Believers in the "chemtrail" theory (the theory that the trails behind high-flying aircraft secretly contain chemicals intended to alter the climate or weather) often point to chemical analyses done on soil, air or water. Typically these tests are done for three metals: Aluminum, Barium, and Strontium (the fallacious "geoengineering signature"). Since these elements are all naturally found in the Earth's crust, they usually find some of each. Experts in geochemistry recently analyzed a typical set of these results, and determined they did not come from a secret spraying program.
However a very common misunderstanding here comes from the fact that these metals are not found in their metallic form in nature. Aluminum needs to be extracted from rock, and once in metallic form it oxidizes (rusts) over the years, and barium and strontium both are highly reactive to air. Typically the chemtrail believer will quote something like Wikipedia:
So the argument goes: if Aluminum (or Barium, etc) is never found in nature, then why are the chemtrail related tests finding it?
The answer is that the tests used for these metals do not distinguish between the A) metal and B) a mineral that contains the metal.
So it does not matter if you test an aluminum soda can, or a piece of rock made of aluminosilicates, or windblown dust that contains tens of different minerals that contain aluminum, the results will all return the presence of significant amounts of aluminum.
The most commonly used test for metals in water is EPA 6010B, a standard test for metals:
The important word there is plasma. Plasma is the fourth state of matter after solid, liquid, and gas. If you take some rock or aluminum and heat it up it will eventually melt. Heat it up more and it will turn to liquid, continue heating and it will become a gas. Finally if you heat it up incredibly high it will become a plasma - a cloud of individual atoms stripped of their electrons. As described in Wikipedia:
Since the sample is just broken down into a soup of atoms, there's no way of knowing (from this type of test) if the individual atoms of aluminum (or other metals) were in their metallic form, or in their mineral (rock) form.
EPA6010B is the most common test used. Another test is EPA 200.8, which is also plasma spectrometry, so the same applies there. Sometimes the test is just described as ICP/MS or ICPMS (Inductively Coupled Plasma/Mass Spectrometry), which is, of course, the same thing.
So when you test samples of dirt, or air or water that has a little dust in it, then you are going to find the elements that are typically found in dirt. You are not finding the metallic form. You are finding minerals that are made from those metals, like feldspar, granite, bauxite, or aluminosilicates. It's just regular dirt.