  Differential settling and growth of dust grains impact the structure of the
radiative envelopes of gaseous planets during formation. Sufficiently rapid
dust growth can result in envelopes with substantially reduced opacities for
radiation transport, thereby facilitating planet formation. We revisit the
problem and establish that dust settling and grain growth also lead to outer
planetary envelopes that are prone to compositional instabilities, by virtue of
their inverted mean-molecular weight gradients. Under a variety of conditions,
we find that the radiative envelopes of forming planets experience
compositional turbulence driven by a semi-transparent version of the
thermohaline instability ('fingering convection'). The compositional turbulence
seems efficient at mixing dust in the radiative envelopes of planets forming at
super-AU distances (say $5$ AU) from a Sun-like star, but not so at sub-AU
distances (say $0.2$ AU). We also address the possibility of compositional
layering in this context. Distinct turbulent regimes for planetary envelopes
growing at sub-AU vs. super-AU distances could leave an imprint on the final
planets formed.
