What is the difference between "red soil" and "black sand"?

[GeoMax]

Hi Forum,

I have a general question. Sometimes, if people refer to soil conditions, they use "red soil" or "black sand". Both are iron-mineralized soil right? What is actually the difference if there is one? Alternatively, is there no difference and it is just a different wording for it.

[Eddy71]

The "red soil" is aluminum compounds, and the "black sand" is a reaction product with iron.

[Skippy]

No doubt Carl will have a more in-depth explanation, but ...

The 'red dirt' is rich in Iron ( III ) oxides, Fe₂O₃ , which is basically rust.
This is what's found in 'Tennessee Ultisol' ( devil dirt ).
The black sands are rich in Iron ( II, III ) oxides, typically magnetite, Fe₃O₄ , but there are several other varieties.

[kt315]

Hi Forum,

I have a general question. Sometimes, if people refer to soil conditions, they use "red soil" or "black sand". Both are iron-mineralized soil right? What is actually the difference if there is one? Alternatively, is there no difference and it is just a different wording for it.

Eric HAD DID this topic on the soil. seek it up.

[Carl-NC]

It really depends on where you are at. Generally, "black sand" is what people call plain magnetite. Red soils are red because of iron oxides which can be magnetite but often have a lot of maghemite. However, black soils/rocks can also have maghemite. Laterites (usually red) can also have aluminum but it's still the iron oxides that make it red. But, again, it doesn't have to be red to have iron compounds. In short, both terms are thrown around pretty loosely.

[Melbeta]

Carl is right, but in Geology classes, they taught us, as the marine (ocean salt water) began to leave the land, the land soil dried out, and the air began to oxidize the ferrous minerals, in the ocean deposits, which Carl covered, and thus the minerals began to turn red (rust like) in color. Lots of Hematite turns red from air oxidization of the Hematite in the ocean deposits. In other words, sand color turns from brown to red like (rust) sand deposits.

Now if the soil did not change from original soil color, to red color, it had more other ferrous minerals in it, and went into more of a black soil.

Also a deep dark soil color, is also indicative of deep mud bottom deposits, which retained more of the calcareous deposits, and also dropped out different mineral ferrous deposits into the soil. After the ocean salt water left, the ground dried out, if it was deep black, and turned hard, it was the deeper mud (bottom muck) deposits, and became a hard cap rock, and then was called "pippey concretions", which is found as a hard cap rock on many of the tops of higher land masses, after erosion.

Oceanic deposits of water invasion went from more coarse deposits into finer and finer deeper deposits. As the ocean invaded the land, it reversed and went from more finer deposts into more and more coarse deposits. thus the deposit of coarse to fine/fine to coarse, indicates an invasion of water, or retreat of water from the land, based on size of granules... Just a general description of coarse and fine deposits. It is how the geologists tell if ocean water was invading the land or ocean water was retreating from the land... The deposits in water show an finer deposits deeper into the deeper water, with more larger granule deposits closer to the shore due to ocean waves. In other words, more coarse sand at shore line, with more finer deposits out deeper into the deep water. It has been so long since I studied Geology not sure which size deposits indicate advancing and which sized indicate retreat of the ocean salt water...
Melbeta

[waltr]

There is the oxidation of ferrous from air but there is also Volcanic and Metamorphic rocks that weather in black sands.
Australia's gold fields are notorious for the black volcanic soil.

[GeoMax]

Hi guys,

thanks to all for input to the topic.

GeoMax