the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Oct 2025
Measurement report: Ice nucleation ability of perthite feldspar powder
Abstract. Feldspars are among the most efficient mineral ice-nucleating particles (INPs) in the atmosphere. However, their nucleation behavior varies significantly across natural samples. This study investigates six feldspar powders selected for their perthitic or anti-perthitic textures and spanning a range of K/Na compositions. All samples were comprehensively characterized in terms of mineralogy, bulk and surface chemistry, and microstructure. Droplet freezing assays revealed consistent onset temperatures between −2 and −4 °C, suggesting the presence of shared active nucleation sites across all feldspar types. Cumulative and differential freezing spectra revealed marked differences in the density and distribution of ice-nucleating sites, which were found to correlate with both feldspar composition and microtexture. Using HUB analysis, different subpopulations of ice-nucleating sites were identified. Perthites showing microcline structures exhibited a continuous increase in nucleation site density with decreasing temperature as subpopulations became active. In contrast, samples lacking dominant microcline structures showed plateaus in the cumulative spectra within specific temperature ranges, indicating a significant reduction in certain subpopulations. These findings highlight the crucial role of exsolution textures and crystallographic structure in regulating feldspar ice-nucleation efficiency. The results have implications for understanding feldspar behavior in the atmosphere and for improving predictive models in cloud microphysics.
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Status: open (until 07 Dec 2025)
- RC1: 'Comment on egusphere-2025-5014', Thomas F. Whale, 07 Nov 2025 reply
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RC2: 'Comment on egusphere-2025-5014', Anonymous Referee #2, 15 Nov 2025
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Canet et al. presented a study on the immersion freezing activity of 6 feldspar samples to characterize their ice nucleation behaviour. The authors analyzed the chemical composition, minerology, surface area and particle size distribution of those samples, in addition to microscopy images for particle visualization. The authors also applied newly reported data analysis method (i.e., Heterogeneous Underlying-Based (HUB) method) to investigate the contribution of different sub-group of ice nucleation sites. Then, the authors compared their results with those in literature. As a measurement report, the amount of work included in this preprint is sufficient. Unfortunately, following ACP guideline, the presentation quality should be improved before acceptance for publication. We briefly list our general comments as below, followed by more detailed comments. We hope that our comments can help the authors improve this preprint and we encourage the authors to do a resubmission.
General comments:
- This preprint is lack of in-depth discussion on interpreting the ice nucleation activity of the samples. For some figures, for instance Figure 4, 6 and 9, even the main information of the data is not sufficiently presented. For example, statements in Line 250-254 are all the text for data in Figure 4, which even did not fully elaborate data in each panel, not mention in-depth discussions for help readers to digest the results.
- There are some (mis-) over-interpretations on the data presented in Figure 3 and 8. For example, the statement in Line 218-219 is not consistent with the data shown in Figure 3a which shows the 0.1wt%-sample freezes almost by 100% at -20°C but not like what the authors elaborated: 90% droplets remained unfrozen. For Figure 8, the statement in line 415-416 goes that the IN active site density of the ‘first subpopulation’ (defined by the authors) appears to be higher in both K-rich and Na-rich perthite feldspars compared to those with more balanced Na/K ratios. However, Figure 8a shows that the peak of the ‘first subpopulation’ of IK1 is similar to that of LNaK1 in Figure 8b.
- Some data was visualized carelessly. There are some obvious mistakes regarding the labeling in figures and sample/measurement information is inconsistent through the text. For example, the authors introduced sample concentrations in section 2 as ‘concentration of 10, 5, 2, 1, 0.5, 0.1, 0.05, 0.01 and 0.001 wt%.’, however, Figure 3a shows results for samples with concentrations from 5 wt% to 0.00001 wt%. What is the sample with concentration of 0.00001 wt%? And Figure 3b for ns should be based on the frozen fraction data in Figure 3a. But Figure 3a does not include data for a sample with concentration 0.001 wt% which is shown in Figure 3b. And what is the sample called FK1 in Figure 2a? Such a sample is not introduced elsewhere in the main text. We suggest the authors carefully going through the preprint for corrections and ensuring the quality of the presentation.
- Discussions on the correlations between sample properties and IN behavior are lacking. The authors first presented the property characterization results for feldspar samples in Figures 1 and 2 and Tables 1 and 2. From Figure 3, the authors started to focus on the IN behavior of the samples. Unfortunately, the authors did not link both parts sufficiently.
- The Discussion section is poorly organized. It should be better structured with subtitles and clearer delivery messages.
- Some literature citations are inappropriate. For example, line 397 ‘Interestingly, similar trends have been observed in this study (Welti et al., 2019),’. The statement refers to this study, why is there a need to refer to Welti et al. 2019? Also in line 455, one did not see it is necessary to refer to Burrows et al., 2022. The literature is relevant but not cited in the correct way.
- Figures generally should be improved for better quality. For example, Figure 1 should have better resolution and Figures 3, 4 and 5 are hard to read because of the poor color scheme.
- Given the high sample concentration (1, 2, 5, 10 wt%) tested for droplet freezing experiments, one critical issue would be the aggregation/agglomeration of particles and even the formation of sediment at the tip of PCR wells, which renders the IN results not relevant for suspended particles anymore. Unfortunately, the effects of particle clusters/sediments were not addressed in the preprint. One would challenge that the freezing activity at warm temperatures of concentrated samples is because of freezing on sedimented sample surface, which corresponds to the first sub-group active sites in Figure 8, while the freezing of suspended particles of less concentrated samples at lower temperatures may be more relevant to the second subgroup below -15C. In the literature, it is rarely reported that mineral dust aerosol particles can freeze at such warm temperatures but more reported freezing for T<-15C (Murray et al. 2012) .
- We suggest the authors to prepare some text figures and tables in the supplementary. Without any text in the supplementary or if not clearly introduced in the main text like now, it is hard for readers to read them.
Detailed comments:
- Line 15: should be ice nucleation
- Line 16: ‘comprehensively’ change the wording. One will expect more objective statements. Also for ‘systematically’ (line 430) and ‘robust and statistically meaningful’ (line 431), etc.
- Line 17-19: the sample mass concentration should be clarified. Otherwise, the statement is misleading readers to assume K-feldspar is as active as biological particles (e.g., bacteria) that single particles can trigger ice formation at warm sub-zero temperatures.
- Line 20: ‘HUB’ should be defined
- Line 26: better to use ‘INP parameterizations’ instead of ‘predictive models’
- Line 31: should be homogeneous freezing but not homogeneous ice nucleation. Please check Vali et al. (2015).
- Line 33: by some of airborne particles. Not all airborne particles are INPs.
- Line 41 and 45 (other parts relevant also): be consistent with ‘ice nucleation efficiency’
- Line 52: give a definition for active sites. There are already some relevant statements in Line 43-45.
- Line 58: ‘specific’? please be explicit and unambiguously develop these statements. Also, there are many statements in this preprint like this in an ambiguous way. Please revise them accordingly.
- Line 61-74: consider revising this part and improving Figure 1 with better schematics to more clearly introduce the mineralogy of feldspar
- Line 127-128: did the samples for IN measurements undergo the same suspension by sonication?
- Line 130: why do the degassing at 393K for 24 hours? There are studies reporting the phase change of feldspar after exposing to temperatures above 100C and leads to increased surface area (e.g., Thomas M. Blattmann, Applied Clay Science 258 (2024) 107477, https://doi.org/10.1016/j.clay.2024.107477).
- Line 135: sample concentrations should be consistent with results in following figures
- Line 135-136: ‘For each dilution, 96 droplets of 3 μL were dispensed into two 384 -well plates,’ is not a clear statement. How were 96 droplets dispensed into two 384-well plates?
- Line 146-147: was background noise subtracted for samples for which there is an influence from the background?
- Line 188-190: It is not intuitive to see aligned porosity of samples in the microscopy images. Please indicate it to guide naked eyes.
- Line 218-219: NO. Almost all droplets freeze at -20C even for 0.05wt%.
- Line 220: should be pure water
- Line 226-227: the results of heated samples should be provided, at least in the supplementary
- Line 240: equation wrong. Please check Vali (2019) for their equation 4
- Line 241: what is NL(T)? did it show up in equation (1) or somewhere?
- Line 247-248: ice nucleation sites
- Line 250-254: no discussions on the date in Figure 4. What do we learn from the data? Any deliverables?
- Line 267-268: what do we learn from the data? Does it mean the size of particles is not important for feldspar ice nucleation?
- Line 276: consistent? How? the data points in Figure 6 spreads for four orders of magnitude around -4C and more than two orders of magnitude at lower temperatures? How can the authors argue that the results are consistent?
- Line 290-291: for which sample concentration was the temperature values calculated?
- Line 291-292: please indicate the temperature values in many publications the authors referred to. However, in Figure 6, LNaK1 in this study shows the lowest onset temperature, which is lower than results from the literature, isn't it? One cannot write down statements out of the blue.
- Line 302-305: we cannot agree. What is the effect of particle agglomeration given such high concentration of particles in the tested samples?
- Line 306: now, go back to figure 4. Why not discuss it when first introduce data in Figure 4?
- Line 328-329: we cannot agree with the authors that size is not important for particle ice nucleation. Also, BET results depends on particle size. Samples with smaller sizes generally show larger BET surface area. This is also supported by the results of LNa2 (Table 2). The authors should consider this point when discussing results in Figure 5.
- Line 334: ‘and correspondingly display higher IN sites densities between −5 to −20 ℃.’ Refers to which figure?
- Line 367: Not very similar. The biggest difference is between IK1 and IK2. Any explanations?
- Line 391-395: please refer to results presented in figures.
- Line 397: It is inapposite to refer to Welti et al. for this study.
- Line 406-413: Are these statements relevant to this study? Does this study include any results with samples having different -OH groups?
- Line 415-416: over interpretation! The first peak of IK1 is similar to LNaK1 in the figure
- Line 419-420: the statement can be supported by results in which figure? Please clearly refer to it
- Line 457: which sample group? Be clear
- Line 455: why refer to Burrows et al., 2022 when mentioning findings in this study
Technical corrections:
- Figure 1: what is Qz? The quality is very poor. Text is hard to read. Is panel a created by the authors or does it need a copyright from other publications?
- Table 1: why are sample names in different colors? to highlight anything?
- Figure 2: what is the sample FK1 in panel a? According to ACP guidelines, this figure should be labeled as 6 panels. And the font size of the text in the figure should be the same as the main text.
- Table 2: the same as for Table 1
- Figure 3: Figure 3b should be based on Figure 3a. However, there is no frozen fraction curve for sample with concentration of 0.001wt%. In panel a, the samples with concentration of 0.0001 wt% and 0.00001wt% were not introduced in the main text at all? Where are they from? The color scheme used is a bad decision.
- Figure 4: very hard to read because of the bad choice of color scheme
- Figure 5: use legend for samples with large and small particles
- Figure 6: there are four font sizes in this figure. Please check ACP guidelines
- Figure 7: add axis scales. What is the thin black line at the top?
- Figure 9: what is the unit on the y-axis for panel b and d?
- Figure S4: change the color scheme for better readability
- Figure S5: use different colors for better readability
References:
Murray, B. J., O’Sullivan, D., Atkinson, J. D., and Webb, M. E.: Ice Nucleation by Particles Immersed in Supercooled Cloud Droplets, Chem. Soc. Rev., 41, 6519–6554, https://doi.org/10.1039/c2cs35200a, 2012.
Vali, G.: Revisiting the differential freezing nucleus spectra derived from drop-freezing experiments: methods of calculation, applications, and confidence limits, Atmos. Meas. Tech., 12, 1219-1231, https://doi.org/10.5194/amt-12-1219-2019, 2019.
Vali, G., DeMott, P. J., Möhler, O., and Whale, T. F.: Technical Note: A Proposal for Ice Nucleation Terminology, Atmos. Chem. Phys., 15, 10263-10270, https://doi.org/10.5194/acp-15-10263-2015, 2015.
Citation: https://doi.org/10.5194/egusphere-2025-5014-RC2 -
RC3: 'Comment on egusphere-2025-5014', Anonymous Referee #3, 20 Nov 2025
reply
This study presents measurements of the ice nucleation activity of 6 different feldspar samples. Immersion freezing experiments were performed with sample powders with particle sizes in the range of several micrometers and suspensions prepared with a broad range of particle concentrations. The samples were chemically and mineralogically characterized to discuss the relevance of different mineralogical features for the ice nucleation activity. HUB analysis was used to divide the samples in subpopulations and to better resolve different stages of ice nucleation behavior. This study confirms the findings of Whale et al. (2017), Welti et al. (2019) and Kiselev et al. (2017) by demonstrating that initial high T freezing is associated with perthitic exsolution features in alkali feldspars. A steady increase of active sites with decreasing temperatures was observed for microcline samples with high degree of Al-Si ordering in contrast to a plateau behavior before active sites increased at much lower temperatures for orthoclase samples with lower Al-Si ordering.
The manuscript is mostly well-structured and clearly written. It presents well conducted experiments and contributes to previous studies by discussing subpopulations with HUB analyses and evaluating the impact of mineralogical differences between the samples. However, especially the mineralogical characterization of the samples needs some additional work to support the presented results and the discussion. I think with a few changes and additions, the presented study is ready for publication.
General comments:
It is one of the strong points of the study that it uses mineralogical information to understand the different ice nucleation activity in different feldspar samples. But I think it needs to be improved to support the conclusions made about the impact of perthite lamellae, state of Al-Si ordering and alkali feldspar polymorphs to the IN activity.
- The distinction of the samples based on Na/K ratios is not sufficient: 1) As shown in figures 2a and S2, the feldspars can have several complex features, that can have relevance for IN. By separating the samples bases on their Na/K ratio, one cannot distinguish the relevance of hydrothermal alteration features from perthite features or polymorphs on IN activity. 2) The chemical analyses given in table 1 and the mineral fractions shown in figure 1b indicate, that IK samples are perthitic alkali feldspars and the Llançà samples are rock samples with pl, kfs, q and ms. The Na/K ratio in the Llançà samples therefore seems to be an arbitrary value resulting from the selected sample volume and the mineral fractions therein and not from feldspar characteristics. If alkali feldspars and plagioclases grains are not separated, I could take 20 samples from the same rock with different fractions of minerals yielding 20 different element ratios, but the alkali feldspars and the plagioclases could be identical. Then the main difference between LNa and LNaK would be that K-feldspar is diluted in LNa yielding different IN activity. Are the differences shown in fig 4 due to different alkali feldspar concentrations or due to different feldspar characteristics? And the provided chemical information does not necessarily say what the actual compositions of the feldspars are. 3) You argue, based on figure 2, that LNa is mostly an anti-perthitic albite. But I do not see any anti-perthites in figure S2. I also see several other complex features in figure 2a that require clarification (similar to point 1)). => (A) provide more photomicrographs (and more BSE images) for all samples to properly identify the feldspar characteristics: perthite lamellae, hydrothermal alteration, twinning, porosity etc. (e.g. see Parsons et al. (2005), DOI: 10.2110/jsr.2005.071), (B) Provide actual feldspar compositions, at best with EPMA since Inclusions, perthite lamellae hydrothermal alteration zones etc. cannot be separated from the host mineral for XRF analyses, if you use XRF analyses, try to use pure alkali feldspar or plagioclase samples.
- It is very crucial to discriminate microcline and orthoclase. You specifically argue that the higher structural ordering in microcline enhances IN activity. Therefore, a more convincing crystallographic characterization would improve the manuscript significantly. In figure 2, you identify the K-feldspar in LNa and LNaK as orthoclase, in figure S3 as microcline. If you use peaks to distinguish them (figure 2b), provide the reflections they correspond to. Peak positions alone are very dependent on Na-K-composition and only specific reflects can be used to identify a microcline as a microcline. Obtain unit cell parameters form your XRD data or if you already have them, show them to convincingly distinguish microcline from orthoclase.
- Do experiments with highly concentrated suspensions still show immersion freezing, i.e. freezing on particles immersed in water, or do they show freezing of water in sediment pores or in pores in re-congregated particles? Especially larger particles settle within minutes. High particle concentration could also result in fast re-congregation. Your freezing curves show bumps for higher concentrated suspensions. You argue that the early freezing events show the initial freezing triggered by perthites. But why do they almost completely vanish in lower concentrations? Your argumentation is understandable and well structured, but to me, the bumps and the subpopulation could be due to pore/interstice freezing on one and immersion freezing on the other side. Could you please elaborate on this point?
Detailed comments:
63: X(Si,Al)Si2O8 is not correct, it should be X(Al,Si)2Si2O8 or the most general formula AT4O8 with the convention T for tetrahedral site or even better A+xA2+(1-x)Al(2-x)Si(2+x)O8. It should be mentioned here that due to the immiscibility between the Ca and the K end member, feldspars can be subdivided into two binary systems being plagioclase feldspars with NaxCa(1-x)Al(2-x)Si(2+x)O8 and alkali feldspars with NaxK(1-x)AlSi3O8.
67: typo: albite is NaAlSi3O8
72-73: “These lamellae typically align with the (100) crystallographic plane and are especially prominent in monoclinic feldspars.” Should be: “These lamellae are typically subparallel to the (100) crystallographic plane.” 1. Lamellae are equally prominent in triclinic microcline. 2. The lamellae are oriented between (-801) and (-601) (Murchison plane) and are therefore subparallel to (100) which for example lead to the hypothesis by Kiselev et al. (2017 & 2021) that perthite lamellae or cracks along the Murchison plane could create steps with exposed (100). But the perthite lamellae are not aligned with (100).
128: “to avoid particle segregation”, probably to avoid particle congregation
176-179: The section should be rewritten. In table 1, the wt%oxide ratios are given, not the atom ratios. If you calculate the atom ratios, Si/Al is in Imerys samples is with 2.93 and 2.97 very close to 3 for pure alkali feldspars. The minor discrepancy is most likely very little muscovite and/or Ca, Ba and Sr in the feldspar (incorporation of 2+ ions is linked to Al-Si substitution). The charge discrepancy from additional Al cannot be balanced with additional K, there is not enough space in the lattice. In the Llançà samples, Si/Al is between 4 and 4.5 and I would ascribe this entirely to quartz. It seems that the Si/Al ratios and the K/Na ratios in Llançà samples are due to the chosen sample volumes not due to specific feldspar properties.
Table 1 (and S1) and Figure 1b: The table and the diagram are not giving “feldspar compositions” for the Llançà samples, but the composition of a rock powder containing plagioclase, alkali feldspar, quartz and muscovite. As shown in previous studies, it can be assumed that the INPs with highest freezing temperatures are alkali feldspars. If the measured powder is representative for the Llançà powders used for the freezing experiments, then the alkali feldspar concentration is of course diluted with plagioclase, quartz and muscovite, making direct comparison with Imerys samples difficult. How do the curves in fig 4d compare with “corrected” alkali feldspar concentrations and surface areas for the LNa and LNaK samples?
Additional to table 1: I would recommend changing the ratios to atom ratios and you could give calculated mineral formulae. Even better would be EPMA measurements for actual feldspar compositions.
Additional to figure 1: If the exact feldspar compositions are known, plot them in the diagram, if the abbreviations for oligoclase, andesine, labradorite, bywtonite, anorthite, anorthoclase and sanidine are in the diagram, they should be explained in the text. The source (maybe Brown and Parsons, 1988) should be mentioned. I think it makes more sense to include 1b in figure 2 and to combine it with photomicrographs.
184-190: I rather see perthite microstructures is figure S2d and S2b. Figure S2d shows a strongly hydrothermally altered perthite (the perthite lamellae are elongated from upper left to lower right) and probably hydrothermal albitization most likely along (001) or (010) cleavage planes (from lower left to upper right). Photomicrographs of samples IK1 and IK2 would be helpful.
Figure 2a: FK1 is probably IK1. It should be mentioned if these are SE or BSE images. I see two elongated albite structures with different orientation in (a) IK1. I cannot tell from this image what they are. Two perthite lamellae with different orientations due two twinning of the host K-feldspar? Perthite lamellae and hydrothermal albitization along a crack? One Perthite lamella with strong hydrothermal alteration? In the LaNaK2-image, I see horizontal Ab features in Or which could be perthite lamellae and the vertical features which are probably something else. For the LaNa1 image, I can believe that I see a patch anti-perthite. Photomicrographs, more clear SEM images or detailed descriptions of what can be seen in the images are needed. Are the images representative for the whole sample? Is LNaK a mixture of perthitic orthoclase and albite and LNa entirely an anti-perthitic albite? If so, it should be stated in the text.
Figure 2b: Did you obtain lattice parameters from XRD measurements? Even if they are note perfectly refined, lattice parameters or any comparison between triclinic and monoclinic structures fitted to the XRD analyses would be way more convincing for distinguishing microcline from orthoclase.
Figure 4: See comments above. Are the K/Na compositions balanced in the feldspars or the only in the sample volume extracted from the rock?
Section 3.4.: I find the comparison very significant. Maybe you could even add a sentence to state that no features that potentially enhance ice nucleation (like defexts at lamellar interfaces) are affected by the studied particle sizes.
296: It should be mentioned that you mean Ab and Mc by Harrison at al. (2016) or use names like AbH and McH, since you use “Ab” and “Mc” elsewhere to describe your samples.
312-323: It could be a strong point of your study, if you can show differences in ice nucleation behavior and relate them to mineralogical properties of the feldspars in samples LNa and LNaK, but it requires back up with a lot more petrographic information. As I mentioned several times, bulk Na/K ratios alone are not sufficient.
317-319: Similar ice nucleating behavior indicates similar IN surface properties, but it does not say anything about the geological conditions or the rock formation.
331-334: Rb is typically incorporated in alkali feldspars, Sr typically more in plagioclase. Welti et al. (2019) compare microcline with microcline (Amazonite) and labradorite with labradorite. The different Rb/Sr ratios between IK and LNa samples might be attributed to different plag/kfs ratios. You should compare IK1 with IK2 and LNa1 with LNa2 (or purer mineral chemistries therein). I think the slightly higher Rb/Sr in IK1 and the slightly higher freezing T does support the statement.
391-392: what do you mean with mineralogical composition? What I understand under ‘mineralogical composition’ is ‘the sample consists of minerals A, B, C etc.’, which does not make sense to me for perthitic feldspar. Do you mean mineral modification (or polymorph), i.e. microcline and orthoclase or mineral composition (chemical composition of the mineral)?
397: “similar trends have been observed in this study (Welti et al., 2019)” do you mean: “similar trends were observed by Welti et al. (2019)” or “We observed similar trends”?
401-402: “structural ordering”, be more specific: “Al-Si ordering” or “Al-Si ordering in the tetrahedral framework” and add a reference.
404: “In feldspars, ice preferentially nucleates on the (100) crystallographic face”, better: “In feldspars, ice appears to preferentially nucleate…” and refer to Kiselev et al. (2017). Also later, change Kiselev et al. (2016) to Kiselev et al. (2017)
412: an “a” is missing: “of a feldspar surface” or “of feldspar surfaces”
415: active site density of this subpopulation?
418-419: “crystal defects at perthite lamellae interfaces” would be clearer than “defects” when first mentioning them. And cite a reference for the statement, that they are associated with exsolution, e.g. Gerald et al. (2006) https://doi.org/10.2138/am.2006.2029, Abart et al. (2009) https://doi.org/10.2475/06.2009.02
Citation: https://doi.org/10.5194/egusphere-2025-5014-RC3 -
RC4: 'Comment on egusphere-2025-5014', Anonymous Referee #4, 02 Dec 2025
reply
This is a relatively well-written well-structured paper that investigates six different feldspar samples of perthitic and anti-perthitic textures with three main K/Na composition ratios, collected from two mining sites. The samples were characterized in terms of mineralogy, bulk and surface chemistry, and microstructure. The authors conducted droplet freezing assays which revealed onset temperatures between −2 and −4 ℃ and concluded the presence of shared active nucleation sites across all feldspar types. Cumulative and differential freezing spectra were discussed and showed differences in the density and distribution of ice nucleating sites. The authors aimed at correlating density and distribution of ice nucleating sites with feldspar composition and microtexture. They also used the Heterogeneous Underlying-Based (HUB) analysis to identify the different subpopulations of ice-nucleating sites. Based on the HUB analysis, the authors suggested that the nucleation activity in the given temperature range may not be directly governed by the degree of crystallographic ordering within the feldspar, but rather by structural imperfections or defects. Samples with microcline structures (Perthites) exhibited a continuous increase in nucleation site density with decreasing temperature as subpopulations became active, while samples lacking dominant microcline structures showed plateaus in the cumulative spectra within specific temperature ranges, indicating a significant reduction in certain subpopulations. The paper aims to highlight the crucial role of exsolution textures and crystallographic structure in regulating feldspar ice-nucleation efficiency which will serve in better understanding of feldspar behavior in the atmosphere.
The paper requires improvements in both presentation and scientific interpretation to reach the ACP level. To prepare this manuscript for publication, the authors need to consider the following points:General comments:
1. The authors depicted and compared the individual modes of nucleation sites for their data and the data from Whale et al. (2017) using the HUB method. Wouldn’t be more convenient to do this comparison with a broader range of data, particularly more recent data (e.g. those in Kiselev et al. (2021))? This would add more strength to the findings in this manuscript.2. The authors are expected to give a statement of the potential aging of their particles.
3. The font size of the axis titles, data labels and legends should be improved and unified for all figures. Sometimes it is not readable.
4. The color schemes of all figures should also be improved and adapted to the ACP standards
5. Please revise and unify the case and format of the acronyms of the “ice-nucleating site densities” used along the manuscript (in both text and figures). (𝑁s, 𝑁s(T), Nm, Nm(T), ns , nm… etc.) Please also add the missing units of these parameters e.g. in the axis title of some figures.
6. The Em dashes “—” are usually used in parenthetical phrase in a text generated or revised by ChatGPT. If the Authors used AI to revise their text, they are requested to mention this in the Acknowledgment to align with the ACP regulations.
Specific comments:
7. L14-15: “However, their nucleation behavior …”
- Replace with “However, their ice nucleation behavior…”8. L16-17: “All samples were comprehensively characterized in terms of mineralogy, bulk and surface chemistry, and microstructure”.
- Along the manuscript, I couldn’t catch the clear correlation between these properties, particularly “microstructure”, and the IN properties. Authors may consider improving their presentation of this correlation.9. L17-18: “Droplet freezing assays revealed consistent onset temperatures between −2 and −4 ℃, suggesting the presence of shared active nucleation sites across all feldspar types”
- Why consistent onset temperatures suggest the presence of shared active nucleation sites?10. L20: “HUB ”
- Replace with “Heterogeneous Underlying-Based”11. L31-33: “However, most natural clouds are mixed-phase, containing both supercooled liquid droplets and ice crystals. In such clouds, ice formation typically occurs at much higher temperatures through heterogeneous IN…”
- Not necessary “much higher”. You can instead write “In such clouds, ice formation typically occurs at higher temperatures...” or “In such clouds, ice formation can occur at much higher temperatures…”.
12. L45: “… can significantly enhance nucleation efficiency at those sites”.
- Replace “sites” by “active sites”.13. L57-59: “Nonetheless, it is well established that feldspars with specific crystal structures and chemical compositions tend to exhibit consistently higher nucleation activity.”
- Give examples and add reference(s)14. L69-74: “The Or-Ab solid solution is fully miscible only at high temperatures. With a decrease in temperature, alkali feldspars unmix, giving rise to an intergrowth of Or- and Ab-rich phases. This immiscibility is produced by the relatively high difference in size between the Na and K atomic radius. One well-known texture is the perthite, which consists of fine, unsolved albite (Na-rich) lamellae within a Mc or Or (K-rich) host crystal. These lamellae typically align with the (100) crystallographic plane and are especially prominent in monoclinic feldspars. The anti-perthite structure, in contrast, features K-rich feldspar lamellae exsolved within a Na-rich albite matrix.”
- Add reference(s)15. L81-82: “Given that alkali feldspars exhibit the highest efficiency in initiating IN at relatively high temperatures, they are especially relevant to mixed-phase cloud formation.”
- Add reference(s)16. L85-87: “Despite this importance, previous studies have reported highly variable freezing spectra for alkali feldspars, even within perthitic samples. Such discrepancies likely arise from uncontrolled differences in sample provenance, crystallographic structure, and microstructural features.”
- Also sample cleaning and experimental conditions (temperature control and cooling rate, and additionally RH control in the deposition freezing studies) may lead to these discrepancies.17. L89: “freezing assays”
- Replace with “immersion freezing assays”18. L89-90: “composition and texture”
- Replace with “composition and surface texture”19. L103-105: “First, portions of each were crushed and homogenized via ball milling to produce fine powders, primarily for ice nucleation experiments and also for granulometric, chemical, and mineralogical analysis.”
- Please briefly explain the milling procedure. What were the milling durations for the different samples?20. L106-107: “This dual approach enabled correlation of ice nucleation behavior with crystallographic texture and surface features of the parent minerals.”
- The crystallographic texture and surface features should have been changed while crushing and ball milling. How can the authors correlate these properties with the ice nucleation behavior?21. L107: “Ultrapure water (resistivity 18 MΩ cm)…”
- Please mention here the measured pH value.22. L177-179: “The absence of mica phases in the Imerys samples implies that Al is incorporated into the feldspar structure via Si–Al substitution, with charge compensation provided by elevated K⁺ content.”
- Please explain in more details or provide a reference.23. L180-181: “The K/Na distribution in selected samples was further investigated through SEM analysis of metallographically polished feldspar surfaces.”
- Metallographic polishing consists of several steps to ultimately produce a deformation-free, scratch-free, and highly reflective sample surface. This process will change the surface physical morphological properties giving ice nucleation properties that are not relevant to the natural particles. Did the author use these polished samples in the freezing assay?
24. L199: Figure (2a):
FK1 should be IK1 (?)25. L109-110: “For the LNa2 sample, two distinct powders were provided, each produced from the same bulk material but subjected to different milling durations, resulting in varying particle size distributions.”
- See my previous comment on the milling procedure.
- What is the relevance of the particle size distributions mentioned here and those exist in nature in the atmosphere?26. L218-219: “As the concentration decreased, freezing shifted to lower temperatures; at 0.1 wt%, 90% of the droplets remained unfrozen until the temperature dropped below −20 ℃.”
- As can be seen from Fig. 3a, at 0.1 wt% less than 80% of the droplets remained unfrozen.27. L219-220: “At the lowest concentrations (≤0.01 wt%), the freezing behavior becomes indistinguishable from that of water”
- As can be seen from Fig. 3a, it is similar but distinguishable with slightly higher on-set temperatures.28. L227-228: “…selected samples were also tested for 𝑓ice(𝑇) after heating at 90 °C and 110 °C for 30 min. Again, no differences were observed, suggesting that biological contamination is unlikely to contribute to ice nucleation in our samples.”
- Please show the results (here or in SI)29. L229: Figure 3:
- The concentrations in panel (b) do not align with the concentrations in panel (a)
- Plotted colors at low concentrations are hard to be distinguished.30. L276-277: “The results presented in this study are consistent with previously reported cumulative active-site density freezing spectra, 𝑁s(𝑇)”
L286-287: “A key observation — both in this study and in previous literature— is the consistent onset of freezing within a narrow temperature range, approximately between −2 and −4 ℃.”
- The term “consistent” could be misleading here. There is a difference of ~ 2°C, Fig. 6, which is not trivial as an onset temperature. It is only consistent with “Dark shap” sample.31. L299-305: “Conversely, for other feldspar types reported in the literature—such as Pl, which show much lower onset temperatures—it is unlikely that increased concentrations would bring their initial freezing into the same range observed for alkali feldspars. These observations suggest that the most active nucleation sites in alkali feldspars are likely shared across all samples, resulting in a common onset temperature. Therefore, the differences in the cumulative spectra (Fig. 5) are best interpreted as a reflection of the density of these active sites (denoted as 𝑁s(𝑇), the number of sites per cm2 , as well as the presence and abundance of other, less active site populations that contribute to nucleation at lower temperatures.”
- This sentence is hard to digest. E.g. Why it is unlikely that increased concentrations of other feldspars would bring their initial freezing into the same range observed for alkali feldspars? Do you have your own measurements? How These observations suggest that the most active nucleation sites in alkali feldspars are likely shared across all samples?32. L317-319: “Despite originating from different extraction fronts, the close similarity of their ice-nucleating behavior suggests that both samples likely formed under comparable geological conditions, resulting in analogous surface properties that govern IN activity.”
- Not necessary. This similarity may occur for samples formed under different geological conditions. A strong evidence or reference should be given here to support this statement.33. L327-329: “Nevertheless, after normalizing by BET derived 𝑆a , the freezing behavior converged across all particle sizes, indicating that IN activity is primarily governed by intrinsic surface chemistry and structural properties rather than by particle size or preparation method itself.”
- This sentence looks incorrect and should be revised. Having the cumulative freezing spectra normalized to specific surface area (Fig. 5b) showing nearly identical Ns(T) cannot be used as an argument that the particle size does not play a role in the IN activity. This statement contradicts with (Fig. 5a).34. L330: “A previous study (Welti et al., 2019)(8) reported…”
- What does (8) here refer to?35. L365-367: “First, the initial IN active population (SubP-A) does not exhibit a clear difference between Na-rich (anti-perthitic) and K-rich (perthitic) feldspars, as both groups initiate freezing at very similar temperatures.”
- What does this indicate? And why this is not the case for corresponding subpopulation in the reference data, published by Whale et al. (2017)? The same for SubP-B and SubP-C.
- In depth discussion is expected here.36. L365-373: “Several trends emerge from the analysis presented in Fig. 8 and Table S3. First, the initial IN active population (SubP-A) does not exhibit a clear difference between Na-rich (anti-perthitic) and K-rich (perthitic) feldspars, as both groups initiate freezing at very similar temperatures. The probability of freezing associated with the first subpopulation appears to be higher in IK samples. In contrast, the second IN population (SubP-B) displays an inverse trend: IK feldspars nucleate at slightly higher freezing temperatures on average (≈ −13.3 to −13.5 ℃) than LNa samples (≈ −13.3 and −15.2 ℃). Notably, between −7 ℃ and −13 ℃, LNa samples, corresponding to an anti-perthitic Or structure, show very low IN activity, with no subpopulation peak assigned to this temperature region, leading to an extended plateau in their 𝑁s(𝑇) cumulative spectra, as observed in Fig. 4d. Feldspars with a similar Na to K ratio, i.e., LNaK samples, exhibit spectra between those of the IK samples and LNa samples (Fig. 4b).”
- The authors show three subpopulations in Fig. 8 while interpret the results in terms of only two subpopulations in the Discussion (?).37. L415-416: “However, the density of this subpopulation appears to be higher in both K-rich and Na-rich perthite feldspars compared to those with more balanced Na/K ratios.”
- Figure 8a and b show similarity between K-rich and balanced Na/K ratios. The three panels of Fig. 8 indicate that the subpopulation (SubP-A) appears to be higher in Na-rich perthite feldspars compared to those in K-rich and balanced Na/K.38. L444-447: “This analysis revealed a fundamental difference between the two nucleation behaviors: plateau-containing spectra are consistent with two well-separated subpopulations—one initiating near onset and another emerging at lower temperatures—whereas continuous spectra require multiple, more gradually activated subpopulations.”
- Again, the authors show three subpopulations in all results, Fig. 8, however, make a conclusion based on two subpopulations. Why SubP-C was not involved in the Discussion and Conclusions?Citation: https://doi.org/10.5194/egusphere-2025-5014-RC4
Data sets
Dataset: Ice nucleation ability of perthite feldspar powder J. Canet et al. https://doi.org/10.5281/zenodo.17396669
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This study reports measurements of the ice nucleation activity of six well-characterized perthitic feldspars. The work adds to, and in many respects confirms, previous findings reported in Whale et al. (2017) and Kiselev et al. (2021) showing that exsolution textures and resulting structural features are responsible for the high temperature ice nucleation activity (INA) of feldspar then clearly showing that the INA observed at around -15°C may have more to do with level of disorder in the aluminosilicate network. The inclusion of differential nucleus spectra for Whale et al. (2017) data is particularly and represents an analysis we should have performed originally. I also like the picking out of individual modes of nucleation sites for the new data and our previous measurements using the HUB method.
The experiments appear to be very carefully and thoroughly conducted. The paper is clearly written and well-structured. I think it should be published with relatively few changes, though I have quite a few points regarding the discussion which the authors may wish to consider.
Comments
1) The authors may find the LDH1 feldspar data from Daily et al. (2023) of interest. It originates from the same location (Mt Malosa, Malawi) as the TUD#3 sample in Harrison et al. This material, described, perhaps a little hyperbolically, as ‘hyper-active’ in Harrison et al. (2016), appears to eliminate supercooling entirely when present in sufficient quantity—even Snomax and AgI don’t do this. Could you compare this measurement to what you already have? Would it change your conclusions regarding the nature of high temperature feldspar ice nucleation sites?
2) It is important to state how long feldspar samples were in contact with water before ice nucleation measurements. Harrison et al. (2016) demonstrated significant changes in activity over time for some feldspars (see Fig. 4). For example, our purest albite sample started highly active but became much less active after 16 months. The hyperactive TUD#3 feldspar also declined, though less dramatically. While I am not suggesting time-consuming aging experiments here, acknowledging this potential variability and complexity would strengthen the discussion.
3) Regarding the impact of aluminosilicate ordering on the nucleation mode at -15°C – it seems likely that this is still something to do with exposure of the feldspar (100)? Can anything be said about likely differences in the relevant features between orthoclase and microcline? I agree that the evidence points to this (and it may be the main new finding in this work) but it’s not obvious to me why aluminosilicate structural disorder itself should have anything much to do with ice nucleation. Maybe I am missing something.
4) The phrase ‘shared active nucleation sites’ is a little unclear I would say. I agree the sites are most likely very similar in nature, but the measured onset temperatures are still different, so they are in some sense ‘different’. What constitutes ‘shared’? Perhaps I am being too picky here but I think it is wrong to imply that the sites are identical in the way that e.g. ice nucleating proteins might be.
5) More discussion of the findings in Kiselev et al. (2021) would be valuable, particularly the results the for FS08-64 samples (o and c). These samples began as gem-quality sanidines but underwent ion exchange and annealing under different conditions to induce cracks, resulting in distinct cumulative spectra. Their processes may have created both the site population at -5 °C and the colder modes identified in this paper. Could you conduct the HUB analysis on that data? I think this would add significant value.
6) The claim that similar IN behaviour implies comparable geological conditions and analogous surface properties may be overstated to my mind? I think Kiselev et al. (2021) shows that you can probably get similar relevant features on a feldspar surfaces in rather different ways.
Minor comments
The statement “In feldspars, ice preferentially nucleates on the (100) crystallographic face” should cite Kiselev et al. (2017) and Keinert et al. (2022).
I’d make sure that the resolution of Fig. 1(a) is improved in the published version.
I’d check the reference list, there are a few mistakes about, e.g B. J. Murray et al., 2021 rather than Murray et al. 2021 in the text. Kiselev et al. (2017) is incorrectly listed as 2016.
Daily, M. I., Whale, T. F., Kilbride, P., Lamb, S., John Morris, G., Picton, H. M., and Murray, B. J.: A highly active mineral-based ice nucleating agent supports in situ cell cryopreservation in a high throughput format, Journal of The Royal Society Interface, 20, 20220682, doi:10.1098/rsif.2022.0682, 2023.
Harrison, A. D., Whale, T. F., Carpenter, M. A., Holden, M. A., Neve, L., O'Sullivan, D., Vergara Temprado, J., and Murray, B. J.: Not all feldspars are equal: a survey of ice nucleating properties across the feldspar group of minerals, Atmos. Chem. Phys., 16, 10927-10940, 10.5194/acp-16-10927-2016, 2016.
Keinert, A., Deck, K., Gaedeke, T., Leisner, T., and Kiselev, A. A.: Mechanism of ice nucleation in liquid water on alkali feldspars, Faraday Discussions, 235, 148-161, 10.1039/D1FD00115A, 2022.
Kiselev, A., Bachmann, F., Pedevilla, P., Cox, S. J., Michaelides, A., Gerthsen, D., and Leisner, T.: Active sites in heterogeneous ice nucleation—the example of K-rich feldspars, Science, 355, 367-371, 10.1126/science.aai8034, 2017.
Kiselev, A., Keinert, A., Gaedecke, T., Leisner, T., Sutter, C., Petrishcheva, E., and Abart, R.: Effect of chemically induced fracturing on the ice nucleation activity of alkali feldspar, Atmos. Chem. Phys. Discuss., 2021, 1-17, 10.5194/acp-2021-18, 2021.
Whale, T. F., Holden, M. A., Kulak, A. N., Kim, Y.-Y., Meldrum, F. C., Christenson, H. K., and Murray, B. J.: The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars, Physical Chemistry Chemical Physics, 10.1039/C7CP04898J, 2017.