Peer review of: Low thermal conductivity of iron-silicon alloys at Earth’s core conditions with implications for the (2020)



RR2 
 REVIEWERS' COMMENTS:
Reviewer #1 (Remarks to the Author):
  The study of thermal conductivity of iron alloy is an important topic for the thermal evolution of the
 planet and the generation of a magnetic field. The authors’ experiment point to much lower thermal
 conductivity for relevant alloys, compared with previous theoretical calculations or experiments based
 on measurement of electrical resistance.
 The authors have done a good job responding to the reviewers comments. The experiments are
 carefully done and well explained. This will certainly help other researchers attempt to reproduce
 these results and consider other possible alloys. Some of the specific predictions likely depend on the
 choice of alloy elements, but the authors choice is reasonable and the conclusions they draw appear to
 be appropriate for their specific choice.
 I think the paper can published in its current form. It will be of wide interest and generate consider
 (constructive) debate.
 Reviewer #2 (Remarks to the Author):
Eiji Ohtani
Reviewer #3 (Remarks to the Author):
 This manuscript provides the new data on thermal conductivity of Fe-Si alloys at high pressure and
 temperature, and showed the thermal conductivity of the alloy is very low compared to pure hcp-iron.
 I have checked carefully the revised manuscript. The authors addressed the reviewers’ comments and
 the manuscript was improved significantly. This low thermal conductivity of the Fe-Si core provides
 very important implications for older formation of the inner core, energy source of geodynamo, early
 initiation of the Earth’s magnetic field, and small heat flow at CMB. The results and implications are
 very significant, therefore this manuscript should be published in this journal.
   This is the second review of this paper. Hence, I do not think that any preface is necessary in this
 review.
 Although the authors claimed that the thermal conductivity difference between Fe0.85Si0.15 and pure
 Fe is well beyond the measurement errors, I disagree to their idea. The values obtained by the
 present method have actual uncertainties larger than a factor 2. This fact is suggested by the data
 points at pressures between 100 and 110 GPa, which range from 28 to 60 W m-1 K-1. If the error
 bars are taken into account, the actual uncertainty is a factor 4 (20 ~80 W m-1 K-1). Although low
 conductivity values (16-30 W m-1 K-1) were obtained at pressures above 120 GPa, these data should
 have the same level of errors. This should be also the case with Konopkova et al.’s study. A
 reasonable conclusion of the present study is therefore “the conductivity of Fe0.85Si0.15 is
 indistinguishable from that of pure Fe within the errors”.
 Although I appreciate the authors’ tremendous efforts for this pioneering study, the robustness should
 not be exaggerated. If the decrease in conductivity above 100 GPa of Fe0.85Si0.15 given in this study
 were the case, the conductivity of pure Fe given by Konopkova et al. should suggest that the
 conductivity decreases by a factor of 2 with increasing pressure from 40 to 70 GPa, and suddenly
 increase by a factor of 2.5 from 70 to 90 GPa, and then remain constant at higher pressures. These

 arguments based on Konopkova et al.’ data are clearly too much, and therefore the authors conclusion
 regarding the difference between pure Fe and Fe0.85Si0.15 is also too much.
 I am afraid that high-pressure experimentalists tend to exaggerate the robustness of their studies. If a
 data set with the same quality is obtained by other methods than high-pressure experiments, no one
 would say that it is robust.
 I think that the present data set is worthwhile to publish in some journal. However, any geophysical
 argument is too early. The present study should be published at some journal of mineralogy such as
 American Mineralogist and Physics and Chemistry of Minerals. The authors should argue the thermal
 history of the Earth’s core after being able to obtain that are more robust.
 For these reasons, I do not recommend this paper for publication at Nature Communications.