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 iron. But 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 cosmological 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 originally accreting - leaving it to form only from heavier elements.
See: Wikipedia
Note that planet formation in general is poorly understood - see : Planet formationplugin-autotooltip__plain plugin-autotooltip_bigPlanet formation
"The origin of planets is a vast, complex, and still quite mysterious subject. Despite decades of space exploration, ground-based observations, and detailed analyses of meteorites and cometary grains (the only space samples available in our laboratories),