  The Al/Si and Mg/Si ratios in non-carbonaceous chondrites are lower than the
solar (i.e., CI-chondritic) values, in sharp contrast to the non-CI
carbonaceous meteorites and the Earth, which are enriched in refractory
elements and have Mg/Si ratios that are solar or larger. We show that the
formation of a first generation of planetesimals during the condensation of
refractory elements implies the subsequent formation of residual condensates
with strongly sub-solar Al/Si and Mg/Si ratios. The mixing of residual
condensates with different amounts of material with solar refractory/Si element
ratios explains the Al/Si and Mg/Si values of non-carbonaceous chondrites. To
match quantitatively the observed ratios, we find that the first-planetesimals
should have accreted when the disk temperature was ~1,330-1,400 K depending on
pressure and assuming a solar C/O ratio of the disk. We discuss how this model
relates to our current understanding of disk evolution, grain dynamics, and
planetesimal formation. We also extend the discussion to moderately volatile
elements (e.g., Na), explaining how it may be possible that the depletion of
these elements in non-carbonaceous chondrites is correlated with the depletion
of refractory elements (e.g., Al). Extending the analysis to Cr, we find
evidence for a higher than solar C/O ratio in the protosolar disk's gas
when/where condensation from a fractionated gas occurred. Finally, we discuss
the possibility that the supra-solar Al/Si and Mg/Si ratios of the Earth are
due to the accretion of ~40% of the mass of our planet from the
first-generation of refractory-rich planetesimals.
