Occurrence of SiC and Diamond Polytypes, Chromite and Uranophane in Breccia from Nickel Laterites (New Caledonia): Combined Analyses

Round 1

Reviewer 1 Report

This paper presents an interesting novelty showing  (potential) preservation of very high pressure phases (SiC polytypes) in rocks that have undergone multi-phase transitions in very wide pressure and temperature regimes -- lower mantle all the way to epithermal deposition!

There are, however, experimental and presentational aspects that gnaw upon the promised scientific merits:

• In Sec 3.2, analyses of SiC moissanite presents Carbon contents when in the same sentence it admits that EDS is NOT a recommended technique because EDS is still XRay Fluorescence that has difficulty quantifying elements with mass numbers less than Na! The Carbon content has to be independently determined to justify this identification.
• The SiC grain in Fig 3b was selected for single crystal diffraction, but its results ARE NOT PRESENTED, only a interpretation of a proposed structure. Even the reference cited Medili et al [15] were taken from rock-salt transformation, NOT REPRESENTING LOWER MANTLE CONDITIONS.
• Diamond crystals were not illustrated and proven despite claims -- if these were clearly documented, this would have provided an independent confirmation of the pressures involved in the formation of the SiC phases.
• It is well known that sample preparation of surfaces for microscopic and SEM examination use silicon carbides as polishing and grinding compounds. Have the authors ruled out that some of these very fine SiC were not entrained in the vugs of the breccia during sample preparation?
• The paper presents a good review and use for raman studies done alongside XRD and this methodology should be pursued, especially for weathering -produced minerals which are not easily characterized by XRD due to near amorphous structures.
• The discussion on the origin of uranophane is somewhat disjointed from the presentation of the high pressure phases. For one, it is quite clear that the U-bearing phases clearly have a hydrothermal origin without having relict imprints such as SiC. It is understandable that a breccia pipe -like origin will somewhat sweep upwards to lower pressures whatever it can take, but this will not be clear to the reader who is not familiar with breccia ore deposition
• The suggestion of the transport of the ultra-reduced phases from a deep mantle source through second stage boninite melts is a HUGE jump! Boninites are formed at much lower pressures and require refractory mantle sources. Only the deposition of volatile and diamond-rich kimberlites could explain the very rapid transport of very high pressure phases to near surface. Multi-stage melting, which is implied in this paper, along with surficial weathering and hydrothermal alteration are the multiple processes that have borne down on these samples, and IF these SiC are Truly preserved high pressure phases transported to near surface, this would truly be novel. Thus, the paper needs to strengthen the basis for the high-pressure phases, beginning with accurate carbon measurements, perhaps C and O isotopes, and an independent pressure determination from observed diamonds to prove its point. As Carl Sagan said famously, great claims require great proof. 
• This is a truly interesting paper, with novel approaches to methodology in raman and XRD combinations that have great future, but the data presentation and conclusions have to be improved.
• There are also some annoying spelling errors and even section heading errors (e.g. see line 147-149)

Author Response

Dear Editor,

Dear reviewers,

The authors would like to thank the reviewers for their relevant questions and a structured peer review, which helped us to improve the content of our manuscript.

The responses are given below the remarks of the reviewer and all corrections in the manuscript and in the supplementary file are highlighted in red color.

 

 

Reviewer comments:

Reviewer #1 (Remarks to the Author):

 

This paper presents an interesting novelty showing (potential) preservation of very high pressure phases (SiC polytypes) in rocks that have undergone multi-phase transitions in very wide pressure and temperature regimes -- lower mantle all the way to epithermal deposition!

There are, however, experimental and presentational aspects that gnaw upon the promised scientific merits:

• In Sec 3.2, analyses of SiC moissanite presents Carbon contents when in the same sentence it admits that EDS is NOT a recommended technique because EDS is still XRay Fluorescence that has difficulty quantifying elements with mass numbers less than Na! The Carbon content has to be independently determined to justify this identification.

Response:

We agree with the reviewer. Neverthless, the purpose of this section is to show the incapacity of the EDS technique to confirm the presence of moissanite, hence the need to use more powerful techniques such as Single-crystal X-ray Diffraction and micro-Raman spectroscopy.

XRD diffraction and Raman analyses presented in section 3.2 confirm well the presence of moissanite. On the other hand, the EDS analyses show the existence of silicon only and thus carbon (by default).

This sentence is added in the 3.2 section: Although EDS is not accurate for low atomic number elements and not recommended as a technique for the quantification of elements lighter than Na, EDS analyzes of moissanite showed that the crystals contained no elements heavier than carbon and Si.

• The SiC grain in Fig 3b was selected for single crystal diffraction, but its results ARE NOT PRESENTED, only a interpretation of a proposed structure. Even the reference cited Medili et al [15] were taken from rock-salt transformation, NOT REPRESENTING LOWER MANTLE CONDITIONS.

Response:

The following paragraph has been added to the text:

Reflection’s splitting can be evidenced in some experimental frames and could be attributed to twinning features. Data were then integrated. The different attempts of integration of reflections using the previous unit cell were not satisfactory: the internal reliability factor Rint = 35%, quantifying the symmetry deviation from the intensity of the reflections expected to be equivalent, excludes unambiguously the hexagonal or trigonal symmetries. The reciprocal space was then interpreted in a different way by considering the orthorhombic unit cell and twin components related by a tri-fold axis parallel to c. Rint = 7.4%. The structure has been determined by charge in the Cmc21 space group and then refined. The final agreement factor is Rf = 0.0329(2) for 194 reflections with 20 refinement parameters and I ≥ 3σ(I).

 

Fig. 5. SCXRD of 6O-SiC grain. a Experimental frame exhibiting SiC diffraction spot and b Part of the (0kl)* reciprocal plan of SiC assembled from the whole experimental frames.

 

• Diamond crystals were not illustrated and proven despite claims -- if these were clearly documented, this would have provided an independent confirmation of the pressures involved in the formation of the SiC phases.

Response:

The following paragraphs have been added to the text. It will in order to clearly demonstrate the presence of diamond and to highlight the effect of pressure in the formation of the SiC:

1. Section 3.2:

 Concerning diamond, a series of polytypes are described and predicted in literature [49-52]. Two of them are observed in natural fabrics, the 3C (cubic diamond) and 2H (hexagonal lonsdaleite) diamond.

The Raman spectra of natural breccia diamonds show a sharp, first order peak of sp3-bonded carbon centered between 1331,4 and 1318 cm^-1 (Fig. 7) with FWHM (Full Width at Half Maximum) from 4.9 to 7.3 cm-1, respectively. Only the 1100-1500 region is illustrated since no other vibrational mode was observed in the region from 60 to 4000 cm^-1. Furthermore, no graphite was detected in our samples.

Fig. 7. a Scanning electron microscope (SEM) image showing a cluster composed of SiC and diamond polytypes and b Raman spectra of five different diamonds from different zones and one commercial cubic diamond.

 

From literature, it is admitted that the well-crystalized cubic diamond peak is observed at 1332 cm^-1 and that the sp3 breathing vibration mode of lonsdaleite (hexagonal diamond) can vary between 1320 and 1327 cm⁻¹ [52].

In comparison, a commercial well-crystalized cubic diamond exhibits sp³ mode centered at 1332 cm-1 with a FWHM of 3 cm^-1. This value corresponds to one of the spectra observed in Fig. 7 (1331.0 ± 0.5 cm⁻¹ with a FWHM of 4.9 ± 0.1 cm⁻¹). The slightly larger FWHM is related to a slightly lower crystallinity of the natural cubic diamond.

Raman spectrum (Fig. 7, pink) exhibiting two overlapped bands was recorded, the first at 1330.5 ± 0.2 cm⁻¹ corresponding to cubic symmetry and the second at 1326.2 ± 0.2 cm−1 assigned to hexagonal diamond. Under the microscope, we could not distinguish individual crystals, and these two contributions can be due to the existence of distorted crystals or to the presence of some sub-micrometric diamond inclusion pockets.

For the sp3 in the 1320-1327 cm^-1 region, the spectra can be attributed to hexagonal diamond polytypes. Indeed, Smith and Godard [52] earlier interpreted Raman spectra of impact diamond-bearing rocks containing the most intense band within 1320-1327 cm^-1 as the lonsdaleite contribution. The presence of lonsdaleite in their samples was also confirmed by X-ray diffraction.

For the spectra with the sp3 mode at values below 1320 cm^-1 (Fig. 7, blue), were also observed before. Nishitani-Gamo et al. [53] suggested that this shift is due to stacking faults of diamond structure, leading to the cubic-hexagonal transition. This structural change from cubic to hexagonal can be explained as the change of stacking sequence of the (111) plane. The FWHM values ranging from 6.4 to 7.3 cm^-1 are also larger, as expected, than those of cubic diamond. We can then hypothesize that the hexagonal structure is kept in these diamonds, however with some distortions explaining its slightly lower Raman shift and its larger FWHM. Finally, Fig. 7 shows a Raman spectrum with three Raman-active vibrational modes around 1212, 1307 and 1328 cm⁻¹. This spectrum is similar to those reported by Goryaninov et al. for lonsdaleite [54].

In literature, although the theoretical calculations of the lonsdaleite vibration spectrum have been the subject of many investigations for years, several ambiguities and contradictions remain present for the vibrational attribution and the position of the bands [54-56].

For this reason, the Raman-active vibrational modes of cubic and hexagonal diamonds were calculated through Density Functional Theory (DFT). We have used the CRYSTAL software [57]. We performed these calculations applying harmonic approximation at the Γ point as it was done by Goryainov et al [54]. for Raman identification of lonsdaleite. We have found for cubic diamond that the position of single Raman vibration band, corresponding to the first-order scattering of F_2g symmetry, is 1331.99 cm^-1. This value is in good agreement with the experimental value. For hexagonal diamond (2H, lonsdaleite), we predict three fundamental vibrational modes: 1207 cm−1 (E_2g), 1307 cm⁻¹ (A_1g), and 1330 cm⁻¹ (E_1g). The theoretical intensity ratio of the Raman modes is: 0.5(E_2g):1(A_1g):0.3(E_1g). Hence, the A_1g mode is expected to be the most intense line in the Raman spectrum of lonsdaleite.

Based on the comparison between calculated and experimental results (Fig. 8), the most intense band in the experimental Raman spectrum at 1307 cm^–1 can be attributed unambiguously to the longitudinal optical vibrational mode A_1g.

The deconvolution analysis of the Raman spectrum let us identify the shoulder as a Raman contribution of the second intensity of the transverse optical vibrational mode (E_2g) observed at 1212 cm^-1. This is in agreement with our ab-initio calculations. DFT calculations also predict that the third Raman-active mode of lonsdaleite E1g (transverse optical) should be observed at 1330 cm^-1, with its theoretically predicted intensity being close to that of the second intensity mode E_2g. The E_2g mode is observed in our spectrum around 1328 cm^-1.

Fig. 8. Raman spectrum of lonsdaleite. a Experimental Raman spectrum of lonsdaleite zoomed from Fig. 2f, b The three Raman-active vibrational modes E_2g, A_1g and E_1g obtained via deconvolution of the experimental spectrum, c Theoretical Raman spectrum of lonsdaleite obtained through ab initio calculations with Lorentzian line shape and FWHM of 10 cm-1 and d The three Raman vibration modes of lonsdaleite.

Experimentally, the lonsdaleite bands are highly broadened (FWHM are in the range of 18–52 cm^-1), compared to the cubic diamond observed in our sample. This effect is due to lonsdaleite imperfection and to small dimensions of crystallites.

The originality of these results comes from the fact that lonsdaleite has been observed so far only in the meteorite from the Meteor Crater [49,54]. Probable contribution of other hexagonal diamond polytypes to Raman spectra can be masked by the presence of large and asymmetric bands [52].

In Fig. 4g, the Raman spectrum of chromite shows four bands associated to the CrO bond-stretching region at 905, 730, 560 and 445 cm^-1 [58]. The very intense and broad band at 730 cm^-1 is assigned to symmetric stretching vibrational mode, A_1g(ν₁). The two peaks at 560 and 445 cm^-1 are attributed to F_2g(ν₄) and E_g(ν₂) modes respectively, and the small band at 215 cm^-1 to the F_2g(ν₃) mode.

 

 

2. Section 4:

 

In nature, lonsdaleite is mainly associated with diamonds. It can form under ultrahigh pressures (>130 kbar), at depths > 150 km, at temperatures above 950°C at PO2 close to IW (Iron-wüstite) [69,70]. Since it is described from meteorites and rocks having experienced shock metamorphism [71], intensive studies have been performed [72]. Lonsdaleite is difficult to analyze as always tightly intergrown with diamond, sometimes graphite, and is of inframicrometric size [72]. These phases often present dislocations, stacking faults, twins and grain boundary disordering. Therefore, microanalysis and structural interpretations of XRD and TEM-SAED may be erroneous due to fuzzy spectra [73].

Although we suggest that these phases are inherited from the peridotites, further studies must be carried out to support the model that reduced phases may have been transported from the deeper mantle through boninite type melts into the upper mantle peridotite. This is beyond the scope of this study. 

In this study, analyzes of spectral data from Micro-Raman spectroscopy were compared and supplemented by results from DFT. This methodology allowed unambiguously to identify lonsdaleite and cubic diamond. The mineral association (lonsdaleite, moissanite, chromite, native chromium) observed in the unaltered chromitite and peridotite from the above-mentioned localities is similar to that observed in the siliceous breccia from the Tiebaghi mine. The moissanite described from peridotites and chromitites in ophiolites are mainly subhedral to anhedral, similarly to those observed in the siliceous breccia pores at Tiebaghi. We therefore suggest that the ultra-reduced phases come from a deep mantle source and are brought to the upper mantle peridotite through a second stage reducing boninitic melt. For the origin of the diamond-lonsdaleite grains, it is suggested that this association is inherited from the serpentinized peridotites.

 

The Title is also changed into: Occurrence of SiC and diamond polytypes, chromite and uranophane in breccia from nickel laterites (New Caledonia): Combined analyses

• It is well known that sample preparation of surfaces for microscopic and SEM examination use silicon carbides as polishing and grinding compounds. Have the authors ruled out that some of these very fine SiC were not entrained in the vugs of the breccia during sample preparation?

Response:

These sections are added in the discussion section to confirm the absence of breccia contamination:

Before discussing the origin of diamond and the associated strongly reducing phases appearing in the pores of the breach, we argue that these phases are artefacts, generated during sample preparation through diamond sawing: (i) the sample preparation was carried out identically for the three other rocks (sandstone, harzburgite, granite), all sawn with the same diamond blade [20]. (ii) all the samples were studied by micro-Raman spectroscopy [20,21], but only the siliceous breccia hosts diamond polymorphs. (iii) the intergrowth of diamond and lonsdaleite is unlikely being a feature for diamond saw derived particles. (iv) clusters of chromite, diamond and SiC polymorph are observed in natural environment [2,5,29,26]. Therefore, the idea of contamination was ruled out.

• The paper presents a good review and use for raman studies done alongside XRD and this methodology should be pursued, especially for weathering -produced minerals which are not easily characterized by XRD due to near amorphous structures.

Response:

We agree with the reviewer. This study highlights the potential and importance of particularly micro-Raman spectroscopy, which provides a powerful, rapid tool without complex sample preparation to achieve the structural properties of weathering-produced minerals.

• The discussion on the origin of uranophane is somewhat disjointed from the presentation of the high pressure phases. For one, it is quite clear that the U-bearing phases clearly have a hydrothermal origin without having relict imprints such as SiC. It is understandable that a breccia pipe -like origin will somewhat sweep upwards to lower pressures whatever it can take, but this will not be clear to the reader who is not familiar with breccia ore deposition

Response:

The 'Breccia' term is explained in the sample description and in order to avoid misunderstanding, in the beginning the descriptive term: 'silica-rich rock' is used.

In addition, the origin of U has been removed from the abstract to avoid misunderstanding.

• The suggestion of the transport of the ultra-reduced phases from a deep mantle source through second stage boninite melts is a HUGE jump! Boninites are formed at much lower pressures and require refractory mantle sources. Only the deposition of volatile and diamond-rich kimberlites could explain the very rapid transport of very high pressure phases to near surface. Multi-stage melting, which is implied in this paper, along with surficial weathering and hydrothermal alteration are the multiple processes that have borne down on these samples, and IF these SiC are Truly preserved high pressure phases transported to near surface, this would truly be novel. Thus, the paper needs to strengthen the basis for the high-pressure phases, beginning with accurate carbon measurements, perhaps C and O isotopes, and an independent pressure determination from observed diamonds to prove its point. As Carl Sagan said famously, great claims require great proof. 

Response:

The scope of the study was to show that reduced phases survived hydrothermal alteration and weathering, therefor we changed the text accordingly:

Although we suggest that these phases are inherited from the peridotites, further studies are needed to support the hypothesis that reduced phases may have been transported from the deeper mantle through boninite type melts into the upper mantle peridotite. This is beyond the scope of this study.

Nevertheless, the presence of diamond polytypes crystals were illustrated and proven (by Micro-Raman combined with DFT), which independently confirms the involvement of high pressures in the formation of SiC phases. Indeed, micrometric porosities observed by SEM and optical microscopy (Fig. 4.a and 7a ) incorporate small crystals and clusters of crystals varying from blue to white and colorless-transparent (Fig. 4c). Interestingly, micro-Raman measurements on these clusters reveal the presence of chromite, micrometric size diamonds and SiC. Individual diamond grains were separated from these clusters. The diamond crystals are usualy transparents and colorless. SiC (moissanite) occurs either as single grains, sometimes having light blue color, or is associated with diamond as a composite inclusion, green to green and colorless irregular flakes or fragments

• This is a truly interesting paper, with novel approaches to methodology in raman and XRD combinations that have great future, but the data presentation and conclusions have to be improved.

Response:

We agree with the reviewer. The figures and the conclusion have been improved.

• There are also some annoying spelling errors and even section heading errors (e.g. see line 147-149)

Response:

The spelling errors were corrected

 

Reviewer 2 Report

Review of the manuscript

Occurrence of SiC polytypes, chromite and uranophane in breccia from nickel laterites (New Caledonia): Combined analyses

Authors: Yassine El Mendili, Beate Orberger, Daniel Chateigner, Jean-François Bardeau, Stéphanie Gascoin, Sébastien Petit and Olivier Perez

The paper describes a rare mineralization in a breccia from nickel laterites  of the Yiebaghi deposit. The rocks and mineralization of this deposit have been described in detail in previous papers. El Mendili et al. (2019) already studied the 6O-SiC polytype in the siliceous breccia and connected its origin with the lower mantle processes. The authors suggested its complicated origin and augured that “6O-SiC results from the 6H-SiC wurtzite to 3C-SiC rock-salt phase transformation as an intermediate state. The 6O-SiC formation requires at least 4 GPa of pressures and high temperatures 2027–2527 °C” (Lines 224–226). The authors believe that “these phases were transported into the upper mantle by a second stage boninitic melt.” (line 25). Later on it was included somehow into porous siliceous rock (siliceous breccia) that was studied in the lower part of a regolith. In this paper, the authors again follow this hypothesis. The merit of this version is the thoroughly performed, detailed and clearly described results of the determination of rare mineral phases with the integrated use of several modern methods.

The work is recommended for publication with minor corrections.

Major comment:

No geologically reliable evidence is proposed for the lower mantle origin of SiC phase. In the paper, it sounds like this one is itself indicator of the lower mantle material. The authors do not discuss possibilities of non-lower mantle origins of moissonite: technogenic, serpentinization-related, impact, or meteorite. In many cases where this mineral phase was described, moissonite formed independently, as compared to associated minerals, in super-reduced conditions.

Minor corrections are required:

Table 1. The EDS results are obviously given in terms of 100% without elements lighter than silicon. For this reason, the Si and Mg values (possibly Fe) should be omitted in the EDS column, because these values are overestimated due to recalculation and cannot be compared with those determined by the XRF method.

Lines 148-150.  The first paragraph of the Results looks like a copied part of the manual about what the section should be.

Line 151. “The chemical analyses by X-Ray Fluorescence (XRF) indicates…”.  What was analyzed?

Lines 273 and 279. H20 change to H2O.

Lines 355–360. The last two sentences in the conclusion are to be reversed or the last sentence should be omitted.

Comments for author File: Comments.pdf

Author Response

Dear Editor,

Dear reviewers,

 

The authors would like to thank the reviewers for their relevant questions and a structured peer review, which helped us to improve the content of our manuscript.

The responses are given below the remarks of the reviewer and all corrections in the manuscript and in the supplementary file are highlighted in red color.

 

 

Reviewer #2 (Remarks to the Author):

Review of the manuscript

Occurrence of SiC polytypes, chromite and uranophane in breccia from nickel laterites (New Caledonia): Combined analyses

Authors: Yassine El Mendili, Beate Orberger, Daniel Chateigner, Jean-François Bardeau, Stéphanie Gascoin, Sébastien Petit and Olivier Perez

The paper describes a rare mineralization in a breccia from nickel laterites  of the Yiebaghi deposit. The rocks and mineralization of this deposit have been described in detail in previous papers. El Mendili et al. (2019) already studied the 6O-SiC polytype in the siliceous breccia and connected its origin with the lower mantle processes. The authors suggested its complicated origin and augured that “6O-SiC results from the 6H-SiC wurtzite to 3C-SiC rock-salt phase transformation as an intermediate state. The 6O-SiC formation requires at least 4 GPa of pressures and high temperatures 2027–2527 °C” (Lines 224–226). The authors believe that “these phases were transported into the upper mantle by a second stage boninitic melt.” (line 25). Later on it was included somehow into porous siliceous rock (siliceous breccia) that was studied in the lower part of a regolith. In this paper, the authors again follow this hypothesis. The merit of this version is the thoroughly performed, detailed and clearly described results of the determination of rare mineral phases with the integrated use of several modern methods.

The work is recommended for publication with minor corrections.

Major comment:

• No geologically reliable evidence is proposed for the lower mantle origin of SiC phase. In the paper, it sounds like this one is itself indicator of the lower mantle material. The authors do not discuss possibilities of non-lower mantle origins of moissonite: technogenic, serpentinization-related, impact, or meteorite. In many cases where this mineral phase was described, moissonite formed independently, as compared to associated minerals, in super-reduced conditions.

 

Response:

The scope of the study was to show that reduced phases survived hydrothermal alteration and weathering, therefor we changed the text accordingly:

Although we suggest that these phases are inherited from the peridotites, further studies must be carried out to support the model that reduced phases may have been transported from the deeper mantle through boninite type melts into the upper mantle peridotite. This is beyond the scope of this study.

Nevertheless, the presence of diamond polytypes crystals were illustrated and proven (by Micro-Raman combined with DFT), which provide an independent confirmation of the pressures involved in the formation of the SiC phases. Indeed, micrometric porosities observed by SEM and optical microscopy (Fig. 4.a and 7a ) incorporate small crystals and clusters of crystals varying from blue to white and colorless-transparent (Fig. 4c). Interestingly, Micro-Raman measurements on these clusters reveal the presence of chromite, micrometric size diamonds and SiC individual diamond grains were separated from these clusters. The diamond crystals are usualy transparents and colorless. SiC (moissanite) occurs either as single grains, sometimes having light blue color, or is associated with diamond as a composite inclusion, green to green and colorless irregular flakes or fragments.

These sections are added in the text to clearly demonstrate the presence of diamond and thus the confirmation of the pressure involved in the formation of the SiC:

1. Section 3.2:

 Concerning diamond, a series of polytypes are described and predicted in literature [49-52]. Two of them are observed in natural fabrics, the 3C (cubic diamond) and 2H (hexagonal lonsdaleite) diamond.

The Raman spectra of natural breccia diamonds show a sharp, first order peak of sp3-bonded carbon centered between 1331,4 and 1318 cm^-1 (Fig. 7) with FWHM (Full Width at Half Maximum) from 4.9 to 7.3 cm-1, respectively. Only the 1100-1500 region is illustrated since no other vibrational mode was observed in the region from 60 to 4000 cm^-1. Furthermore, no graphite was detected in our samples.

Fig. 7. a Scanning electron microscope (SEM) image showing a cluster composed of SiC and diamond polytypes and b Raman spectra of five different diamonds from different zones and one commercial cubic diamond.

 

From literature, it is admitted that the well-crystalized cubic diamond peak is observed at 1332 cm^-1 and that the sp3 breathing vibration mode of lonsdaleite (hexagonal diamond) can vary between 1320 and 1327 cm⁻¹ [52].

In comparison, a commercial well-crystalized cubic diamond exhibits sp³ mode centered at 1332 cm-1 with a FWHM of 3 cm^-1. This value corresponds to one of the spectra observed in Fig. 7 (1331.0 ± 0.5 cm⁻¹ with a FWHM of 4.9 ± 0.1 cm⁻¹). The slightly larger FWHM is related to a slightly lower crystallinity of the natural cubic diamond.

Raman spectrum (Fig. 7, pink) exhibiting two overlapped bands was recorded, the first at 1330.5 ± 0.2 cm⁻¹ corresponding to cubic symmetry and the second at 1326.2 ± 0.2 cm−1 assigned to hexagonal diamond. Under the microscope, we could not distinguish individual crystals, and these two contributions can be due to the existence of distorted crystals or to the presence of some sub-micrometric diamond inclusion pockets.

For the sp3 in the 1320-1327 cm^-1 region, the spectra can be attributed to hexagonal diamond polytypes. Indeed, Smith and Godard [52] earlier interpreted Raman spectra of impact diamond-bearing rocks containing the most intense band within 1320-1327 cm^-1 as the lonsdaleite contribution. The presence of lonsdaleite in their samples was also confirmed by X-ray diffraction.

For the spectra with the sp3 mode at values below 1320 cm^-1 (Fig. 7, blue), were also observed before. Nishitani-Gamo et al. [53] suggested that this shift is due to stacking faults of diamond structure, leading to the cubic-hexagonal transition. This structural change from cubic to hexagonal can be explained as the change of stacking sequence of the (111) plane. The FWHM values ranging from 6.4 to 7.3 cm^-1 are also larger, as expected, than those of cubic diamond. We can then hypothesize that the hexagonal structure is kept in these diamonds, however with some distortions explaining its slightly lower Raman shift and its larger FWHM. Finally, Fig. 7 shows a Raman spectrum with three Raman-active vibrational modes around 1212, 1307 and 1328 cm⁻¹. This spectrum is similar to those reported by Goryaninov et al. for lonsdaleite [54].

In literature, although the theoretical calculations of the lonsdaleite vibration spectrum have been the subject of many investigations for years, several ambiguities and contradictions remain present for the vibrational attribution and the position of the bands [54-56].

For this reason, the Raman-active vibrational modes of cubic and hexagonal diamonds were calculated through Density Functional Theory (DFT). We have used the CRYSTAL software [57]. We performed these calculations applying harmonic approximation at the Γ point as it was done by Goryainov et al [54]. for Raman identification of lonsdaleite. We have found for cubic diamond that the position of single Raman vibration band, corresponding to the first-order scattering of F_2g symmetry, is 1331.99 cm^-1. This value is in good agreement with the experimental value. For hexagonal diamond (2H, lonsdaleite), we predict three fundamental vibrational modes: 1207 cm−1 (E_2g), 1307 cm⁻¹ (A_1g), and 1330 cm⁻¹ (E_1g). The theoretical intensity ratio of the Raman modes is: 0.5(E_2g):1(A_1g):0.3(E_1g). Hence, the A_1g mode is expected to be the most intense line in the Raman spectrum of lonsdaleite.

Based on the comparison between calculated and experimental results (Fig. 8), the most intense band in the experimental Raman spectrum at 1307 cm^–1 can be attributed unambiguously to the longitudinal optical vibrational mode A_1g.

The deconvolution analysis of the Raman spectrum let us identify the shoulder as a Raman contribution of the second intensity of the transverse optical vibrational mode (E_2g) observed at 1212 cm^-1. This is in agreement with our ab-initio calculations. DFT calculations also predict that the third Raman-active mode of lonsdaleite E1g (transverse optical) should be observed at 1330 cm^-1, with its theoretically predicted intensity being close to that of the second intensity mode E_2g. The E_2g mode is observed in our spectrum around 1328 cm^-1.

Fig. 8. Raman spectrum of lonsdaleite. a Experimental Raman spectrum of lonsdaleite zoomed from Fig. 2f, b The three Raman-active vibrational modes E_2g, A_1g and E_1g obtained via deconvolution of the experimental spectrum, c Theoretical Raman spectrum of lonsdaleite obtained through ab initio calculations with Lorentzian line shape and FWHM of 10 cm-1 and d The three Raman vibration modes of lonsdaleite.

Experimentally, the lonsdaleite bands are highly broadened (FWHM are in the range of 18–52 cm^-1), compared to the cubic diamond observed in our sample. This effect is due to lonsdaleite imperfection and to small dimensions of crystallites.

The originality of these results comes from the fact that lonsdaleite has been observed so far only in the meteorite from the Meteor Crater [49,54]. Probable contribution of other hexagonal diamond polytypes to Raman spectra can be masked by the presence of large and asymmetric bands [52].

In Fig. 4g, the Raman spectrum of chromite shows four bands associated to the CrO bond-stretching region at 905, 730, 560 and 445 cm^-1 [58]. The very intense and broad band at 730 cm^-1 is assigned to symmetric stretching vibrational mode, A_1g(ν₁). The two peaks at 560 and 445 cm^-1 are attributed to F_2g(ν₄) and E_g(ν₂) modes respectively, and the small band at 215 cm^-1 to the F_2g(ν₃) mode.

 

 

2. Section 4:

 

In nature, lonsdaleite is mainly associated with diamonds. It can form under ultrahigh pressures (>130 kbar), at depths > 150 km, at temperatures above 950°C at PO2 close to IW (Iron-wüstite) [69,70]. Since it is described from meteorites and rocks having experienced shock metamorphism [71], intensive studies have been performed [72]. Lonsdaleite is difficult to analyze as always tightly intergrown with diamond, sometimes graphite, and is of inframicrometric size [72]. These phases often present dislocations, stacking faults, twins and grain boundary disordering. Therefore, microanalysis and structural interpretations of XRD and TEM-SAED may be erroneous due to fuzzy spectra [73].

Although we suggest that these phases are heritated from the peridotites, further studies must be carried out to support the model that reduced phases may have been transported from the deeper mantle through boninite type melts into the upper mantle peridotite. This is beyond the scope of this study. 

In this study, Micro-Raman spectroscopy coupled with DFT was used. These methodologies allowed unambiguously to identify lonsdaleite and cubic diamond. The mineral association (lonsdaleite, moissanite, chromite, native chromium) observed in the unaltered chromitite and peridotite from the above-mentioned localities is similarly to that observed in the siliceous breccia from the Tiebaghi mine. The moissanite described from peridotites and chromitites in ophiolites are mainly subhedral to anhedral, similarly to those observed in the siliceous breccia pores at Tiebaghi. We therefore suggest that the ultra-reduced phases come from a deep mantle source and are brought to the upper mantle peridotite through a second stage reducing boninitic melt. For the origin of the diamond-lonsdaleite grains, it is suggested that this association is inherited from the serpentinized peridotites.

 

The Title is also changed into: Occurrence of SiC and diamond polytypes, chromite and uranophane in breccia from nickel laterites (New Caledonia): Combined analyses

Minor corrections are required:

• Table 1. The EDS results are obviously given in terms of 100% without elements lighter than silicon. For this reason, the Si and Mg values (possibly Fe) should be omitted in the EDS column, because these values are overestimated due to recalculation and cannot be compared with those determined by the XRF method.

Response:

We agree with the reviewer. For this reason, we decided to delete the EDS analyses and keep only the composition obtained by XRF which is more precise in particular for oxygen.

• Lines 148-150.  The first paragraph of the Results looks like a copied part of the manual about what the section should be.

Response:

This paragraph is removed.

 

 

• Line 151. “The chemical analyses by X-Ray Fluorescence (XRF) indicates…”.  What was analyzed?

Response:

This sentence is replaced by ‘The chemical analyses by X-Ray Fluorescence (XRF) of the breccia surface indicates ~88 wt.% of silica and 1.9 wt.% Fe₂O₃ (as total iron). Traces of CaO and MgO (0.1 and 0.7 wt.%, respectively). In term of elemental composition, XRF indicate the presence of Si, Mg, Fe, and other trace elements such as Ni and Cr (Table 1).’

• Lines 273 and 279. H20 change to H2O.

Response:

We replaced H₂0 by H₂O

• Lines 355–360. The last two sentences in the conclusion are to be reversed or the last sentence should be omitted.

Response:

We agree with the reviewer. The conclusion has been improved:

In this paper, we reveal for the first time the association of SiC and diamond polytypes, chromite and uranophane in a nickel laterite profile at Tiebaghi (New Caledonia), unambiguously defined by Micro Raman spectroscopy and XRD. The diamond and moissanite polytypes most likely crystallized in the lower mantle together with chromite, and were transported through boninite type melts into the upper mantle peridotite.

Based on these studies, the breccia at Tiebaghi needs to be further investigated and may present exploration potential, as it acts as a trap for weathering-resistant valuable minerals, and elements migrating in low temperature silica rich environments, such as U, Ni, Cr.

This study also highlights the potential and importance of micro-Raman spectroscopy to achieve the structural properties of porous materials. This technique is thus crucially important for mining companies to rapidly access detailed mineralogical compositions without any sample preparation.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Revisions largely answered the questions raised after the review. Although I would argue that the responses can still be shortened/simplified, they have helped to clarify the issues raised. This makes this paper significant especially in the methodology of identifying high pressure phases preserved in surficial rocks. 

Author Response

The authors would like to thank the reviewer for it relevant questions and a structured peer review, which helped us to improve the content of our manuscript.

All corrections as suggested in the manuscript are highlighted in red color.