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The ability of Earths mantle to conduct heat by radiation is determined byoptical properties of mantle phases. Optical properties of mantle minerals athigh pressure are accessible through diamond anvil cell experiments, butbecause of the extensive thermal radiation at T above 1000 K such studies arelimited to lower temperatures. Particularly uncertain is thetemperature-dependence of optical properties of lower mantle minerals acrossthe spin transition as the spin state itself is a strong function oftemperature. Here we use laser-heated DACs combined with a pulsed brightsupercontinuum laser probe and a synchronized time-gated detector to examineoptical properties of high and low spin ferrous iron at 45-73 GPa and to 1600 Kin FeO6, one of the most abundant building blocks in the mantle. Siderite(FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron andexhibits a sharp optically apparent spin transition at 44 GPa, simplifying datainterpretation. We find that the optical absorbance of low spin FeO6 issubstantially increased at 1000-1200 K due to the partially lifted Laporteselection rule. The temperature-induced low to high spin transition, however,results in a dramatic drop in absorbance of the FeO6-unit. The absorption edge(Fe-O charge transfer) red-shifts (~ 1 cm-1/K) with increasing temperature andat T above 1600 K becomes the dominant absorption mechanism in the visiblerange, suppressing radiative conductivity. This implies that the radiativeconductivity of analogous FeO6-bearing minerals such as ferropericlase, thesecond most abundant mineral in the Earths lower mantle, is substantiallyreduced approaching the core-mantle boundary conditions. Finally, our resultsemphasize that optical properties of mantle minerals probed at room temperatureare insufficient to model radiative thermal conductivity of planetaryinteriors.

Reduced radiative conductivity of high and low spin FeO6 octahedra in the Earth's lower mantle