The rarest chemical element naturally ocurring in the Earth's crust is...
rhenium
iridium
osmium
astatine
Astatine is a radioactive chemical element with symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. The total amount of astatine in the Earth's crust is estimated to be less than one gram at any given time.
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