Another videogame species, this time the gorons from the Legend of Zelda franchise http://zelda.wikia.com/wiki/Goron . They are a silicon based species (though they can grow hair for some reason) that feed on rocks, but for some reason are particular about the rocks they eat (which explains the plot hole in their chapter in Ocarina of Time). Could something like this evolve in nature?
Exotic biology! Let us make these creatures eat minerals because really they are heat eaters.
Let us sidestep the question of silicon based life form. Yes they eat rocks, but that doesn't mean they are rocks. Let us assume a robust, thermally durable carbon based life form. Perhaps the Goron are colonial archaebacteria?
We like to eat partly oxidized carbon like sugar, because that is what is available on the surface. Maximally reduced carbon yields even more energy per mole when oxidized, because there is room for oxygen. Coal would be a fine foodstuff for a subterranean creature. A problem - oxygen might be in short supply.
Here comes some made up biochemistry. Calcium carbonate is limestone and is abundant in the earths crust. Thermal decomposition of calcium carbonate drives off the carbonate as carbon dioxide and leaves calcium oxide: quicklime.
it releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of CaCO3), to form calcium oxide, commonly called quicklime, with reaction enthalpy 178 kJ/mole: CaCO3 (s) → CaO (s) + CO2 (g)
I propose these creatures fill up with carbonates when they find them. Carbonate rocks might be stored in the tissues - calcium carbonates are very biologically compatible which is how limestone accumulated in the first place- millions of years of shells. Full of carbonate minerals, the Goron then retire to depth where the earth is hot. They can catalytically (=enzymatically) facilitate the decomposition of CaCO3, driving off the CO2 and returning to the surface full of CaO.
The formation of CaO is a way of storing the earths heat energy. On the surface, the careful, gradual addition of water to CaO releases energy as heat (slaking) much in the way that in our bodies we carefully add oxygen the sugars. Any exothermic reaction can in theory be harnessed enzymatically to phosophorylate ATP - for us the oxidation of sugar, for the Goron the hydration of lime.
This is why they eat rocks.
1: Pure carbon like coal, because it is easy to oxidize on the surface for energy. Of course for a snack they might also like fats, waxes, and liquid alkanes.
2: Carbonate rocks, because they can thermally decompose them and store the heat energy as CaO, later hydrating it for energy.
This is also why too much water is not a good thing for these creatures. If you slake a large amount of quicklime at once, things get extremely warm.
Silicon based life could exist, silicon and carbon are chemically quite related. The problem is in the food: we cannot consume carbon dioxide, because you cannot metabolize it with energy gain (plants metabolize it, but they put solar energy into it, converting it to sugars, which they then use for energy (and we use those, too)). So the Goron would either use some energy-source (magic, sunlight, pressure,...) to essentially up-convert the silicon dioxide from stones, and then feed on that product.
Well, it is pretty straightforward, if you think about it.
We are carbon based life forms, and we rely on food based on carbon. A silicon based life form is therefor reliant on silicon based food for nutrition.
Rocks are mostly based on silicon, therefore are a suitable food. Based on the different metabolism, each Goron has preference for different silicates, meaning different rocks.
The world of Hyrule has some bizarre and obscure zoogony, so the first step to building an answer is nailing down this issue. For this answer, we'll mix some Empedoclism, Lamarckism, and Darwinism, with some divine intervention on top. This seems like we'd get a good evolutionary approximation of Hyrule's creative forces
We will start at the creation of the planet. When the forms and natures of minerals were being decided, one of the divinities decided that it'd be a wonderful idea for some minerals to be able to spread like plants from the heat of the core. And it was wonderful, at least until it started consuming the rest of the minerals and destroying all their lovely work
They needed a solution, and needed it fast. What they settled on is grabbing a piece of this mineral and animating it with muscles, creating a sort of magma worm. This worm, now capable of motion, quickly settled into an animal role, feeding on the plant-like ancestral mineral. Their geology saved, they could move on to other creative endeavors
Once they had nurtured the intelligent races of Hyrule, the divinities realized that there was no species capable of surviving in the burning heat of the great volcanoes. While they had some ability to retrofit animals for these temperatures, plants were out of the question, and so any ecosystem was doomed to fail. Then, they remembered the deep-dwelling magma-worms they had made long ago, which by now had diversified into countless forms. One of the surface-dwelling species was selected, and transformed into a humanoid mineral
There were still problems with this approach. Specifically, that the minerals would go hard and die when it got too cold. This wouldn't be an issue if not for the fact that for them, 'too cold' was synonymous 'not incandescent'. To solve this issue, the divinities had an idea, to combine these minerals with an animal species to get the best of both worlds: The animal resistance to low temperatures, with the mineral ability to live on stone. They found an armoured terrestrial ape dwelling in the volcano, and replaced its viscera with the geological counterparts
Now, with this union of ape and mineral, the Gorons arise