Life requires a variety of chemicals and elements. The question of water, how it was delivered to Earth and how it would be delivered to exoplanets, has taken a central position. However, other chemicals and elements are also important. Earth is significantly (~80%) depleted in elements important for life like sodium and potassium, both much less volatile than water, but much more volatile than silicon, magnesium, or iron. I will show how part of the composition of rocky planets is set early during their formation process, when the solids are still in sub-cm dust grains. Thermal processing of these grains during accretion events will alter both their composition and geometries, promoting the rapid formation of planetesimals with compositions unexpected at their radial location. This can explain the Earth’s puzzling depletion of moderately volatile elements. Relatedly, magnetically mediated interactions between small dust grains can explain why Mercury is extremely iron rich. Both processes should play major roles in exoplanet formation, with consequences for their composition and chemistry.
