Deformation and Transformation Textures in the NaMgF₃ Neighborite—Post-Perovskite System

Round 1

Reviewer 1 Report

This is a well written manuscript that presents useful experimental data for deformation and transformation textures in the analog of perovskite to post perovskite system. The manuscript adequately explains the experimental and analytical methods used in this study. The introduction and results are clearly laid out except some typos, and in a logical sequence. The discussion well covers the results, but it seems that some sentences are required to be more clearly. The conclusions are supported by the results and seems reasonable. I suggest publishing this manuscript with some modifications and clarifications detailed as follows: 

1. In this study, the texture strength was reported by comparing multiple of a random distribution (m.r.d.) value in the inverse pole figure map. However, to compare the texture strength, the misorientation index (M-index by Skemer et al., 2005) and/or the texture index (by Bunge, 1982, or generally used as J-index by Mainprice and Silver, 1993) is more appropriate than m.r.d. value. I suggest the calculation M-index and/or J-index to comparing the development of texture by increasing compressional pressure (you can easily calculate these values by MTEX toolbox).

 2. In this study, during compression, the differential stress in pPv increase more rapidly than in pPv in run #1. However, authors mentioned this trend in opposite way: diff. stress in Pv increase more rapidly than in pPv. Please check this trend, and modified the section 3.1 and 4.1.

 3. In this study, authors used the term as 100 texture and 001 texture from the IPF map, but I think that it's acceptable to represent the main axes such as 100, 010, and 001 as the [100], [010], and [001], or (100), (010), and (001) because their crystal symmetries are orthorhombic. Instead, using the 100 texture and 001 texture appears more unusual. I suggested changing these terms to (100) texture and (001) texture, or something similar.

 4. As I known, the IPF show the reference axis/pole at specific position regardless of the axis length. So please check the position where the 45 degrees between the (100) and (010) is whether (130) pole or (110) pole/axis in the orthorhombic crystal symmetry.

 In addition, there are several minor comments in the PDF file. Please check the annotations for mismatched data, references, and typos and comments. Below are not all minor comments, so please check the PDF file.

In Lines 279-280,

Which mechanism? dislocation creep?

 For reference 28,

Please check the reference. I think the reference was cited incorrectly (not GRL paper, but EPSL paper is proper maybe).

 In Lines 283-284,

- Please check the first orientation relationship: maybe [110]pv//[010]ppv is correct.

- Please check weather [001]pv or [100]pv. I don't know which one is correct, but you mentioned that [100]pv//[1-10]ppv in section 4.3 and Fig.6.

- Please check whether [100]pv or [010]pv. I think [010]pv is correct.

 In line 296,

Perhaps the latter is (010)<001> following the Table2?

 In line 331,

Does this mean that actual differential stress was applied in the extensional direction (since the differential stress is negative in Table 1)?

 In lines 338-339,

I think this seems to be written in the opposite way. In the neighborite phase, diff. stress increased by 1.79, 1.52, and 2.02 GPa as an absolute value with increasing compressional pressure. However, in the pPv phase, it increased by 2.76, 1.86, and 3.49 GPa with increasing compressional pressure.

 In Fig.3A,

Is there any reason that you didn't plot the stress in Pv in run #2? In addition, the stress in the pPv at 28GPa in run #1 is only positive in the Table1, but in Fig 3A is likely to show the same quantity (negative). Is it correct? or you just show the absolute value of the differential stress?

 In lines 379-380,

Is it true? As I known, the IPF show the reference axis/pole at specific position regardless of the axis length. If the 45 degrees between the (100) and (010), this shows the (110) axis/pole in the orthorhombic crystal symmetry. Please check this.

 In Fig.4,

- Where is the reference frame? Dose the IPFs in Fig4 indicate relative to the compression direction?

- Is there any reason the maximum m.r.d. value is fixed at (maybe) 2.3? To show the weakening trends of m.r.d. with increasing compressional pressure, I think that it is better to set the maximum value at 5.0 m.r.d. or a value similar to the strongest m.r.d. in these IPFs.

- I think that it's acceptable to represent the main axes such as 100, 010, and 001 as the [100], [010], and [001], or (100), (010), and (001) because their crystal symmetries are orthorhombic.

 In line 423,

As I mentioned above, it seems that the diff. stress in pPv increase more rapidly than in Pv.

 In line 473-475,

As I mentioned above, please check the position is whether (130) pole or (110) pole. As I known, the IPF show the reference axis/pole at specific position regardless of the axis length.

 In Fig.6,

Please add the reference frame in the caption Dose the IPFs in Fig6 indicate relative to the compression direction?

 In lines 602-604,

I poorly understand this sentence. What means the Pv>pPv? In addition, do you mean that if pPv is transformed from Pv, which primarily has the (100) plane oriented at high angles to the compression direction, the initiation of deformation in pPv requires the (100) slip system? Please describe more detail.

 In lines 622-623,

Dose this part deal with the (001)pv? As I understand, these references suggested the (001)pv can contribute to the V_SV>V_SH at the CMB, while (001)pPv can contribute to the V_SH>V_SV at the CMB. So, please confirm this more clearly.

 In lines 671-672,

Dose this part also deal with V_SH>V_SV region? As I understand, the reference 112 suggested the (010)pPv or (100)pPv can contribute to the V_SV>V_SH regions such as the below the central Pacific and South Africa. On the other hands, the reference 113 suggested the (010)pPv can contribute to the regions of palaeosubduction, but these regions somewhat mixed area and hard to tell. So, please confirm this more clearly.

 In lines 699-702,

This sentence is somewhat confusing. Maybe this is your intent: Transformation mechanisms may be different from those of the NaNiF3, NaCoF3, or MgGeO3 systems; nevertheless they remain coherent and preserve the c-axis between the Pv and pPv structures.

 Lastly, I checked the reference list and some missing points. Please check these references.

Missing page number (or paper number): References 2, 9, 15, 17, 39, 41, 65, 66, 67, 72, 74, 77, 94, 112

Wrong format of journal title: References 19, 26, 90

Wrong format of published year: References 56, 70

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Ledoux et al. combine new experimental observations with DFT calculations of elastic properties and modeling of plastic flow of NaMgF3, an analog for MgSiO3 in Earth’s lower mantle. The deformation of Pv + pPv mixtures is important to Earth’s D” region, and this work contributes needed experimental constraints on this type of phase assemblage on an analog at accessible condtions. A strength of the work is how well the DFT and EVPSC simulations complement the experiments. Agreement between experiment and theory for lattice parameters and ambient moduli is excellent. The agreement in differential stresses obtained for coexisting pv and ppv phases based on calculated elastic moduli and experimental strain observations is also good, supporting the elasticity measurements. This work reinforces the importance of differences between the CPO developed through crystallization from the Pv to pPv structure and through deformation along slip systems for comparing results of experiments to observations of seismic anisotropy. It contributes substantial discussion and review of the current consensus on deformation mechanisms of MgSiO3 pPv and analogs. However, the goal of the experiments on NaMgF3 is a bit vague: it’s not clear how experiments on NaMgF3 can resolve the discrepancies between results from the past decades of experiments on other post-perovskite analogs reviewed in the introduction. The discussion of mantle textures and texture inheritance is important but not novel. I recommend trimming and revising with attention to clarity and focus on the scope of this study.

A couple of holes remain in our understanding of the results due to the limitations of the two experimental runs. We know that both experiments produced a 2-phase mixture of Pv and pPv. The 2-phase mixture is likely relevant to regions of Earth’s deep mantle. But we don’t know what the microstructure of the 2-phase mixture is, or whether we would also see the same results with pure pPv. As the authors note, we also don’t know whether heating path affects the crystallization mechanism of the pPv (line 532). The finding about the relative strength of Pv vs. pPv structured NaMgF3 is not supported by the strain data, which are reported with unrealistically small uncertainty.

Overall, this is novel data worth publishing and supported by complementary simulations. The work could be strengthened by adding an experiment with complete transformation to pPv, or with information about microstructure of the recovered sample, but both would be challenging to obtain, and there has been little similar published work on this material. Revision to focus the text and revise illegible Fig 3 is needed.

Abstract: conclude with implications of observations: confirmation that NaMgF3 deformation mechanisms matches MgSiO3 and other analogues except CaIrO3, and any implications of this for D” and future studies. Line 17-18 “as previously observed” – clarify whether this matches previous observations of deformation of NaMgF3 or MgSiO3 or both.

Introduction: Please clarify D” region and lowermost mantle terminology. The text refers to D” region above the CMB, and it would be helpful to define this as reaching from the CMB to the D” discontinuity. This would allow explanation that the D” discontinuity is the observation that has been suggested to vary in height (topography) above the CMB. The D” region is not necessarily a “layer,” since in some regions no D” discontinuity is observed at all. In pg 2 line 63, it’s not just the global presence of post-perovskite that’s debated, but also the global continuity of a D” region or layer. If Brg is stabilized over pPv (line 64), that region isn’t in D” (unless you mean to define D” based on the 1-D PREM profile, in which case the region doesn’t have topography). LLSVPs overlap with but extend far above the D” region. D” layer should also not be confused with the thermal boundary layer at the base of the mantle. Although the thermal boundary layer is also mostly in the same place where D” is, since the thermal boundary layer is defined by geotherm and the D” is defined by a seismic discontinuity, they’re not the same thing either.

The key point of pg 2 line 73 – page 3 line 163 in particular can be much more concise to more simply summarize the slip systems best supported by the literature for MgSiO3 and the case that NaMgF3 is a promising analog for MgSiO3. Lines 140-141 are a key point too. Although >70 references have been cited in the introduction (surely these can be reduced), the one previous published study of deformation of NaMgF3 (ref 99) is an odd omission from the introduction.

Fig 1B: splitting in 023-130 peak perpendicular to compression is not replicated in the fit. Any explanation for this?

Fig 2: DFT results for pressure dependence of lattice parameters and elastic moduli are reported as continuous curves from 0-80 GPa. Were physical properties refined at discrete pressures and fit to curves, extrapolated using an equation of state, or something else?

Another experimental study that would be appropriate to compare to DFT results is Li and Weidner PEPI 2012. There is also a previous published study by Arar et al. (2015) in a materials science journal of high pressure elasticity of NaMgF3.

Table 1: why is t presented in the table as negative, but in the text described using positive numbers? Differential stresses can’t really be constrained with 0.01 GPa precision. The stated uncertainty seems unlikely, especially since pressure is reported to 1 GPa precision and few-GPa differences in pressure are observed between the Pt calibrant and the sample. How do we reconcile this difference in reported precision?

Fig 3: labels are too small to see on the legend. Due to poor resolution, I can’t even read them zoomed in. Comparison between A and B may be easier if the axes are scaled to match.

Section 4.1: it seems unlikely that the 0.4 GPa difference in differential stress between Pv and pPv, 3.99 vs 3.57 GPa (line 425), or possibly even the 1 GPa difference (line 434), is significant based on likely uncertainty in measurements of differential stress by this method. As noted above, I don’t think differential stress is known to 3 significant digits. Differential stress in Pv and pPv is nearly the same in both runs. It’s likely that the softer pPv is present in sufficient amounts in both runs that it takes up the strain and limits the deviatoric stress transmitted to the Pv. The authors may want to elaborate that “microstructure” (line 428) includes this possibility.

More discussion of the microstructure could also be important to interpretation of Fig 4. Fig 4 shows that despite the big differences in the relative abundances of Pv and pPv in the 2-phase mixtures in the 2 runs, the observed texture in pPv is always the same. The presence of 80% vs 30% Pv does not appear change the deformation mechanism of pPv. However, there’s still a significant amount of incompletely transformed Pv in the 2^nd run. If Pv is interconnected, the results may be different relative to pure pPv. However, if Pv is not interconnected, pPv may be effectively a pressure-transmitting medium, and the differential stress differences between the two phases are due to uncertainty in differential stress measurement.

Section 4.3: lines 537-539 note that transformation texture of MgSiO3 from pv to pPv has not been measured by experiments, only by the two computational studies cited. This is important context needed to motivate all of the preceding discussion of analog studies, and really should be emphasized to support the importance of the experimental finding in this work.

Line 632: isn’t the reason why texture inheritance isn’t observed for CaIrO3 simply that CaIrO3 experiments start with the post-perovskite structure rather than perovskite?

Line 514 please define “variant selection”

Typo line 623 mantle

Typo line 692 where

Typo line 694 slight

Author Response

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Reviewer 3 Report

This is the excellent and well-written manuscript on the pressure-induced texture evolution of neighborite NaMgF3 system. It can be accepted subjet to some minor revisions as outlined below.

Line 54

Please note that the IMA accepted mineral symbol for bridgmanite is Bdm, not Brg, for perovskite - Prv

Refer to: Warr LN. IMA–CNMNC approved mineral symbols. Mineralogical Magazine. 2021;85(3):291-320. doi:10.1180/mgm.2021.43

Lines 90-91

Since you refer to Burgers vectors, they should be written in bold letters

Line 160

Pbnm In the space-group symbols, letters should be in italics

Table 1

The letters for unit-cell parameters should be in italics

Author Response

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Reviewer 4 Report

General: This paper is well written, informative, and addresses an important topic. The authors present a thorough analysis of carefully conducted and well documented experiments. This paper should be published after a minor revision. I have some comments and questions which should be addressed in the revision: The authors write 'The phase transition at the D” discontinuity may hence introduce a trans- 60 formation texture in the newly-nucleated pPv. This process needs to be understood in 61 order to correctly interpret seismic anisotropy in the D” region. ' Comment: This is a very good point. I have two basic questions: 1) Texture may evolve over time - slow creep processes may modify patterns that formed initially upon the transition 2) In the diamond cell experiment crystallites are probably much smaller than in the mantle and diffusion creep at grain boundaries is expected a much stronger contribution than in mantle rock - say of the grainsize of garnet peridotites. Table 1: The uncertainties of the unit cell parameters are unrealistic. Not even single crystal diffraction on an unstrained specimen would give that low uncertainties. Besides, temperature is probably not that stable to allow for such accurate cell parameter determination. Is there some way of assessing the uncertainties of the average cell parameters independent from MAUD? What does a LeBail or Pawley fit with GSAS, Fullprof etc. give? Is the MAUD refinement uncertainty influential on the texture analysis? Table 1: Crystallite size: Wrong dimension or unclear. Do you report a volume or a radius? stress t of 4-6 GPa in an ionic salt compressed at 300 K to 30-70 GPa is a very plausible number for a diamond celll experiment. However, in the mantle stress gradients are orders smaller than 4-6 GPa over 100 micron (or whatever the diamond cell chamber width has been). Is there a possibility to test if the assessed slip systems are still dominant at lower stress levels? I’m not expecting a definite answer within the frame of this study but a comment on this issue is appreciated. Abstract: indicating that {100}Pv transforms to ~{130} pPv. not quite clear - rephase: indicating that the {100} orientation axis of pv is mapped onto the {130} axis of ppv - or a similar phrase. line 21/22: with some contribution of glide on (100)[010] and (001)<110>. add: in ppv MgSiO3 bridgmanite (Brg), t Please use the abbreviations recommended by the IMA: Bdm. (see L. Warr Mineralogical Magazine (2021), 85, 291–320 doi:10.1180/mgm.2021.43 ) One may debate which abbreviation is better but it is certainly best to establish a single one and that should be the one recommended by the IMA. line 160: At ambient conditions NaMgF3 neighborite is isostructural with bridgmanite (Pbnm per- ovskite) change to: is isotypic with… line 178: Mao-Bell type cell with large openings for radial diffraction. That is: with large openings perpendicular to the beam direction. . Transfor- 699 mation mechanisms may be different than for the NaNiF3, NaCoF3 or MgGeO3 systems, 700 but nevertheless is coherent and preserves the c-axis in between the Pv and pPv structures. change to ‘. Transformation mechanisms may be different for NaMgF3 than for the NaNiF3-, NaCoF3-, or MgGeO3-systems but nevertheless ARE coherent and preserve s the c-axis in between the Pv and pPv structures.’ or similar.

Author Response

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Reviewer 5 Report

see the attachment

Comments for author File: Comments.pdf

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Round 2

Reviewer 1 Report

The authors have thoroughly revised this manuscript and have effectively addressed most of the comments.

I believe that this manuscript can be acceptable after minor corrections. Please check the annotations for mismatched figure numbers and typos in the PDF file.

In line 268, 

figure 2 → figure 4

In line 295, 

[010]Prv // [110]pPrv → [110]Prv // [010]pPrv

In line 353,

Figure 3A → Figure 3

In line 364,

Figure 3B → Green solid line in Figure 3

In line 368,

in figure 3B → as the green solid line in figure 3

In lines 378–379, and 382,

Please remove the (A), (B), A), and B) in figure caption for Fig.3.

In line 595,

figure 3m → figure 4m

In the references list,

I added some missing issue numbers and page numbers. Please check these in the PDF file.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments:

1. Summary

We appreciate the thorough verification of the manuscript by the reviewer and are sorry for missing these typos out during the last revision. We are glad that the reviewer finds our manuscript ready for publication after the correction of some minor comments.

2. Point-by-point response to Comments and Suggestions for Authors

In line 268, 

figure 2 → figure 4

This has been corrected.

In line 295, 

[010]Prv // [110]pPrv → [110]Prv // [010]pPrv

This has been corrected.

In line 353,

Figure 3A → Figure 3

This has been corrected.

In line 364,

Figure 3B → Green solid line in Figure 3

 This has been corrected.

In line 368,

in figure 3B → as the green solid line in figure 3

 This has been corrected.

In lines 378–379, and 382,

Please remove the (A), (B), A), and B) in figure caption for Fig.3.

 It seems to me they were already removed, maybe it didn’t show as removed in the PDF version. I will be careful for the next conversion.

In line 595,

figure 3m → figure 4m

This has been corrected.

In the references list, I added some missing issue numbers and page numbers. Please check these in the PDF file.

The changes in the PDF file has been added.

Reviewer 2 Report

Response to review has improved the manuscript and it can be published.

Author Response

We thank the reviewer for the review of the manuscript and the interesting comments that have been raised to improve this work, and are glad the reviewer finds it ready now for publication.

Reviewer 4 Report

The authors responded to the comments by the reviewer and revised the manuscript accordingly. I recommend it to be accepted in the present version.

Author Response

We thank the reviewer for their help improving the manuscript and are glad they now find it ready for publication.