Under what conditions does Hexavalent CrOx form on NiChrome?

Compare/contrast the gases in the atmosphere during the Silurian with the conditions under which iron ore form?

• We acknowledge that vast iron ore deposits were created during the Precambrian [the Superior-type]. In the Silurian [mid-Paleozoic] iron ore deposits known as "oolitic hematite" were formed [they are today present from Alabama in the south, in WV and even in Wisconsin]. Compare and contrast the gases in the atmosphere during the Silurian with the conditions under which the Superior type iron ore was formed.

• Answer:

  It took an hour to come up with this info and I hope there is enough material to provide an answer. PRECAMBRIAN Most of the world's iron ore was formed during the middle precambrian period. This was a time of massive erosion that leveled the earlier mountain range, releasing iron and silica particles into a new sea. It is theorized that the appearance of abundant marine algae at this time began to create large quantities of atmospheric oxygen, which combined with the iron and precipitated in layers of iron formations known as the Mesabi, Gunflint, and Cuyuna iron ranges. http://www.dnr.state.mn.us/snas/naturalhistory.html SILURIAN The most extensive iron ore deposits in the Southeast are the middle Silurian Clinton Formation deposits and equivalents that extend along the eastern side of the Appalachian Basin from New York to Alabama. Iron weathered from the eroding Taconic Mountains was deposited in various forms along the edge of the ancient seaway to the west, and occurs as oxides, carbonates, and silicates in sandstones, shales, and limestones. Weathering increases the grade of the ores and makes them more easily mined. The Clinton iron ores are especially rich and thick near Birmingham, Alabama, where the dominant iron mineral is hematite. Most is the product of weathering, formed as iron oxide was concentrated as a residuum from carbonate rocks that were chemically weathered and eroded away. http://geology.teacherfriendlyguide.org/index.php?option=com_content&view=article&id=196&Itemid=207