  Earth/s lowermost mantle displays complex geological structures that likely
result from heterogeneous thermal and electromagnetic interaction with the
core. Geophysical models of the core-mantle boundary (CMB) region rely on the
thermal and electrical conductivities of appropriate geomaterials which,
however, have never been probed at representative pressure and temperature
(P-T) conditions. Here we report on the opacity of single crystalline
bridgmanite and ferropericlase, which is linked to both their radiative and
electrical conductivity, measured in dynamically- and statically-heated diamond
anvil cells as well as computed from first-principles at CMB conditions. Our
results show that light absorption in the visible spectral range is enhanced
upon heating in both minerals but the rate of change in opacity with
temperature is a factor of six higher in ferropericlase. As a result,
bridgmanite in the lowermost mantle is moderately transparent while
ferropericlase is highly opaque. Our measurements suggest a very low (< 1
W/m/K) and largely temperature-independent radiative conductivity in the
lowermost mantle, at odds with previous studies. This implies that the
radiative mechanism has not contributed significantly to cooling the Earth/s
core throughout the geologic time and points to a present-day CMB heat flow of
9-11 TW. Opaque ferropericlase is electrically conducting and mediates strong
core-mantle electromagnetic coupling, explaining the intradecadal oscillations
in the length of day, low secular geomagnetic variations in Central Pacific,
and the preferred paths of geomagnetic pole reversals.
