Pyrite Photo

Plate 15.11 I ron oxide formation by atmospheric weathering of a pyrite vein in a limestone (photo courtesy, Ph. Jaesche). 

Prospectors used the physical properties of gold and pyrite to distinguish between them. Can you tell which of these photos shows gold and which shows pyrite ... 

In addition to SO2 self-shielding many other possible sources of S-MIF can be identified. The model A S/results for the case of a I0W-O2 atmosphere (e.g.. Figure 5.7b) are in qualitative but not quantitative agreement with the ancient rock record. Elemental S, derived from S(D, is predicted here to have > 0 and < 0, which is consistent with observations of most pyrites [4], but the magnitude of the A S/A S ratio is about a factor of 2 to 3 too high ( -2.5 vs. 0.9). This is a significant discrepancy, and may indicate that MI processes in addition to SO2 photodissociation are at work. One such MI process almost certain to be important in a I0W-O2 atmosphere is SO photodissociation. Isotope-selective photolysis will occur in SO at wavelengths 190-230 nm, but rotationally-resolved spectra, either laboratory or synthetic, are needed to estimate the MI effect. In addition to S-MIF due to SO photolysis, SO2 photoexcitation ( 280-330 nm) and SO3 photolysis [18] must also be considered as possible contributors to S-MIF in the ancient atmosphere. S-MIF due to these photo-processes will be considered in future work. 

Hydrogen sulfide can be oxidized in less than an hour in seawater. This removal can be through oxidation by oxygen or iodate. There is a possibility of oxidation, by hydrogen peroxide, but it is probably a minor pathway (Radfordknoery and Cutter, 1994). Photo-oxidation is also possible (Pos et ai, 1998), along with oxidation by Fe(III) oxide particles. This latter process is dependent on the way in which the particle forms and on pH with a maximum near 6.5. The Fe(III) oxide route gives mostly elemental sulfur as a product, which may have implications for pyrite formation (Yao and Millero, 1996). 

The name comes from thallos, Greek for green twig. It was discovered by William Crookes using spectroscopic analysis, who named it after the color of the spectra line. The metal was isolated by Crookes and independently by Claude-Auguste Lamy (1820-1878) in 1862. The elemental metal does not occur naturally and is extracted as a by-product from refining pyrites, lead, or zinc. In its pure form it is shiny but oxidizes quickly. It resembles lead. The element is toxic and should be handled carefully. It primary commercial use is in rodent poison and ant killer, as well as in photo cells. 

Plate 14 Yellow Ore from Matsuki Kuroko Deposits, Akita, northern Japan. About 20 cm in length (Shikazono 1988 Photo by M. Shimizu). The ore is mainly composed of pyrite and chalcopyrite (see Chap. 2)... 

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