fission, fusion; confusion?
There seems to be some confusion of terms here, and I may have contributed to it.
Fusion: like what happens in the core of the Sun, an H-bomb, etc. Main fusion reactions are, in summary, conversion of protons (and deuterons) to
4He and
3He. This only happens in a sustained way (power out > power in, in bulk, over more than ~1s) in gravitationally confined objects ('stars'), tho' many believe it will be possible in something like ITER. On Earth, the only natural fusion which takes place is (maybe) in cosmic ray collisions with atoms in the atmosphere or oceans.
Fission: the splitting of a heavy nucleus into two (or more?) lighter nuclei (alpha decay excepted). We usually also mean 'with a net production of kinetic energy'. This happens in A-bombs, in nuclear reactors (both man-made and natural*), and in some isotopes ('spontaneous fission').
Spontaneous fission, along with other forms of radioactive decay, generates heat if it occurs in atoms within rocks (and elsewhere). Radioactive decay (including spontaneous fission) is the primary source of the heat which results in the Earth's core being much hotter than its crust; the primary nuclides are
40K,
238U and
232Th.
It doesn't matter in what physical form a radionuclide is - pure element, chemically-bound, alloyed; concentrated (e.g. crystalline salt), impurity - it generates the same amount of heat when it decays.
*in the early days of the Earth,
235U was a more common isotope than it is today. There were some deposits of uranium - in the form of oxides? - in which sustained fission reactions took place, very similar to man-made reactors. Oklo is the best-known example:
http://antwrp.gsfc.nasa.gov/apod/ap021016.html
Such natural reactors are highly unlikely on, or in, the Earth today.
Finally, iron (and nearby elements in the periodic table) cannot undergo exothermic fission or fusion; they are the most stable nuclei.
Andre, that link is just sooo incredible