Temperature transformation of calcium and potassium Martian sulfates as seen by an Exomars 2022 RLS-like Raman instrument

1. Introduction

Mars orbiters have detected different sulfates in the new landing sites of the upcoming Mars missions, Jezero crater (Mars2020) [1] and Oxia Planum region (Exomars2022) [2]. Both missions incorporate the Raman instrument to study the mineral phases in the planet, but the Raman Laser Spectrometer (RLS) has the capability to spot at microscopic scale of 50 microns. Thus, the possibility to detect sulfate minerals with the RLS instrument in the drilled samples, taken at different depths in the Martian sub-surfaces, must be considered. The expected sulfate mineral phases could contain crystallized water, and it is known that hydrated compounds have Raman responses sensitive to the temperature, especially when decreasing [3] but also when increasing due to mineral transformations. For this reason, some Raman essays at different temperatures were made for the sulfates gypsum [CaSO₄·2H₂O], syngenite [K₂Ca(SO₄)₂·H₂O] and görgeyite [K₂Ca₅(SO₄)₆·H₂O] at high and low temperatures.

2. Sample description

The samples gypsum, syngenite and görgeyite used were previously synthesized  by García-Florentino et al. [4]. The gypsum and syngenite samples were obtained at room temperature by mixing the adequate liquids containing the ions at concentrations to attain saturation for each salt. Görgeyite was obtained by using hydrothermal conditions (90 ºC during 8 hours) with the appropriate ion concentrations.

3. Materials and Methods

The sulfate synthesized samples [4] were analyzed at spot sizes of 50 microns using a Renishaw inVia micro-Raman spectrometer, equipped with the 532 nm excitation laser and a highly sensitive CCD detector, with a mean spectral resolution of 1 cm^-1. The spectrometer was coupled to a temperature-controlled stage THMS600/HFS600 Linkam Scientific Instrument (UK) for the automatic control of the measurement temperature. The high temperature program used consisted of an increment of 20 ºC.min^-1 up to 400 ºC. Whereas the low temperature program consisted of a decrease of 10 ºC.min^-1 up to -100 ºC. Each ramp was followed by a one-minute hold to allow the stabilization of the mineral phase under study. And the spectra were collected both at the end of the ramp and at the end of the hold

4. Results and Discussion

As a consequence of the temperature increase, the shift to lower or higher wavenumbers of the main Raman bands were observed in all the spectra. However, the drop in temperature did not cause any shift in the position of the sulfate Raman bands although changes in the form of the Raman bands were observed. Therefore, the following features were noted:

Gypsum. The transition from gypsum to anhydrite III (AIII) at a temperature of 180 ºC was observed. The transformation was detected by the shift of the 1008 cm^-1 main Raman band to 1025 cm^-1 and together with the disappearance of the hydration bands. The AIII form is metastable, reaching its stable form (Anhydrite II and I) above 800 ºC [5]. Although the AI/II could not be completely isolated because the experiment only reached 400 ºC, the presence of both compounds can be easily observed by a deconvolution of the main Raman band collected at 400 ºC, obtaining the bands at 1017 and 1025 cm^-1, the main Raman bands of AI/II and AIII, respectively.

When temperature decreases, the Raman bands due to the sulfate modes do not change. However, the two water bands at 3405 and 3487 cm^-1 change in shape (both are more thin when temperature increases) but not in the wavenumber of the maximum.

Syngenite. This compound is stable when temperature increases until 100 ºC where the transformation to soluble anhydrite starts. Anhydrite(III) was not obtained in pure form, suggesting a kinetic control of the dehydration process. Above 360 ºC the main Raman band of anhydrite I/II (1017 cm^-1) is observed. In addition, from 380 ºC the 981 cm^-1 and all the hydration bands disappeared, because the syngenite suffered the transformation to arcanite [K₂SO₄] and langbeinite [K₂Ca₂(SO₄)₃], which main Raman bands are located at 985 and 978 cm^-1, respectively [6].

The temperature decrease do not affect the maximum of any bands of syngenite. However, the broad water band at room temperature is splitted in two bands at 3107 and 3298 cm^-1 when temperature decreases below -40 ºC, being both more thinner when temperature continues decreasing.

Görgeyite. Above 160 ºC the görgeyite Raman band at 1012 cm^-1 decreased, due to the fact that part of this sulfate was transformed to syngenite (980 cm^-1) and gypsum (1007 cm^-1). The corresponding bands due to the crystallized water also decreased.

The temperature decrease do not affect Raman bands related to sulfate modes, but the broad water band at room temperature splits at <-10 ºC, appearing two bands at 3527 and 3582 cm^-1, being the second one less intense than the first, in the opposite trend to other compounds. Thus, although görgeyite was hydrothermally obtained, it is quite stable when temperature decreases at Martian levels.

5. Conclusions

The increase of temperature causes the shift of the main sulfate Raman bands to other wavenumbers due to the formation of other anhydrous mineral phases. Above 180 ºC the gypsum loses its water molecules, forming the different anhydrite. Whereas, syngenite decomposes to arcanite [K₂SO₄] and langbeinite [K₂Ca₂(SO₄)₃] while görgeyite decomposes first to gypsum plus syngenite and then follows the respective decomposition.

However, the effect of low temperature does not produce any change in that bands position although the water bands are more clearly observed.

6. References

[1] Salvatore, M. R., et al.: Bulk mineralogy of the NE Syrtis and Jezero crater regions of Mars derived through thermal infrared spectra analyses, Icarus, 301, 76-96, 2018.  

[2] Dugdale, A., et al.: Development of Oxia Planum simulant relevant to the Exomars mission, 51^th LPSC, Abs. 2590, The Woodlands(TX), 2020.

[3] Iriarte, L.M., et al.: Reference Raman spectra of synthetized CaCl₂.nH₂O solids (n=0, 2, 4, 6), J. Raman Spectrosc., 46, 822-828, 2015

[4] García-Florentino, C., et al.: Transformations in the Gypsum-Syngenite-Görgeyite system to describe their possible formation on Mars, Astrobiology, submitted, 2020.

[5] Prieto-Taboada, N., et al.: Raman Spectra of the Different Phases in the CaSO₄-H₂O System, Analytical Chemistry, 86, 10131-10137, 2014.

[6] Chung, C-W. and Lee, J-Y.: Premature Stifferning of Cement Paste Caused by Secondary Gypsum and Syngenite Formation (False Set), Architectural Research, 13, 25-32, 2011