Start from a stellar remnant, it is theoretically possible that a planet composed largely of iron and heavier elements could be formed as the result of the destruction of a star. A fragment of the core of a Sol-like star near the end of it's life would be mostly Iron from stellar fusion mixed with the heavy elements that were pulled into the star from the original Molecular Cloud. Such a world would be extremely radioactive and would start out white hot but the surface would eventually cool and solidify.
I would expect that the Crust of such a world would be largely Iron, the way the Earth's crust is largely Basalt with the equivalent of Continental Crust being composed of the lighter Transition Metals like Titanium and Chromium, and some lower density compounds formed by heavier elements. The mantle would indeed have a good percentage of Mercury but would almost certainly contain a lot more Lead than any other single component formed by the decay of many heavier radioactive elements that were originally in the stellar core and will also have a lot of still radioactive isotopes that haven't yet decayed.
I would expect this world to be extremely geologically active, the mantle is going to be much hotter than Earth's and the world as a whole will have far greater heat reserves for far longer. It will be hotter because it's composed of denser minerals with higher heat capacities and because more heat is constantly being pumped in by far more radioactive decay than in the Earth. The world will have more heat reserves because of three mechanisms; higher energy crystallisation reactions, repeated crystallisation and dissolution due to radioactive decay destabilising crystal structures, and, because of the relatively high levels of radioactive isotopes (as a percentage of the whole), more radiogenic heating, including more long half-life isotopes.
The mantle is probably going to have a similar viscosity to that of the Earth, if not being even more fluid due to the high percentage of Lead and other low melting point heavy metals. If those assumptions hold then tectonics will be almost identical to the Earth but with higher temperatures and different "rock" forming minerals/alloys.
Life as we know it, i.e. carbon-based and water soluble, wouldn't stand a chance on such a world, at least not initially. There's too much ionising radiation, too many heavy metals in the crust, and too little of the elements we need to function, Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, and Phosphorus account for 99% of the mass of the Human body (Source) and all of them, with the possible exception of Hydrogen, will be vanishingly rare on this remnant world.
Now it is possible that such a world might be able to accumulate large volumes of lighter elements later in it's evolution through collision with debris from the planetary system, if any, of the star system that centred on the star it was part of. This material would initially coat the world forming a, geologically, short lived "life layer" that could support Carbon-based life for a time before tectonic activity mixed this material into the mantle.
Let me know if you want me to go in depth on any of this stuff, I've gone pretty lightly over a lot of bits.
