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

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The quality of the crystallographic data is quite impressive, and the authors are to be congratulated on 
their successful efforts to obtain such data. However, I am disappointed that much of the data collected 
(Supplementary Table 1) is not reported in full, nor used in this manuscript. I am also disappointed that 
the authors do not discuss the mechanisms of the coordination changes at high pressure, and whether 
they are related to previously reported elastic softening and recovery in feldspars at high pressure. Were 
all of the data to be reported and analysed, this would make an excellent contribution. 
 
However, I disagree fundamentally with the two major interpretations by the authors of the results 
reported here: 
 
(1) One of the precepts of this manuscript is, as stated in the abstract, that ‘Previous structural studies of 
feldspars are limited to ~10 GPa, and it is commonly believed that their pressure-induced deformations 
are realized through tilting of AlO4 and SiO4 tetrahedra in a tetrahedral framework’. First, belief is for 
religion not science. Second, this is not true. For example, figure 4 in reference 22 (from 1996) shows 
the experimentally-measured compression of the tetrahedra in microcline up to 7 GPa, and the DFT 
simulations of reference 13 show the compression of the tetrahedra in albite. Curetti et al (2011) Amer 
Min, 96, 383-392, also measure tetrahedral compression in a feldspar. Thus, the amount of compression 
of the tetrahedra is small and therefore difficult to measure experimentally, also because the suppression 
of the anisotropic thermal motion of the bridging oxygens masks some of the compression as measured 
by Bragg diffraction. But the shift under pressure of Raman lines due to internal modes of the tetrahedra 
also indicates that there is tetrahedral compression. Therefore, as stated in many studies, and stated 
here on page 3 ‘the pressure-induced compression of the framework is primarily accommodated by 
altering the TO- T bond angles’ but nobody ‘believes’ that the tetrahedra are rigid. The use of models 
with rigid tetrahedra, which dates back to Megaw in the 1960s is only to help understand the role of 
tetrahedral rotation in the structural response of feldspars and to identify the role of tetrahedral 
distortions and compression. 
 
Therefore the abstract needs correction. And the following phrase on page 3 ‘while the TO4 tetrahedra 
remain as rigid units’ is false and must be deleted. 
 
The text at the bottom of page 5 and the top of page 6 also has this mistake by implying previous studies 
did not detect tetrahedral compression. Then the new structural phase transitions are discussed in terms 
of a ‘compression mechanism’. I think that it is wrong to consider transition mechanisms as the same 
type of physical process as that of phase transformations and to do so will only confuse the reader; one 
would not consider polymorphic phase transitions (e.g. from andalusite to sillimanite) as a change in 
compression mechanism. I would therefore recommend re-writing the text in page 6 to avoid this 
confusion. 
 
(2) I am not convinced that the heating experiments demonstrate conclusively that feldspars could 
remain stable at pressures above 3 GPa in subducting slabs on geological timescales, as the experiments 
only lasted 2 hours. One also imagines that the kinetics of transformations to the stable phases would be 
greatly accelerated in the presence of water: were water-present experiments performed? 
  
I also have a number of technical questions and comments: 
 
What is the evidence for the space groups reported for these structures? Could they actually be acentric, 
and what statistical tests were performed to test this possibility? 
 
Were single crystals preserved through the transitions? If not, what procedures were used to separate 
the diffraction contributions from individual grains? Were they related by twin operations, etc? 
 
Supplementary Table 1 indicates that data were collected at many more pressures than those discussed 
in the manuscript, which focusses on the new polymorphs. 
• Does the evolution of the unit cell parameters (not reported) agree with previous measurements below 
10 GPa? • Were full intensity datasets collected at these lower pressures? If so, do the refined structure agree 
with previous determinations or, if not, what are the differences? 
• If full structure refinements were performed on these other datasets, they would be extremely valuable 
for understanding the mechanisms of structural compression of framework materials and should be 
published in full. 
• Did the authors detect the elastic softening in albite (references 13 and 20)? Can they shed light on the 
mechanism of the elastic recovery above 9 GPa? Is the coordination change they report the consequence 
of this softening and the reason for recovery? Is it already present in the DFT simulations of ref 13? Is 
the coordination change the reason for the changes in the Raman spectra in the 8 – 11 GPa pressure 
range (Aliatis et al (2016) Phys Chem Minerals, doi: 10.1007/s00269-016-0850-5)? 
• What is the criteria for determining whether an atom-atom distance constitutes a bond? Bond valence 
sums, or some other method? 
• What is the mechanism of the transformations involving coordination change, and is there hysteresis? 
This could be addressed if there were more closely-spaced data points around the transition pressures. Is 
the mechanism the same for the coordination change for Si in CaSi2O5 which occurs around 1 kbar, or in 
the structures described in references 25-27? What are the general features of this type of coordination 
change? 
 
 
Other corrections: 
 
Figure 1: This shows well the topology of the structure, but the unit-cell of a feldspar depends on the 
Al:Si ratio and the state of Al/Si order. While the differences in the alpha and gamma angles between the 
monoclinic and triclinic polymorphs is probably not significant at the scale of the drawing, the doubling of 
the c-axis in anorthite compared to anorthite would be obvious. Therefore, the statement in the figure 
caption ‘Black lines outline the unit cell’ is misleading. Either say the ‘unit-cell of alkali feldspars’ or ‘of 
the aristotype structure’, or reword in another way. 
 
Page 3, line 1: ‘Commonly, feldspars are believed to be stable at pressures...’. Again, belief is for 
religions not science. Better to state that ‘Feldspars are thermodynamically stable at pressures up to 3 
GPa ....’ The highest thermodynamic stability is for K-feldspar, at about 3 GPa. 
 
Reference list: requires copy-editing. Many chemical formula are given without subscripts. Some journal 
names ate not written correctly, etc 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
It is not often that new polymorphs of the most important rock forming minerals are discovered, so this 
paper will certainly be of interest across a wide area of mineralogy, petrology and geophysics. The 
authors have demonstrated that each of the three principal end-members of the ternary feldspar system 
undergo phase transitions to previously unknown high pressure phases. Of particular interest and 
importance is the increase in coordination of Al and Si from [4], typical of the crust, to [5] or [6]. 
Whether this turns out to be important in the context of the properties of the crust at depth remains to 
be seen, because these minerals usually react with other minerals before pressures of 14-15 GPa are 
reached. That does not reduce the impact of the work, however. It is likely that there will be a whole 
family of structures with compositions comparable with the many different natural and synthetic feldspars 
that are known. 
 
The paper is clear and well written. The quality of the data appears to be good from what was probably 
quite a difficult series of experiments, from a technical point of view. 
 
In my opinion, this is the kind of work that should appear in Nature journals to broaden the rather narrow 
range of geological papers that are currently published.