Peer review of: Polymorphism of feldspars above 10 Gpa (2020)


RR2
REVIEWERS' COMMENTS:
Reviewer #1 (Remarks to the Author):
  The revision addresses most of the criticisms that I had of the original submission, and the
 addition of the crystallographic data strengthens the arguments made by the authors in their
 manuscript.
 I still think that the criteria used to define what constitutes a bond, and thus a change in
 coordination number, is somewhat arbitrary, but the revised manuscript does at leastt make clear
 what criteria have been used and the deposited data allow others to make their own analysis.
 One new request. On closer inspection of the data, I realise that the authors have used for the
 new phases a choice of unit-cell orientation that is not obviously related to the conventional unit-
 cell setting of feldspars. It would be very useful if they could provide the transformation matrices
 between the conventional cell setting and the settings used for these high-pressure polymorphs.
 The authors can determine these unambiguously from the orientation matrices used to index their
 diffraction patterns, and they should be added to the supplementary materials.
 With respect to the relative rigidity of the tetrahedra as determined in previous studies, the text is
 still not exactly correct at several points, and I would suggest the following specific rewording at
 various points:
 Line 17-19: Still poorly written and misleading. I would suggest �...to ca. 10 GPa, and have shown
 that the dominant mechanism of pressure-induced deformation is the tilting of....�
 Line 60: Still incorrect. The tetrahedral compression is not �insignificant�. That would imply that it
 is less than the experimental uncertainties and therefore cannot be determined. However, that is
 not true, as tetrahedral compression has been measured experimentally (just above the levels of
 uncertainty) and is very clear in DFT simulations. Therefore, rewrite as �while the TO4 tetrahedra
 show very little compression and behave as relatively-rigid units.�
 Line 186: �while the tetrahedra undergo little distortion and compression�. Line 188: I would leave in the original text �a different mechanism, i.e.�
Minor corrections:
Line 105: Change to �by the absence of single crystal reflections from....�
Line 134: Change to �bond length�
Line 140: Cut and paste error? I think this should be �albite-III� not �albite-II�
Line 151: geometry
Line 189: hyphenate �nearly-rigid� as a compound adjective. ...the rule is that if you use two words
  I am not sure now that the argument on lines 190-194 about ionic potential and coordination
 change are correct, as one might expect that the ability to change coordination is actually related
 to the energy change involved in the change of orbital hybridisation. But I would leave in the
 current text, as it might provoke discussion of this issue.
       
as a compound adjective, you must hyphenate them. But not if the words are used as an adjective
 plus noun. So, �high-pressure structure�(compound adjective) and �a structure at high pressure�
 (adjective �high�, plus noun �pressure�)
 Line 190: delete �soon�
Line 215: change to: �insufficient to draw conclusions about pre-transitional...�
Line 216: change to: �However, increases in the distortion parameters of the AlO4...� Line 220 �connectivities� if the referenced structures are of more than one topology Line 220-221: change to �Similar to feldspars, ...�
Line 221: �continuous�
Line 223 hyphenate �newly-formed�
Line 232: hyphenate �previously-reported� Line 255: �the stability�
       Line 229: delete �well�; you cannot qualify an absolute adjective like �pronounced�. Something is
 either �pronounced� or it is not! I would also rewrite this part: �a pronounced jump in the unit cell
 parameters, as also found in the current study and, accordingly, an increase in the densities of the
 feldspar-derived phases (Figure 4).�
   Line 341-343. Rewrite. This sentence makes no sense to me. Same for lines 357-359; what is
 �Earth�s paradigm�?
 Ross Angel
 IGG-CNR Padova, 13-March-2020