Formation of solid bituminous matter in pegmatites: Constraints from experimentally formed organic matter on microporous silicate minerals

The early stages of burial diagenesis involve the reactions of various oxidizing agents with organic matter, which is the only reducing agent buried with the sediment. In a system in which a local equilibrium is established, thermodynamic principles indicate that, inter alia , manganese, iron and sulphate should each be consumed successively to give rise to a clearly characterized vertical zonation. However, ferric iron may not react fast enough and the relative rates of reduction of Fe III and sulphate not only control the formation of iron sulphide and associated carbonate but also may lead to extreme chemical and isotopic dis-equilibrium. This produces kinetically controlled ‘micro -environments’. On a larger scale, sulphide will diffuse upward to a zone in which its oxidation leads to a reduction of pH. The various dramatic changes in chemical environment across such an interface cause both dissolution and precipitation reactions. These explain common geological observations: the occurrence of flint nodules (and their restriction to chalk hosts) and the association of phosphate with glauconite.

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Sr, Nd and Pb stable isotopes of surface dust on Ürümqi glacier No. 1 in western China

Annals of Glaciology ◽

10.3189/172756411795931895 ◽

2010 ◽

Vol 51 (56) ◽

pp. 95-105 ◽

Cited By ~ 15

Author(s):

Naoko Nagatsuka ◽

Nozomu Takeuchi ◽

Takanori Nakano ◽

Emi Kokado ◽

Zhongqin Li

Keyword(s):

Organic Matter ◽

Tien Shan ◽

Western China ◽

Residual Fraction ◽

Isotopic Ratios ◽

Silicate Minerals ◽

Surface Dust ◽

Desert Sand ◽

Significant Difference ◽

The Difference

AbstractStable-isotopic ratios of strontium (Sr), neodymium (Nd) and lead (Pb) provide a means of identifying a geological source of substances and are used as tracers of elements in biological and geochemical processes. We analyzed these isotopic ratios of surface dust (cryoconite) collected on Ürümqi glacier No. 1 , Tien Shan, China. The dust was separated chemically into five fractions (four minerals and organic matter), and the isotopic ratios of each fraction were measured. The Sr and Nd isotopic ratios in the fractions extracted with ultrapure water (saline minerals), hydrogen peroxide solution (organic matter) and acetic acid (carbonate minerals) were low and invariable, whereas those extracted by hydrochloric acid (phosphate minerals) and the residual fraction (silicate minerals) were higher. The difference was likely due to the original source of each fraction. The isotopic ratios of the surface dust collected from different sites showed no significant difference, suggesting that they were spatially uniform across the glacier. The isotopic ratios of the silicate fraction were closer to those of desert sand reported in China than those of the soil and bedrock around the glacier. This suggests that the silicate minerals on the glacier were derived from distant deserts. The isotopic ratios in saline, carbonate and phosphate fractions were also close to those of evaporites and apatite in that desert region, suggesting that these minerals were also derived from that source. The Sr isotopic ratios in the organic fraction were closer to ratios in the saline and carbonate fractions rather than the silicate or phosphate fractions and may therefore reflect the isotopic ratios of the elements when they are incorporated into living microbes on the glacier.

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Electron energy-loss near-edge structure in silicate minerals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100130924 ◽

1992 ◽

Vol 50 (2) ◽

pp. 1258-1259

Author(s):

D W McComb ◽

R S Payne ◽

P L Hansen ◽

R Brydson

Keyword(s):

Solid State ◽

Energy Loss ◽

Electron Energy ◽

State Energy ◽

Local Environment ◽

Edge Structure ◽

Silicate Minerals ◽

Two Samples ◽

Electron Energy Loss ◽

Careful Processing

Electron energy-loss near-edge structure (ELNES) is an effective probe of the local geometrical and electronic environment around particular atomic species in the solid state. Energy-loss spectra from several silicate minerals were mostly acquired using a VG HB501 STEM fitted with a parallel detector. Typically a collection angle of ≈8mrad was used, and an energy resolution of ≈0.5eV was achieved.Other authors have indicated that the ELNES of the Si L2,3-edge in α-quartz is dominated by the local environment of the silicon atom i.e. the SiO4 tetrahedron. On this basis, and from results on other minerals, the concept of a coordination fingerprint for certain atoms in minerals has been proposed. The concept is useful in some cases, illustrated here using results from a study of the Al2SiO5 polymorphs (Fig.l). The Al L2,3-edge of kyanite, which contains only 6-coordinate Al, is easily distinguished from andalusite (5- & 6-coordinate Al) and sillimanite (4- & 6-coordinate Al). At the Al K-edge even the latter two samples exhibit differences; with careful processing, the fingerprint for 4-, 5- and 6-coordinate aluminium may be obtained.

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