Behaviors of Gas-Rich Crystalline Fluid Inclusions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Summary: The authors report the transformation of fluid inclusion (FIs) from faceted to holly-leaf shaped during the rapid cooling of DCP crystals. They show that the holly-leaf shape is obtained when dissolved gas, in this case CO2, nucleates a bubble within the FI, resulting in a near instantaneous increase in concentration because the volume of solution decreases, being displaced by the bubble. The curvature of the holly leaf seems to relate to the size of the bubble. The explanation for the holly leaf morphology is the inability of diffusion to deliver solute to the corners of the negative crystal fast enough to keep up with rapid growth in the middle of the facets. Interestingly, a similar dynamic appears to be found in olivine formed in volcanos.

Comments: 

• Is the bubble made of solvent vapor or dissolved gas? Increased temperature from exothermic crystallization should dissipate during annealing, does gas bubble shrink or redissolve on annealing at low temperature?
• For schematic figures made in MS PowerPoint (or MS Word), save the figure as a picture rather than take a screenshot so that typos are not underlined in red (Figure 1, 5, 7 and 9) and text boxes are not selected (Figure 5)
• Does Figure 3 show the FI holly leaf phenomenon with or without adding additional CO2? CO2 is not as inert as most other gases, and it would be nice to show the effect is due to the gas, not a competing effect like acidification by CO2 reaction with water to form carbonic acid.
• Is the cause of the concentration increase due to an increase in pressure when the bubble nucleates? The bubble sizes suggest the between the bubble and the fluid is ~0.1 atm.
• In the first paragraph of page 8, is the argument that the holly leaf appears because fast growth depletes solute concentration faster than diffusion can deliver the solute to the corners of the negative crystal? The paragraph is a little unclear.
• Why don’t the holly leaf inclusions in olivine anneal the way they do in DCP?
• Are the supplemental videos real time or are they sped up?

Comments on the Quality of English Language

There are several typos, some of which make understanding the work more difficult. Please proofread the manuscript again. Some instances of typos are listed below.

• Line 40: is this a section heading?
• Line 47: what is Tr?
• Line 90: “exposition” should be “exposure”
• Line 149: “more visible at the lower tip of the top FI”, this region is blocked by the temperature labels in the figure, please move the temperature label.

Author Response

• Is the bubble made of solvent vapor or dissolved gas? Increased temperature from exothermic crystallization should dissipate during annealing, does gas bubble shrink or redissolve on annealing at low temperature? The gas bubble contains solvent vapors (ethanol and water). Dicumyl peroxide is probably negligible in the vapor phase. When crystal growth occurs in a mother liquor with added CO₂, the vapor phase also contains CO₂. This is consistent with a gas bubble size proportionally larger than those observed when no CO2 is added.

On annealing, at low temperature we never noticed a change in the volume of the gas bubble.

• For schematic figures made in MS PowerPoint (or MS Word), save the figure as a picture rather than take a screenshot so that typos are not underlined in red (Figure 1, 5, 7 and 9) and text boxes are not selected (Figure 5). Done
• Does Figure 3 show the FI holly leaf phenomenon with or without adding additional CO2? In figure 3 the crystal was not enriched in CO2. CO2 is not as inert as most other gases, and it would be nice to show the effect is due to the gas, not a competing effect like acidification by CO2 reaction with water to form carbonic acid. Indeed, we plan to enrich the FIs with other gas. But CO2 has a much higher solubility in aqueous solution than the other gas. We agree that higher concentration in CO2 will result in acidification of the solution however, in the present case, DCP has no ionizable moiety therefore, little effect -in any- is expected from pH variation.
• Is the cause of the concentration increase due to an increase in pressure when the bubble nucleates? The bubble sizes suggest the between the bubble and the fluid is ~0.1 atm. The variation of pressure inside the FI before and after the nucleation and growth of the gas bubble is not that great and the estimation of the reviewer is aligned with that. This difference in pressure (ca. 0.1 atm) cannot justify such variations of crystal growth kinetics. By contrast, the variation in volume of the solution leads to a variation in concentration which help in understanding the sudden surge in crystal growth immediately after the apparition of the gas bubble.
• In the first paragraph of page 8, is the argument that the holly leaf appears because fast growth depletes solute concentration faster than diffusion can deliver the solute to the corners of the negative crystal? The paragraph is a little unclear. Reviewer 1 got very well the point. As suggested, we have re-written this paragraph.
• Why don’t the holly leaf inclusions in olivine anneal the way they do in DCP? In Olivine, the viscosity of the silicate liquid is much greater than that of DCP’s FI. The fast cooling (may be quenching) of the Olivine crystals bring them at a temperature below the glass transition (below Tg) where mobility is far too low to contemplate any relaxation of the IFs.
• Are the supplemental videos real time or are they sped up? Actually, the two videos have been accelerated 10 times. This has been mentioned in the manuscript.

 

Comments on the Quality of English Language

There are several typos, some of which make understanding the work more difficult. Please proofread the manuscript again. Some instances of typos are listed below. Thank you for detecting those typos.

• Line 40: is this a section heading? Sorry, that was a personal note that I left. These words have been discarded.
• Line 47: what is Tr? (temperature of relaxation: Figure 1)
• Line 90: “exposition” should be “exposure”: done
• Line 149: “more visible at the lower tip of the top FI”, this region is blocked by the temperature labels in the figure, please move the temperature label.

Reviewer 2 Report

Comments and Suggestions for Authors

1. The resolution of Figures 6 and 8 needs to be improved
2. What does negative crystal means?
3. Figure 4 should indicate which figure 4a and 4b correspond to, respectively
4. If this behavior can be quantitatively correlated with crystal growth rate, this study will be even more interesting
5. Will gas concentration have a significant impact on this behavior?
6. Can the author explain the reason why intact single crystals can still form at a very fast cooling rate. This looks very interesting.
7. The authors claim that this shape transition far away from equilibrium may have universality, but I think people may be more concerned about whether the critical cooling rate and critical gas phase concentration that cause this transition exist? How can we regulate this behavior and obtain some unexpected effects from it. Of course, we also need to consider the impact of this behavior on purity, but in practical applications, we may not adopt such a fast cooling rate.

Author Response

 

1. The resolution of Figures 6 and 8 needs to be improved. We tried to improve the resolution of Figures 6 and 8, without success. When the IFs are deeply embedded in single crystals, it is very difficult to improve, because several optical effects negatively impact the quality of the photos. Nevertheless, we think that the facts are clearly enough illustrated with that quality of the figures.
2. What does negative crystal mean? This expression comes from a Fluid Inclusion (FI) which is well-faceted in the shape of a crystal but the surrounding of this FI is the single crystal itself and the interior is composed of fluids: solution with or without bubble of gas. Thus, it corresponds to an inverted situation compared to the usual one where, the solution surrounds the single crystal. In organic crystals this is commonly observed at ‘high’ temperature’. This is detailed in the introduction: top of page 2 and in figure 1.
3. Figure 4 should indicate which figure 4a and 4b correspond to, respectively. Done, thank you for this remark.
4. If this behavior can be quantitatively correlated with crystal growth rate, this study will be even more interesting. Indeed, the next step of that research will be to be more quantitative. Constant temperature is relatively easy to control, cooling rate is easy to program but it is more difficult to know the exact value of the gradient and the amount of gas dissolved in every FI is even much more difficult to know. This study has shown that FIs few tens microns apart can contain a significantly different amounts of dissolved gas at HT and size of the bubbles at low T. This will constitute the challenge for the next study. 
5. Will gas concentration have a significant impact on this behavior? Yes, and we are sure about that. A high concentration in gas (together with a high cooling rate) will prompt the transient apparition of the holly-leaf-shaped FIs in Dicumyl peroxide (DCP). In experiments where the mother liquor (Water and Ethanol) is not supplemented in CO2, we observed very few, if any, holly-leaf-shaped FIs and sometime none.
6. Can the author explain the reason why intact single crystals can still form at a very fast cooling rate? This looks very interesting. Actually, negative single crystals can be formed regardless the cooling rate. The transient holly-leaf-shaped FIs or the ellipsoidal FIs will finally get faceted after long annealing.
7. The authors claim that this shape transition far away from equilibrium may have universality, but I think people may be more concerned about whether the critical cooling rate and critical gas phase concentration that cause this transition exist? External crystal growth is supposedly well understood, unlike nucleation, which is subject to heated debates. This study shows that internal crystal growth exhibits diverse, unexpected behaviors. We fully agree on the value of determining gas concentration and cooling rate thresholds for each material.  and How can we regulate this behavior and obtain some unexpected effects from it. Of course, we also need to consider the impact of this behavior on purity, but in practical applications, we may not adopt such a fast cooling rate. We have not yet found an application for this behavior. However, if we can accurately determine the amount of CO₂ in olivine crystals, the cooling rate could be estimated, and vice versa.