Structural Effects of Aluminum and Iron Occupancy in Minerals of the Jarosite-Alunite Solid Solution

The alunite supergroup of minerals contains several hydroxysulfate mineral phases that commonly occur in acidic natural and engineered environments. The main division of the mineral supergroup defines two minerals, jarosite and alunite, based on the relative structural occupancy by Al or Fe, respectively. However, intermediate members of the jarosite-alunite solid solution have not been extensively characterized, especially in the environment. Here, we link the mineral unit cell sizes measured by X-ray diffraction, peak shifts in Raman spectra, fitting parameters in Mössbauer spectroscopy, and elemental quantification by EDX spectroscopy to known amounts of Al substitution in two synthetic series of Al-substituted jarosite (up to Al-for-Fe substitution of 9.5%) and unknown Al substitution in a natural jarosite isolated from an acid sulfate soil. Strong correlations were observed between the Al substitution of the jarosite samples and unit cell size, position of several vibrational peaks in Raman spectroscopy, and the temperature of magnetic ordering. In addition, elemental mapping provided a robust way to characterize the Al content of jarosite. As the techniques were effective in quantifying the Al or Fe content of jarosite-alunite supergroup mineral samples, without the need for sample dissolution, the findings support the application of these spectroscopy techniques to characterize natural jarosite-alunite samples. Using these techniques, we demonstrate at least 5% Al-for-Fe substitution in a jarosite sample from an acid sulfate soil. Application to environmental samples is especially useful in cases where it is otherwise difficult to directly measure the Al content of a mineral sample or when Al-for-Fe substitution influences the spectral responses to substitution at other sites in the crystal structure.

1 Soil profile Figure S1: Soil profile from which the natural jarosite sample was collected.Jarosite can be seen in the profile as yellow flecks between 68 cm and 135 cm depth.Figure S3: Elemental distribution maps of unsubstituted room-temperature-synthesised jarosite (RT-Jrs).Panels A-E show the high angle annular dark field image (HAADF), distribution of S, distribution of Fe, distribution of Al and distribution of K and distribution of Si, respectively.The spectrum in the bottom panel represents to the integrated signal intensities from the area marked by the red box in panel A, from which elemental ratios were calculated (Table S3).
Figure S4: Elemental distribution maps of a sample of 0.6%-Al-for-Fe-substituted room-temperaturesynthesised jarosite (RT-Jrs).Panels A-E show the high angle annular dark field image (HAADF), distribution of S, distribution of Fe, distribution of Al and distribution of K and distribution of Si, respectively.The spectrum in the bottom panel represents to the integrated signal intensities from the area marked by the red box in panel A, from which elemental ratios were calculated (Table S3).S2A, S3A, S4A and S5a. a Elemental content presented as normalised atom percent abundance.b S-norm refers to the abundance of key elements in the jarosite structure, normalised to the expected molar ratio of S in jarosite (2 mol/mol).c The Al-for-Fe substitution is calculated as the content of Al normalised to the sum of Al and Fe and presented as a percentage (100×Fe/(Al+Fe)).3 Mineral colours       with measured substitution also indicated (Figure 3). a Corrections in the right-hand columns were carried out as described in the main text, assuming H3O-for-K substitution to the level of measured K deficiency and full Fe occupancy.6 Raman spectroscopy Figure S11: Raman spectra of all HT-Jrs, RT-Jrs and natural jarosite samples.Spectra were measured using the 1800 l/mm grating for display of spectra along a maximum possible domain.All spectra are normalised to their maximum range (peak intensity) for comparison.The natural jarosite spectrum is the average of two spectra measured using the method described in the main text.Spectra that were measured with a 3000 l/mm grating, used for the determination of peak locations (Table S5), are not plotted in this document.

Sample
Table S5: Peak positions of Raman spectra of room-temperature jarosite (RT-Jrs) and hydrothermal jarosite (HT-Jrs), measured using 3000 l mm -1 grating.Not all fitted peaks are reported, but the largest peaks are listed.Peak assignment is based on Maubec et al 3 . a The relationship between the Al substitution and measured peak locations are analysed by linear regression with gradient, m, and coefficient of determination, R 2 .b Regressions exclude samples with 0% and 0.37% Al substitution, as they are outliers.Alunite was excluded from all regression calculations.c These peaks were present in jarosite and attributable to previously reported peaks but did not match observed peaks in alunite (see explanation below).d May be associated with another O/OH-B vibration as identified in jarosite samples (see explanation below).The measured peaks for Al-substituted jarosite were well matched to previously reported peak locations for jarosite. 3The major peaks in the alunite sample could be attributed to previously reported peak locations for alunite. 3Exceptions occurred for the 'O-B(1) / framework vibration' and 'O-B(2)' peaks (Table S5), which were previously measured in alunite samples at 345 cm -1 and 381 cm -1 , respectively. 3In our alunite sample, three small peaks occurred in this range, at 327.00 cm -1 , 360.97 cm -1 and 389.78 cm -1 , and none could be attributed to the previously identified peaks.Another exception occurred for the 'O/OH-B(5)' peak.In jarosite, this peak corresponds to a peak that was previously observed near 573 cm -1 . 3The alunite peak that we observed at 567.94 cm -1 has been placed in the same column in Table S5, but may be associated with a different vibration mode.Another O/OH-B vibration, which we did not observe in our jarosite samples, has previously been identified in jarosite near 551 cm -1 . 3The peak measured in alunite at 567.94 cm -1 may be related to the 551 cm -1 vibration in jarosite.

Mössbauer spectroscopy
Figure S12: Mössbauer fits for synthetic hydrothermal jarosite with two levels of Al substitution (low substitution) at a series of temperatures between 4.2 K and 77 K.The doublet components of the fits of jarosite undergoing magnetic ordering are superimposed in grey on total spectra.All plots are normalised by integrated area of the respective fitted curve.Further plots of Mössbauer spectral fits are available in Figure S13, S14 and S15, line plots of the relative doublet/sextet areas are presented in Figure 6 and spectral data is available in Table 1 (main text) and Table S6.
Figure S13: Mössbauer fits for synthetic hydrothermal jarosite with two levels of Al substitution (high Al substitution) at a series of temperatures between 4.2 K and 77 K.The doublet components of the fits of jarosite undergoing magnetic ordering are superimposed in grey on total spectra.All plots are normalised by integrated area of the respective fitted curve.Further plots of Mössbauer spectral fits are available in Figure S12 S14, and S15, line plots of the relative doublet/sextet areas are presented in Figure 6 and spectral data is available in Table 1 (main text) and Table S6.
Figure S14: Mössbauer fits for synthetic room temperature jarosite with two levels of Al-for-Fe substitution at a series of temperatures between 4.2 K and 77 K.The doublet components of the fits of jarosite undergoing magnetic ordering are superimposed in grey on total spectra.All plots are normalised by integrated area of the respective fitted curve.Further plots of Mössbauer spectral fits are available in Figure S12, S13 and S15, line plots of the relative doublet/sextet areas are presented in Figure 6 and spectral data is available in Table 1 (main text) and Table S6.
Figure S15: Mössbauer fits for natural jarosite at a series of temperatures between 4.2 K and 77 K.The doublet components of the fits of jarosite undergoing magnetic ordering are superimposed in grey on total spectra.All plots are normalised by integrated area of the respective fitted curve.

S21
Table S6: Fitting parameters of all Mössbauer spectra plotted in Figures S12, S13, S14 and S15.Parameters for ≤5 K and 77 K spectra are also presented in Table 1.The parameters reported for each fitted phase are: Proportional area of the measured curve (Rel.spec.area), the centre-shift (CS), quadrupole splitting (QS) or quadrupole shift (), distribution of QS or  (one standard deviation; σQS or σ), hyperfine field (H) and distribution of the hyperfine field (one standard deviation; σH).When sextets were fitted with two hyperfine field components, the relative proportion of the first component is listed in the column marked 'P', and the parameters of the components from the first and second components are denotated as 'H_1' with 'σH_1' and 'H_2' with 'σH_2', respectively.Red-χ 2 is the reduced chi squared statistic to estimate the goodness of fit.Uncertainty of the last quoted significant figure is notated in parentheses or replaced with an asterisk (*) when the parameter was not floated during the fit.Uncertainties were calculated using the covariance matrix.Parameters that were not applicable to a particular component are fixed as 0*.Other than jarosite (Jrs), mineral phases identified in natural jarosite samples were goethite/ferrihydrite ('Gt/Fh'), 4 a paramagnetic Fe(II) soil phase consistent with Fe in phyllosilicate clays or organic complexes ('soil Fe(III)'), 5,6 and collapsed features associated with minerals undergoing magnetic ordering ('partially ordered').a Two techniques were applied to fit natural jarosite spectra at 77 K and 60 K, as described in the main text, and denoted here as fit v1 and v2.

Figure S2 :
Figure S2: Elemental distribution maps derived from EDX analyses of a sample of 7.3% Al-substituted hydrothermally-synthesised jarosite (HT-Jrs).Panels A-E show the high angle annular dark field image (HAADF), distribution of S, distribution of Fe, distribution of Al and distribution of K respectively.The spectrum in the bottom panel represents to the integrated signal intensities from the area marked by the red box in panel A. The red arrows in Panels B to D indicate the location of a particle that produced a signal in the HAADF image but had low S and K content, as discussed in the main text.

Figure S5 :
Figure S5: Elemental distribution maps of a sample of natural jarosite from Thailand.Panels A-F: High angular dark-field image (HAADF), distribution of S, distribution of Fe, distribution of K, distribution of Al and distribution of Si, respectively, reproduced from Figure 2. Panels G & H: The EDX spectra extracted from the areas indicated by red and blue boxes in Panel A.

Figure S6 :
Figure S6: Secondary electron (SE) images of natural jarosite.Images of synthetic minerals are found in Figure 1 (main article).

Figure S7 :
Figure S7: Substitution of Al in synthetic jarosite-alunite minerals as related to the Al content of the starting solution.

Figure S8 :
Figure S8: Fitted XRD patterns of hydrothermally synthesised jarosite minerals.Blue lines are plots of measured data and red is the best fit using a single jarosite or alunite model structure, as described in the main text.

Figure S9 :
Figure S9: Fitted XRD patterns of jarosite minerals synthesised at room temperature.Blue lines are plots of measured data and red is the best fit using a single jarosite or alunite model structure, as described in the text.

Figure S10 :
Figure S10: Rietveld fit of the XRD pattern for natural jarosite.Blue lines are plots of measured data and red is the best fit using model jarosite and quartz (Qz) structures.Additionally, the background is shown in black and the residual in grey.The goodness of fit (GoF) and Rwp are indicated on the plot with the estimated phase proportions.

Table S1 :
Elemental composition of individual jarosite particles derived from EDX measurements in areas marked by coloured squares in Figures

Table S2 :
Co2ours of jarosite samples investigated in this work, as determined by comparison of dried mineral samples with a physical Munsell colour chart.For quick comparison, sample colour squares for these colours are provided in the table from an online reference.1,2Toensure accurate colour visualisation, the reader is advised to view this table in digital format on a calibrated display, or examine a certified physical reference.

Table S3 :
Elemental analysis of jarosite samples by ICP-OES and AAS, reported as the ratios of each element to the theoretical 2 mol/mol S content of jarosite.a Jarosite synthesised by the hydrothermal (HT) and room temperature (RT) methods are identified by the ratio of Al to Fe in the synthesis solution.

Table S4 :
Parameters of the Rietveld fitting of XRD patterns of HT-Jrs, RT-Jrs and natural jarosite.The table presents unit cell sizes and crystallite sizes for all jarosite samples, as well as goodness-of-fit and Rwp as indicators of the fitting quality.All data and fitted curves are also plotted in Figures S8, S9 and S10.Each sample is identified by the ratio of Al to Fe in the synthesis solution (nominal substitution),