Subject: Why is planet Mercury's core considered to be iron?

Mercury is very massive considering its small volume, meaning that it is 
very dense.
I can understand how scientist hypothesize that an iron core would have to 
be very large for mercury (75% of the planet's total size).  I think that 
maybe that is incorrect.
How large is Earth's core in relation to Mercury's, larger or smaller?
If Mercury's core were that large wouldn't it be volcanicly active as a 
result of energy being relesed by all that Iron squeezing on it's own 
center of gravity?
Why can't Mercury's core be composed of heavier elements like Platinum, 
Gold, or even its namesake element, Mercury?
From what I understand of planetary formation these heavy elements 
wouldn't have been pushed outward so far by the sun's ignition flare, like 
hydrogen and helium(major constituants of the gas giant planets), iron was 
pushed out to Venus's current orbit to the asteroid belt and three 
planets, Earth, Venus, and Mars formed from it. 
Am I correct about this or way off?

Thanks for tolerating my proposed explainations...
-Matt

Re: Why is planet Mercury's core considered to be iron?