Why does sulfur dictate a planets magnetic field? I've been reading up on planet composition and I saw this little gem:

&gt; The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.

Trying to do some Google-Research resulted in this interesting tidbit: 

&gt; depends essentially on the concentration of sulfur in the core, which influences the core's density and as a result its size.

Why specifically does sulfur have this effect?  Why can't other minerals/filler adjust the density?