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:

  The Mesabi range "banded iron" ores were created by the oxidation of ferrous ions in ocean water by the invasion of the novel gas oxygen (created by blue-green algae) and is dissolution into the oceans. The bands were created by alternate episodes of oxidation and precipitation and normal ocean floor ooze and mud deposition. In the Silurian, oxygen content in the atmosphere was nearing its modern level, maintaining an fairly even 15%.