at distances greater than 2 m (>6.6 ft) from sandstone–mudstone contacts. Therefore, short-distance diffusive transport is inferred to have been the predominant transport mechanism associated with carbonate cementation. Large-scale mass transfer between sandstones and adjacent mudstones occurred in a relatively open geochemical system on a very local scale. Numerical model results show that low porosity zones (2.6%–5.1%) exhibit coherence with high abundances of carbonate cements (13.9%–21.2%). Tightly cemented intervals were created by different generations of carbonate cementation and resulted in destruction of sandstone reservoir porosity.

Seasonal changes in the mineral compositions of tropospheric dust in the industrial region of Upper Silesia, Poland

Abstract: The tropospheric dust loading in Upper Silesia (Poland) shows a steady, annually averaged supply of minerals from natural and anthropogenic sources, industrial dust emitters, domestic heating, transportation, but with superimposed seasonal changes for some dust types Samples of airborne and deposited particles were collected at monthly intervals between 1996 and 2001 in several cities of Upper Silesia Dust samples were examined by X-ray powder diffraction, analytical transmission electron microscopy, analytical scanning electron microscopy, and electron microprobe microanalysis The most common dust included quartz, gypsum, coke, soot, hematite, magnetite, wustite, bassanite, graphite and various K-, Fe- and Mg-bearing aluminosilicates, in order of decreasing abundance Minor phases included α-iron, sulphur, sphalerite, halite, sylvite, hercynite, franklinite, baryte, dolomite, ankerite, apatite, olivine and feldspars Quartz, and specific industrial minerals, eg spinels, sphalerite, olivine and iron, occurred throughout the year in almost constant abundances The amounts of all other dust components show seasonal variations; gypsum, baryte and other sulphates are particularly abundant in winter In general, minerals related to low-emission sources are abundant in the winter time, while both natural dusts and dust from high-emission sources are predominant during the summer