9/9/2023 0 Comments Oolite composition![]() The most important hematite deposits formed in sedimentary environments. It can also form during contact metamorphism when hot magmas react with adjacent rocks. It can crystallize during the differentiation of a magma or precipitate from hydrothermal fluids moving through a rock mass. Hematite is found as a primary mineral and as an alteration product in igneous, metamorphic, and sedimentary rocks. Instead, if the specimen is magnetic and has a reddish streak, it is most likely a combination of hematite and magnetite. If the investigator checks the streak, a reddish streak will rule out identification as magnetite or pyrrhotite. The investigator must check other properties to make a proper identification. This can lead to an incorrect assumption that the specimen is magnetite or the weakly magnetic pyrrhotite. However, many specimens of hematite contain enough magnetite that they are attracted to a common magnet. Hematite is not magnetic and should not respond to a common magnet. They quickly learn that the reddish streak is the most important clue for identifying hematite. Students in introductory geology courses are usually surprised to see a silver-colored mineral produce a reddish streak. It occurs in many forms that include micaceous, massive, crystalline, botryoidal, fibrous, oolitic, and others.Įven though hematite has a highly variable appearance, it always produces a reddish streak. Its color ranges include red to brown and black to gray to silver. ![]() Its luster can range from earthy to submetallic to metallic. Hematite has an extremely variable appearance. ![]() The streak on the right still has a trail of glittery fragments that must be gently removed for proper evaluation. In the photo above, the streak on the left has been cleaned of fragments, and you can see that it is a reddish brown. This leaves behind the powder that is embedded within the textured surface of the streak plate. So, to assess the streak, the loose particles must be gently shaken free from the streak plate or very lightly brushed off. That debris is not a powder - it is a trail of fragments. These specimens are often brittle and leave a trail of debris along with the streak. Care is needed when testing a specimen of hematite with a metallic luster. Some specimens of hematite will produce a brilliant red streak, others will produce a reddish brown streak. The streak of a mineral is its color in powdered form when scraped across a streak plate (a small piece of unglazed porcelain used to produce a small amount of mineral powder). We propose that carbonate precipitation in marine oolitic biofilms is spatially and temporally controlled by a complex consortium of microbes with diverse physiologies, including photosynthesizers, heterotrophs, denitrifiers, sulfate reducers, and ammonifiers.Hematite's Streak: All specimens of hematite will produce a reddish streak. EPS showed a gradient increase from active to microbially stabilized communities, and when combined with functional gene analysis, which revealed genes encoding EPS-degrading enzymes (chitinases, glucoamylase, amylases), supports a putative role of EPS-mediated microbial calcium carbonate precipitation. Differences in functional genes were detected among the environments, with higher diversity associated with non-active and microbially stabilized environments in comparison with the active environment. In addition, a broad diversity of genes was related to organic carbon degradation, and N2 fixation implying these communities has metabolic plasticity that enables survival under oligotrophic conditions. Among these, gene-encoding enzymes for denitrification, sulfate reduction, ammonification, and oxygenic/anoxygenic photosynthesis were abundant. Functional gene analysis, which employed a microarray-based gene technology, detected a total of 12,432 of 95,847 distinct gene probes, including a large number of metabolic processes previously linked to mineral precipitation. Oolitic sediments ranging from high-energy 'active' to lower energy 'non-active' and 'microbially stabilized' environments were examined as they represent contrasting depositional settings, mostly influenced by tidal flows and wave-generated currents. To fill this gap, we assess the metabolic gene carriage and extracellular polymeric substance (EPS) development in microbial communities associated with oolitic carbonate sediments from the Bahamas Archipelago. Despite the importance of oolitic depositional systems as indicators of climate and reservoirs of inorganic C, little is known about the microbial functional diversity, structure, composition, and potential metabolic processes leading to precipitation of carbonates.
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