  The bulk chemical compositions of planets are uncertain, even for major
elements such as Mg and Si. This is due to the fact that the samples available
for study all originate from relatively shallow depths. Comparison of the
stable isotope compositions of planets and meteorites can help overcome this
limitation. Specifically, the non-chondritic Si isotope composition of the
Earth's mantle was interpreted to reflect the presence of Si in the core, which
can also explain its low density relative to pure Fe-Ni alloy. However, we have
found that angrite meteorites display a heavy Si isotope composition similar to
the lunar and terrestrial mantles. Because core formation in the angrite
parent-body (APB) occurred under oxidizing conditions at relatively low
pressure and temperature, significant incorporation of Si in the core is ruled
out as an explanation for this heavy Si isotope signature. Instead, we show
that equilibrium isotopic fractionation between gaseous SiO and solid
forsterite at 1370 K in the solar nebula could have produced the observed Si
isotope variations. Nebular fractionation of forsterite should be accompanied
by correlated variations between the Si isotopic composition and Mg/Si ratio
following a slope of 1, which is observed in meteorites. Consideration of this
nebular process leads to a revised Si concentration in the Earth's core of 3.6
(+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary
bodies.
