The planet Mercury is the second most dense in the Solar System. Its density is currently calculated at 5.427 g/cm3 ( Earth is 5.514 g/cm3 )
Based on this, the assumption is that Mercury, like Earth, probably has a core composed mainly of (molten) iron.
Cosmologists are in disagreement as to how this came about. There are three main theories :
1) That, like Earth, it originally had a rocky outer mantle with an inner iron core, but the outer mantle was destroyed (removed) in a collision with another very large astronomical object.
2) As above, the planet was originally Earth-like, but the outer mantle was evaporated away by intense heat during its formation near the Sun.
3) The Sun's 'solar nebula' caused gravitational drag on the lighter particles from which Mercury was accreting - leaving it to form only from heavier elements.
Also see :
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