TAILIEUCHUNG - Tiếng anh chuyên ngành địa chất part 6

Tham khảo tài liệu 'tiếng anh chuyên ngành địa chất part 6', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | weathering continental surface at the time that the free oxygen content of the atmosphere was still nil or very low. A lateritic type weathering would largely retain Al ferric iron and phosporous and also part of the silica while ferrous iron and much of the silica would be transported by groudwater to the oceans - see chart below. Later at the time the atmosphere did contain free oxygen the weathering processes could produce somewhat different effluents containing Ca and Mg and the resulting precipitates are less pure Fe Si It should be realized that the original preccipitates were probably gels or amorphous water - rich masses and colloidal particles. During early diagenesis these preccipitates change into discrete minerals such as quartz hematite etc these minerals will show only primary textures. Later low - grade metamorphism changed the mineral composition and caused the appearance of metamorphic minerals with a secondary or replacement texture such as stilpnomelane minnesotaie etc. Sedimentary iron deposits The average crustal abundance of iron is mass iron occurs in most rocks of the earth s crust but the distribution is highly variable Ultramafic rocks Sandstone 0 98 - Basic rocks Carbonate - Granites - Sea water 0 01 Syenites Average lithosphere Shale There are two main rock types Ironstone - an iron - rich sedimentary rock either deposited directly as a ferruginnous sediment of mixed clastic - chemical origin or resulting from chemical replacement. The term is customarily applied to hard coarsely banded or non - banded and non - cherty sedimentary rocks of Phanerozoic age. They are often of oolitic texture e g. minette iron ore from Luxembourg and France Iron-formation - a general term for layered bedded or laminated rocks that contain 15 by mass or more of iron in which the Fe minerals are commonly interbanded with quartz chert or carbonate and where the banded structure industrial development .

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