[vmelkon]

Hi,

Companies don't extract compounds from silicate rocks because they are rather nonreactive, right?
Such as Potassium feldspar which can be found in some granite rocks. http://en.wikipedia.org/wiki/Potassium_feldspar

I'm not an expert on rocks and specially that "silicate" anion which seems to have all sorts of forms (example : Potassium feldspar = KAlSi3O8) but acids such as H2SO4 and HCl and HNO3 won't react with most silicate rocks, right?

Would HF react with it? Anything else?

I imagine that companies prefer oxides/hydroxide/carbonate (such as iron oxides) and chlorides/halides (NaCl for getting Na and Cl2) for extracting elements or preparing other compounds.

[Eudoxus]

Well it's certainly possible to extract materials from silicate rocks, it just isn't cost effective. For instance, why go through the horribly complex process of breaking down, for instance, the silicate mineral mica for its magnesium, when you can simply take the otherwise near-worthless stone dolomite ([Mg,Ca]CO₃) and dissolve away the carbonate for it? Or why try extracting iron from silicate minerals when there's so much hematite, magnetite, limonite, ilmenite, chromite, goethite, and other iron-bearing ores aplenty that require one tenth the effort to process?

One day if Earth's mineral resources become depleted enough, we may find ourselves needing to extract metals from bodies with mere trace amounts of metal and nothing you could call ore--but we're not there yet, and won't be for another century or two.

[vmelkon]

Thank you for the reply. It must be a cost issue otherwise they would have done it by now.

Am I right in assuming that HF is one possible way to dissolve all silicate materials. Is there another acid besides HF?
I would love to give it a try but I don't have HF and I hear that it is very toxic.

[Eudoxus]

Well, I'm mainly a geologist, not a chemist, so I'm not sure. We mainly dissolve things from around silicate bodies. A lot of the time you'll get silicious fossils preserved in limestone, so you dissolve the limestone away with HCl and you get perfect, undamaged, unweathered fossils. Heck, in that piece of limestone I'm turning into calcium acetate, I've recently spotted what may be a partial, silica-replaced crinoid preserved in it. Can't be sure until the limestone finishes dissolving, but that would be a cool bonus.

HF is indeed very toxic and very dangerous and can kill you dead. I don't know how much chemistry experience you have, but unless you're a very advanced home chemist with a fume hood and lots of experience dealing with noxious stuff, I'd stay away from it.

[SirRoderick]

HF is nasty stuff indeed.
I have also never heard of it being used in that fashion.

[Eudoxus]

I did some reading and apparently HF is used for dissolving silicate rocks to observe carbonaceous fossils, much as HCl is used for the reverse. But no one I know has done it before, most fossils in silicate rocks are silicate as well so it's rarely used, at least in comparison to the HCl method.

[zaphraud]

What is it you want out of there? If its the aluminum, then yeah companies prefer the hydroxide. They don't use HF to extract it though, rather, NaF is the flux to make a mixture that melts low enough for economic electrolysis of the metallic aluminum to occur.

[vmelkon]

I'm not sure. I guess I am thinking of most of the rocks out there under the streets, in the park, in our backyard.
They probably have a lot of Al, Fe, Ti, Ca, K, etc. Probably Al and Fe are the biggest need of society.

I guess the ordinary citizen chemist hasn't tried to extract anything from ordinary rocks.

[zaphraud]

I guess the ordinary citizen chemist hasn't tried to extract anything from ordinary rocks.

I'm in Arizona so I have a lot of rocks to play with. I've found that aspirin is really good at removing some sort of pink solute from rocks, that isn't removed much at all by water, 70%IPA, conc. vinegar (25%), ammonia, or boiling salt water. Not sure exactly what it is, however. Just that its really pink, but not quite pepto-pink so I don't really think its bismuth, or if it is its not anywhere near pure.

I've also played with melting a variety of them in a microwave furnace - some are good susceptors (heat up on their own), some are not (need to be helped out by another item in the oven to heat up). Tables of what absorbs how much seem to be scarce to non-existent, little wonder considering the same data could be used to predict the ability of a variety of things (i.e. types of fiberglass composite) to absorb radar. Sigh.. the microwave is such a powerful but oppressed scientific tool.

The main amateur motivator for extracting things from rocks seems to be to beautify the rock itself, actually. There is a lot of solid-state chemistry going on in the world of gemology.

[Eudoxus]

Many of the rocks that heat up easily in a microwave are probably hydrated to some extent. That might actually be a useful technique for determining how hydrated a rock is; I'll do some research into that.

[vmelkon]

If a rock is hydrated, what would that mean? It would be some silicate (SiO4?). Would it be a hydroxide?

[Eudoxus]

A hydrated rock is a mixture containing water. The chemical formula looks like XXXXX · nH₂O, n being the number that determines just how hydrated the mineral is, from barely at all to being practically a solution. So hydrated silica (aka Opal) has the formula SiO₂ · nH₂O.

A hydroxide would be different. For instance, the iron ore Goethite is an oxyhydroxide with the formula FeO(OH). Or just your typical lye, NaOH.

[zaphraud]

Many of the rocks that heat up easily in a microwave are probably hydrated to some extent. That might actually be a useful technique for determining how hydrated a rock is; I'll do some research into that.

Definitely, although hydrated rocks generally don't make it to glowing red without dehydrating first at 1atm. They tend to start heating really slow somewhere between 100-250c, as steam is lost. Or, sometimes you get one that makes it to a bit hotter, then cracks open, sometimes with quite a bit of force; if the goal isn't to split it open, it pays to make sure the rock is very nearly close to dehydrated before intense heating, you can best do this in a microwave by using a fractional power setting - what this does in real life is not to lower the power but instead to switch the microwave on and off. The result is a pattern of slight expansion and contraction that will help any generated vapor pressure to exit the rock, if it is at all possible in the first place.

[zaphraud]

A hydrated rock is a mixture containing water. The chemical formula looks like XXXXX · nH₂O, n being the number that determines just how hydrated the mineral is, from barely at all to being practically a solution. So hydrated silica (aka Opal) has the formula SiO₂ · nH₂O.

A hydroxide would be different. For instance, the iron ore Goethite is an oxyhydroxide with the formula FeO(OH). Or just your typical lye, NaOH.

At some much hotter point, hydroxides (and carbonates) will dehydrate (and decarbonate) to simple oxides; these are often caustic even at room temperature, and many will suck water or CO2 back out of the air.

It is possible to make some really pretty looking things out of rocks that will end up as mere crumbly/dusty/fuzzy things only a few short weeks later when this is the case, and the crumbly dust can be quite an irritant.