Ankerite

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

Mineralogical and Geochemical Characteristics of the Early Permian Upper No. 3 Coal from Southwestern Shandong, China

Abstract: The Upper No. 3 coal of the Early Permian age is a major workable seam in the southwestern Shandong coalfield, which is located in the eastern part of North China. From Early Jurassic to Neogene, the coalfield was subjected to intensive tectonic processes, leading to a significant rearrangement in depth of coal seams. In this paper, three Upper No. 3 coals occurring at −228, −670 and −938 m in the Luxi, Liangbaosi, and Tangkou mines, respectively, were collected to investigate their mineralogical and geochemical characteristics, with emphasis on modes of occurrence and origin of epigenetic minerals. The three coal seams are similar in vitrinite reflectance, volatile matter yield, and maceral components, suggesting insignificant influence from the tectonic activities on coal rank. Terrigenous minerals (e.g., kaolinite and quartz) are comparable in both types and distribution patterns in the three coals. The presence of siderite and pyrite of syngenetic or penecontemporaneous origin indicate they were emplaced during peat accumulation. The distribution of epigenetic minerals (e.g., calcite, ankerite, and dolomite) are associated with the underground water activities, which were Ca (Mg, Fe)-bearing.

Effect of CO2 on mineralogical differences in some low-grade metamorphic iron formations.

Abstract: Mineral assemblages and the Fe/(Fe+Mg) ratio in minnesotaite are evidently different for some low-grade metamorphic Precambrian iron formations in spite of very similar metamorphic conditions (temperature, probably pressure, and rock chemistry). Since carbonates, siderite and ankerite are commonly found, such differences are considered to have been caused by a difference in the degree of the escape of carbon dioxide during metamorphism. In comparison with the iron formations, the greatest degree of the escape is found in the Sokoman and Biwabik Iron Formations, the lowest degree in the Brockman Iron Formation, and the intermediate in the Gunflint Iron Formation. The variation of Fe/(Fe+Mg) in minnesotaite in each iron formation reflects that of oxygen fugacity during metamorphism. The ratio increases with decreasing oxygen fugacity. The fugacity change is probably due to the degree of consuming carbonaceous matters at constant temperature. Fe/(Fe+Mg) in greenalite probably reflects that in a primary one. Greenalite seems to be easily oxidized and to be decomposed to form iron talc with increasing PCO2 at lower temperatures because of very conservative substitution of Fe2+ by Mg. The ratios in siderite and ankerite do not change so significantly as that in minnesotaite, although no sufficient data is available. But they might have resulted from the ratios in carbonates, which were involved prior to recrystallization or ankeritization. It is therefore proposed that Fe/(Fe+Mg) in minnesotaite is a good indicator to reveal the magnitudes of partial pressures of oxygen and carbon dioxide during low-grade metamorphism of Precambrian iron formations.

Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France)

Abstract: In the Lower Aptian historical stratotype area (Cassis-La Bedoule, SE France), a geochemical study of the Gargasian (Middle Aptian) marl-limestone alternations of the La Marcouline quarry complements data already obtained from Bedoulian (Early Aptian) sediments there Nannoconids are the main carbonate producers in both limestones and marls Although diagenetic minerals, such as ankerite (25%) are present in small amounts, the trace-element content of bulk carbonate is very close to that of Nannoconus spp so geochemical sequences can be defined The long-term evolution of trace-element content was not affected by diagenetic processes, variations in carbonate mineralogy, or a change of carbonate producers An increase of around 500 ppm in the strontium content of bulk carbonate occurs between the base of the Cabri zone (late Bedoulian) and the Algerianus zone (late Gargasian) This evolution is linked to fluctuations in seawater Sr/Ca ratios caused by variability in the influx of hydrothermal and river waters, by changes in the ratio of aragonite/calcite production and by shifts in sea level The eustatic sequence Aptian 4, its parasequences and its key surfaces (sequence boundaries, maximum flooding surface) are clearly reflected in the evolution of the bulk-carbonate contents of manganese