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Plenty of iron in the earth is in the form of iron oxide, which is already oxidized and won’t oxidize further upon exposure to the atmosphere.

by ryao 2y ago

pfdietz 2y ago
It's mostly in the form of ferrous oxide (Fe(+2)). Fully oxidized to rust, it goes into the ferric form (Fe(+3)). It's very common in weathered rock to see the red color of ferric iron. Ferrous iron occurs in common minerals like pyroxene, hornblende, pyrite, and olivine. Olivine is the most common mineral in the upper mantle.
adrian_b 2y ago
As the other poster has said, about half of the iron in the Earth is as elemental iron in the core and the other half is as ferrous Fe(II) oxide in the mantle, which both are reduced forms of iron that are unstable in the Earth atmosphere. The fully oxidized iron, i.e. ferric Fe(III) oxide, i.e. rust, forms a negligible fraction of the iron in the Earth, being restricted almost entirely to the upper crust of the Earth. A small amount of ferric Fe(III) oxide (which forms magnetite with the remainder of ferrous oxide) is formed at great depths by the reaction with water, where this is present in the rocks, which releases some free dihydrogen, which may remain trapped in the rocks and which is the source of the hydrogen discussed in this article. Here, on the surface of the Earth, almost all iron is oxidized as a consequence of being exposed to the air, but when iron comes from the deeper regions of the Earth, through volcanic eruptions, it is reduced and it begins to oxidize after being exposed to the air. So the volcanic rocks with iron are unstable in the air, they transform slowly into rocks with oxidized iron, in the same way as the artificial objects made of metallic iron, which rust. In general, saying just "reduced" and "oxidized", is ambiguous for many chemical elements, because, like iron, they can have several degrees of reduction or oxidation. Most frequently, using "reduced" and "oxidized" without any other qualifier is taken to mean unstable or stable in the presence of an oxidizer. Some oxidizers are stronger than others, so a given degree of reduction/oxidation may be stable or unstable depending on the oxidizer that is used. However, without other mentions, it is understood that the implicit oxidizer is air. In the presence of air, metallic iron and Fe(II) are reduced, while Fe(III) and Fe(VI) are oxidized. Fe(VI) can be created only by a stronger oxidizer than air, but once created it can no longer be oxidized by air, like the Fe(III) of rust, so both are oxidized forms of iron that are stable in air.
- pfdietz 2y ago
  An example of oxidation of iron in rocks can be seen in the Hawaiian island of Kauai. This island is 5 million years old and is heavily eroded. The dark black volcanic rocks typical of the younger islands have turned bright red, for example in Waimea Canyon State Park. https://en.wikipedia.org/wiki/Waimea_Canyon_State_Park https://en.wikipedia.org/wiki/Waimea_Canyon_State_Park

Plenty of iron in the earth is in the form of iron oxide, which is already oxidized and won’t oxidize further upon exposure to the atmosphere.

by ryao 2y ago

Plenty of iron in the earth is in the form of iron oxide, which is already oxidized and won’t oxidize further upon exposure to the atmosphere.

pfdietz 2y ago

It's mostly in the form of ferrous oxide (Fe(+2)). Fully oxidized to rust, it goes into the ferric form (Fe(+3)). It's very common in weathered rock to see the red color of ferric iron. Ferrous iron occurs in common minerals like pyroxene, hornblende, pyrite, and olivine. Olivine is the most common mineral in the upper mantle.

adrian_b 2y ago

As the other poster has said, about half of the iron in the Earth is as elemental iron in the core and the other half is as ferrous Fe(II) oxide in the mantle, which both are reduced forms of iron that are unstable in the Earth atmosphere. The fully oxidized iron, i.e. ferric Fe(III) oxide, i.e. rust, forms a negligible fraction of the iron in the Earth, being restricted almost entirely to the upper crust of the Earth. A small amount of ferric Fe(III) oxide (which forms magnetite with the remainder of ferrous oxide) is formed at great depths by the reaction with water, where this is present in the rocks, which releases some free dihydrogen, which may remain trapped in the rocks and which is the source of the hydrogen discussed in this article. Here, on the surface of the Earth, almost all iron is oxidized as a consequence of being exposed to the air, but when iron comes from the deeper regions of the Earth, through volcanic eruptions, it is reduced and it begins to oxidize after being exposed to the air. So the volcanic rocks with iron are unstable in the air, they transform slowly into rocks with oxidized iron, in the same way as the artificial objects made of metallic iron, which rust. In general, saying just "reduced" and "oxidized", is ambiguous for many chemical elements, because, like iron, they can have several degrees of reduction or oxidation. Most frequently, using "reduced" and "oxidized" without any other qualifier is taken to mean unstable or stable in the presence of an oxidizer. Some oxidizers are stronger than others, so a given degree of reduction/oxidation may be stable or unstable depending on the oxidizer that is used. However, without other mentions, it is understood that the implicit oxidizer is air. In the presence of air, metallic iron and Fe(II) are reduced, while Fe(III) and Fe(VI) are oxidized. Fe(VI) can be created only by a stronger oxidizer than air, but once created it can no longer be oxidized by air, like the Fe(III) of rust, so both are oxidized forms of iron that are stable in air.

pfdietz 2y ago
An example of oxidation of iron in rocks can be seen in the Hawaiian island of Kauai. This island is 5 million years old and is heavily eroded. The dark black volcanic rocks typical of the younger islands have turned bright red, for example in Waimea Canyon State Park. https://en.wikipedia.org/wiki/Waimea_Canyon_State_Park https://en.wikipedia.org/wiki/Waimea_Canyon_State_Park