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.