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I live very near an extinct Andesite volcano and hiking in that area I've noticed bands of soil that have a dark red coloration. I've also been in Kauai, one of the volcanic islands of Hawaii, but those soils appear to be a lighter, almost orange color which I've heard is attributed to iron content.

So for the soil in my local neighborhood, what is the likely constituent that gives it the dark red color? Is there a simple chemical test I can use to confirm it's iron?

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tl;dr

  1. If it's red, it's iron.
  2. If it's soil, it probably has iron even if it's not red.
  3. Therefore there's no point testing it for iron (in a simple way), because you can be confident it's there.

I'd like to add a bit to Jezibelle's answer. First of all, you could do a simple chemical test to test for iron, but that would not give you much. Iron oxides on the surface are extremely insoluble in water. You would either have to use LOTS of soil and water to test it, or use acids that will dissolve it and then it's not a simple test anymore.

Another reason is that assuming you do that test, it is going to be positive for iron. Andesite has iron in it. This and just about every other rock you can find have iron in it. Those that don't, are not red. The colour that you're seeing is enough proof that there's iron in it.

Now the question is which iron oxide is it. Pure hematite ($\ce{Fe2O3}$) is deep dark red. Pure goethite ($\ce{FeO(OH).H2O}$) is yellow. But then you have everything in between, and mixtures (given the general name: limonite). Now remember, iron is not the only constituent of soil. Silicon is the most abundant metal in soils, followed by aluminum, sodium, calcium, magnesium and potassium (not necessarily in that order). These elements can combine with iron to form stuff that doesn't even have to be red. It can be grey, black, green and everything in between.

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Soil redness is typically from hematite (an iron oxide). The mixture of other minerals and organic products and how well-drained the soil is determines the exact shade.

You can buy soil test kits that include iron, or, alternatively, run some distilled water through a soil sample over a cheese cloth or two and use a water test kit on the filtrate (water test kits with iron testing tend to be cheaper than soil ones - although depending on the limit of detection for the kit, you might need to use a lot of water and soil and then boil down the water to get a high enough concentration to test positive).

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