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Research ArticleReceived: 11 March 2009 Accepted: 27 September 2009 Published online in Wiley Online Library: 13 January 2010

(wileyonlinelibrary.com) DOI 10.1002/jrs.2547

Raman spectra of mirabilite, Na2SO4·10H2Oand the rediscovered metastableheptahydrate, Na2SO4·7H2OAndrea Hamilton∗ and Robert I. Menzies

Salt crystallisation in pores is known to cause serious damage to masonry. Sodium sulphate, often regarded as one of themost damaging salts, has a rich hydrate chemistry including one rediscovered metastable hydrate and a new high pressureoctahydrate plus five known polymorphs of the anhydrous phase. The difficulty in working with these hydrates lies in theirstrong tendency to dehydrate or to convert to the stable phase, in the case of the heptahydrate. We present Raman spectraand a table of peak wavenumbers for randomly oriented crystals of mirabilite and the metastable heptahydrate, sufficientto distinguish between these phases that have SO4 ν1 values of 989.3 and 987.6 cm−1, respectively. Mirabilite has a Ramanspectrum very similar to the free sulphate anion in solution, which is probably due to the mobility of oxygen atoms within thesulphate tetrahedron. The oxygen atoms in the heptahydrate sulphate groups have no partial occupancy, and predicted peaksplitting is observed in the region 400–1200 cm−1. Copyright c© 2010 John Wiley & Sons, Ltd.

Keywords: sodium sulphate heptahydrate; mirabilite; Raman spectroscopy; thenardite; stone deterioration

Introduction

Sodium sulphate is regarded as one of the most damaging saltsfor masonry, also affecting works of art, the built environment[1 – 3]

and terrestrial[4] landforms through crystallisation in the pores ofstone.[5] At ambient pressure and temperatures below 32.4 ◦C,mirabilite (sodium sulphate decahydrate) is the stable phasein contact with moisture. It dehydrates to the more solubleanhydrous phase, thenardite, when exposed to dry air (RH<75% at 20 ◦C). Thenardite is stable at temperatures above32.4 ◦C, up to 185 ◦C, and is one of at least five reportedpolymorphs of the anhydrous phase. The other polymorphs arestable at higher temperatures,[6] and four of them have knowncrystal structures.[7] Nineteenth-century and early twentieth-century literature discusses a metastable heptahydrate of sodiumsulphate,[8 – 10] which has been largely absent from chemicalliterature until very recently. There are several reasons for this: theheptahydrate shows a strong tendency to convert to mirabilitewhen exposed to ambient laboratory air and the current X-raydiffraction (XRD) pattern in the JCPDS database (pdf card no.40–0727) seems to be in error. Card no. 40–0727 is referencedto a short report by Mehrotra,[11] which gives no details ofsample preparation for XRD. Given the ease of conversion tomirabilite and the subsequent formation of thenardite throughdehydration when exposed to the atmosphere, some care hasto be taken in preparing and covering the sample prior toand during analysis. Heptahydrate has been shown to form inporous materials saturated with solution by 23Na NMR[2], whichallowed detection of the solid phase through comparison ofsolution concentration with literature solubility data.[12] Morerecently, the first full powder XRD pattern for heptahydrate[13] hasbeen obtained using energy dispersive synchrotron XRD alongwith direct evidence of its formation in porous materials.[14] Itscrystal structure has been determined through laboratory single-crystal diffraction[15] as well as the crystal structure of a previously

unknown octahydrate, formed under pressure. It has also beenpostulated that heptahydrate is the stable phase under conditionsof low temperature and water vapour pressure,[16] consistent withthe atmosphere on Mars.[17]

Raman spectroscopy is a powerful and well-developed toolfor investigating phases in the liquid or solid state. Advancesin instrumentation have produced portable spectrometers forin situ use on Earth and possibly Mars through development ofthe Mars Microbeam Raman Spectrometer.[18] Portable Ramanspectrometers have been used to study deteriorating buildingstones[19] and glazed ceramics.[20] The usefulness of Ramanspectroscopy in detecting hydrated salts on Mars and in thepores of stone requires an accurate database of all solid phaseslikely to be encountered. To date, there are no accurate spectrapresented together with peak wavenumber values for both ofthese sodium sulphate hydrates in the literature. Edwards et al.[21]

report values for sodium sulphate heptahydrate but, as these wereobtained from a commercial reagent, it cannot possibly be theheptahydrate and is presumably mirabilite. However, they reportmirabilite peak wavenumbers, which are inconsistent with themirabilite data presented here. Also, the S–O stretching modevalue they report for thenardite at 990 cm−1 is approximately3 cm−1 lower than that described by Choi and Lockwood[22] andthe RRUFF database.[23] A heptahydrate spectrum by Theimer[24]

and referenced by Gmelin gives only two peak positions at 996and 3434 cm−1, which do not agree with data presented here.Ohno et al.[25] show a mirabilite spectrum as part of their analysisbut only give a value for the SO4 symmetric stretching mode.Sarmiento et al.[26] give a list of peak wavenumbers between 452

∗ Correspondence to: Andrea Hamilton, School of Engineering, University ofEdinburgh, Edinburgh EH9 3JL, UK. E-mail: andrea.hamilton@ed.ac.uk

School of Engineering, University of Edinburgh, Edinburgh EH9 3JL, UK

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Raman spectra of Na2SO4·10H2O and Na2SO4·7H2O

and 1152 cm−1 only; therefore, there is some need for a full list ofpeak wavenumbers in the literature.

Experimental

A Renishaw InVia Raman microscope was used with a 785 nmdiode laser and a power range of 5–100 mW. For examining theregion of water activity, a second InVia Raman system with a514 nm argon ion laser was used. All spectra were recorded usingthe 20× objective (both lasers) from a Leica microscope and aPeltier cooled CCD detector. Spectra were acquired over the range100–4000 cm−1 and precision was ±1 cm−1. Count time variedbetween 10 and 120 s for examining the region of water stretchingand bending. The vast majority of the work was recorded using the785 nm laser, which is particularly suitable for analysis of geo- andbiomarkers[27] and may be the best overall choice for exploration ofboth organic and inorganic phases on the surface of Mars. Samplesfor analysis were stored and analysed in sealed glass bottles(10 mm diameter) to prevent dehydration. The heptahydrate wasstored and analysed surrounded by some supernatant as completeremoval of solution at ambient temperature often results inconversion to mirabilite. For these reasons, polarised single-crystalstudies were not attempted.

Calibration and smoothing

Calibration of the 785 nm laser was carried out in two stages. First,the absolute wavelength of the scattered radiation was related topixel position through atomic emission from a Ne lamp, suppliedby Renishaw. Second, the data points were related to the Ramanshift scale through excitation of polystyrene.[28] In principle, itis possible to calibrate the Raman shift axis directly throughuse of Raman wavenumber standards but the available Ramanbands are few in comparison to a Ne lamp.[28] The recordedspectrum was smoothed using a five-point adjacent averagingalgorithm in Origin. Peak positions were detected using code(‘Peakfinder’) written by T. C. O’Haver for use in the Matlab softwarepackage (The Mathworks Inc. Natick, MA), which locates peaks bysearching for downward zero crossings in the smoothed thirdderivative, which exceed user input values. Neon emission lineswere fitted to values taken from National Institute of Standardsand Technology (NIST)[29] using a third-order polynomial, thecoefficients of which were then applied to the polystyrene emissionlines. Raman wavenumbers were calculated from the assessedlaser wavelength, which was determined by comparing thecalculated Raman wavenumber with eight polystyrene (Aldrich,average mol wt = 290 000) peak positions from a spectrumrecorded directly in wavenumbers. Comparing the calculatedRaman wavenumber peak positions with American Society forTesting and Materials (ASTM) values[30] produced a root meansquared error of 0.15 cm−1. Less accurate values are given forthe water region because a Si calibration only was carried outfor the 514 nm laser line and the data was collected directly inwavenumber (no calibration) with the 785 nm laser. Observingthe well-defined ν1 peak of thenardite, full calibration (785 nmlaser), data collected only in wavenumber and the 514 nm lasergives peaks at 993.1, 993.7 and 994.9 cm−1, respectively. Thedifference at most is 2 cm−1, which is insignificant when thebroadness of peaks in the water region is considered. Waterregion peaks were fitted as Lorentzians and deconvolved usingcode (‘InteractivePeakFitter’) written by T. C. O’Haver for use inMatlab.

Crystal structures

Single-crystal XRD studies have determined the crystal structuresof mirabilite[31] and the metastable heptahydrate.[15] Mirabilitebelongs to the monoclinic space group C5

2h with Z = 4 in theBravais lattice. The unit cell[31] has the dimensions: a = 11.512(3),b = 10.370(3), c = 12.847(2) Å and β = 107.789(10). All atomsare in general positions 4(e) occupying sites of symmetry C1

with two independent sodium atoms, one independent sulphategroup and ten independent water molecules. Mirabilite has a layerstructure with chains of sodium atoms bonded to each otherthrough bridging water molecules that are hydrogen bonded tothe next layer. Sulphate groups and two free water moleculesare also hydrogen bonded to the water molecules of the sodiumchains. There is notable disorder in the sulphate tetrahedra asthree out of four of the oxygens can occupy more than one site,suggesting considerable mobility within this group. The sameapplies to four out of ten of the water molecules within theformula unit where oxygen atoms can occupy more than onesite simultaneously. This is in contrast to the isostructural mineralborax (sodium tetraborate decahydrate), which has no atoms withpartial occupancy and no free water molecules not bonded directlyto Na.

The heptahydrate crystallises in the tetragonal space group D194h

with two formula units in the Bravais lattice. The unit cell[15]

has the dimensions: a = 7.1413(2) and c = 22.1101(11) Åand Wyckoff positions for the individual atoms were obtainedthrough the Bilbao crystallographic server.[32] Like mirabilite,the heptahydrate has a layer structure with sodium atomscoordinated to other sodium atoms through bridging waters,which are hydrogen bonded to uncoordinated sulphate groups.Unlike mirabilite, the layers are not solely hydrogen bondedtogether but joined by a bridging sodium atom as well ashydrogen bonding. Mirabilite is known to have perfect cleavagein (1 0 0), which cuts through the hydrogen bonded water layers.In the heptahydrate, the presence of bridging sodium betweenthe otherwise weakly bonded layers may reduce the cleavageproperties so that it is perhaps unlikely to have perfect cleavagein any direction.

Factor group analysis

Structural vibrations of both mirabilite and heptahydrate arecomposed of lattice modes (translational and rotational) andinternal bending and stretching modes of sulphate and water.

Mirabilite

Mirabilite has 441 allowable optical modes, which can berepresented by[33 – 34]

�trans = 39Ag + 38Au + 39Bg + 37Bu (1)

�rot = 33Ag + 33Au + 33Bu + 33Bg (2)

�sulphate = 9Ag + 9Au + 9Bg + 9Bu (3)

�water = 30Ag + 30Au + 30Bg + 30Bu (4)

The principle of mutual exclusion applies, leaving a total of 222Raman-active g modes and 219 infrared-active u modes. Factorgroup analysis of sulphate and water is given in Table 1 and peakwavenumbers in Table 3.

J. Raman Spectrosc. 2010, 41, 1014–1020 Copyright c© 2010 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs

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Table 1. Factor group analysis of internal water and sulphate modesin mirabilite

Free symmetry Site symmetry Factor group

SO4 Td C1 C2h

ν1 A1 A Ag , Au , Bg, Bu

ν2 E 2A 2Ag , 2Au , 2Bg , 2Bu

ν3 F2 3A 3Ag , 3Au , 3Bg , 3Bu

ν4 F2 3A 3Ag , 3Au , 3Bg , 3Bu

H2O C2v C1 C2h

ν1 A1 A Ag , Au , Bg, Bu

ν2 A1 A Ag , Au , Bg, Bu

ν3 B1 A Ag , Au , Bg, Bu

Heptahydrate

The chemical formula of heptahydrate permits 165 opticalmodes, but, using the correlation method of Fateley et al.,[33]

half modes result as one of the water molecules (O3) has multipleoccupancy and occupies sites of low symmetry leading to greatersplitting than the actual number of water molecules suggests.To compensate, the water molecules were allowed to occupy allpossible positions simultaneously, which correlates to more modesthan 3N. Therefore, the actual vibrations calculated from factorgroup analysis, which could in theory be observed, correspond toa formula of Na2SO4· 9H2O, which allows 201 optical modes andcan be represented by

�trans = 4A1g + 2A1u + 6B1g + 2B1u + 4A2g + 6A2u + 5B2u

+ 4B2g + 9Eg + 9Eu (5)

�rot = 2A1g + 3A1u + 2B1g + 5B1u + 6A2g + 3A2u + 3B2u

+ 4B2g + 8Eg + 8Eu (6)

�sulphate = 2A1g + A1u + 2B1g + 2A2u + 2B2u + B2g

+ 2Eg + 2Eu (7)

�water = 7A1g + 3A1u + 5B1g + 2B1u + A2g + 4A2u + 6B2u

+ 2B2g + 6Eg + 6Eu (8)

The Raman-active modes are A1g, B1g, B2g and Eg, which wouldleave a total of 91 Raman-active modes. Factor group analysis ofsulphate and water is given in Table 2 and peak wavenumbers inTable 3.

Results and Discussion

Mirabilite – internal SO4 modes

The spectral region corresponding to the internal sulphatebending and stretching modes is shown in Fig. 1(A). The freesulphate tetrahedron has Td symmetry, producing one doubly andtwo triply degenerate modes that factor group analysis predictswill undergo site splitting, relieving all degeneracy. Table 1 showsthat the sulphate symmetric stretch (ν1), which is not degenerate,should split into an Ag and a Bg mode. However, only one modeis visible at 989.3 cm−1. Four modes (two Ag and two Bg) arepredicted to result from the sulphate ν2 bending mode and twoare clearly visible at 446.7 and 457.9 cm−1 with a much smallerpeak to the left at 423.2 cm−1 and a broad shoulder to the right at494 cm−1, which comes from the glass sample container. The anti-symmetric stretch and bending modes, ν3, ν4 respectively, shouldproduce six modes each (three Ag and three Bg) of which four (ν3)and two (ν4) are visible at 1086, 1108, 1119, 1129 and 616 and627 cm−1, respectively. It is very clear from Fig. 1(A) that, althougha lot of peak splitting is predicted, comparatively little is observed.This arises from the noted disorder of the sulphate tetrahedron,[35]

which is also apparent in polymorph I of the anhydrous phase andexplains why the predictions from factor group analysis and the

Table 2. Factor group analysis of internal water and sulphate modes in the heptahydrate

Free symmetry Site symmetry Factor group

SO4 Td D2d D4h

ν1 A1 A1 A1g , B2u

ν2 E A1, B1 A1g , A1u , B2g , B2u

ν3 F2 B2, E A2u , B1g , Eg , Eu

ν4 F2 B2, E A2u , B1g , Eg , Eu

H2O(1) C2v D2d D4h

ν1 A1 A1 A1g , B2u

ν2 A1 A1 A1g , B2u

ν3 B1 B1 A1u , B2g

H2O(2) C2v C2 D4h

ν1 A1 A A1g , A1u , B1g , B1u , Eg , Eu

ν2 A1 A A1g , A1u , B1g , B1u , Eg , Eu

ν3 B1 B A2g , A2u , B2g , B2u , Eg , Eu

H2O(3) C2v Cυs D4h

ν1 A1 A′ A1g , A2u , B1g , B2u , Eg , Eu

ν2 A1 A′ A1g , A2u , B1g , B2u , Eg , Eu

ν3 B1 A′′ A1g , A2u , B1g , B2u , Eg , Eu

Water occupies three sites of different symmetries as indicated by superscript numerals.

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Raman spectra of Na2SO4·10H2O and Na2SO4·7H2O

Table 3. Raman wavenumbers observed from randomly oriented samples

Assignment andfree ion values

Mirabilite(Na2SO4·10H2O) cm−1

Heptahydrate(Na2SO4·7H2O) cm−1

Solutioncm−1

Thenardite(Na2SO4) cm−1

SO4 ν1 − 981 989.3 987.6 981.9 993.1

SO4 ν2 − 451 446.7 464.4 450.9 451.8

457.9 482.7 – 466.6

SO4 ν3 − 1104 1086.3 1106.6 1115.2 1102.1

1108.4 1134.9 – 1132.1

1118.9 – – 1152.7

1128.9 – –

SO4 ν4 − 613 615.9 592.5 617.0 620.5

627.0 644.6 – 632.8

– – – 647.2

H2O ν2 − 1595 ∼1671 ∼1711 ∼1638 –

H2O ν1 and ν3 3652 and3756

∼3092 (common tomirabilite and solution)3242, 3343, 3381 and 3506

∼3337, 3384, 3418, 3466 3092, 3208, 3257, 3299 and3448

–

Other (combination) 2162 2157 and 2181 2162 (as mirabilite) –

The 785 nm laser was used to determine all internal SO4 lines and a combination of 785 nm (120 s count time) and 514 nm (10 s count time) for theinternal H2O lines. Laboratory temperature was maintained at 22 ◦C. Free ion values from Ref. 39 (SO4) and Ref. 37 (H2O).

recorded spectrum do not agree. In fact, the overall wavenumbervalues of ν2 –ν4 are close to free sulphate in solution. Bendingmode freedom may be enhanced by the multiple sites availableto oxygen and the anti-symmetric stretch is possibly only lightlyconstrained by the lattice. The symmetric S–O stretch is the onlymode with any real wavenumber shift from the free ion value.According to Myneni et al.,[36] a reduction in the average S–Obond length (1.475 Å for mirabilite) from the value of the freeion in solution at 1.49 Å results in vibrations shifting to higherwavenumbers as the H-bonding between SO2−

4 and structuralH2O is weaker than that in aqueous solution. However, it shouldbe noted that there is no obvious correlation between the averageS–O bond length and ν1 value in the common Ca, Mg and Nasulphate hydrates except the general observation that the S–Obond length is shorter and ν1 is higher than solution.

Mirabilite – internal H2O modes

Sixty internal water modes are predicted to be Raman active,but considerably fewer than 60 are apparent in the spectrum.Using the 514 nm argon ion laser line, which is more suitablefor detecting water, bands are observed at 1638 and 1671 cm−1.The lower value of 1638 cm−1 corresponds to a small amount ofsolution in the sample bottle and the higher value of 1671 can beattributed to ν2 of water in the solid. Krishnamurthy and Soots[37]

report that the two water ν2 wavenumbers of gypsum are higherin the solid than in free water by about 35 and 85 cm−1, whilethe ν1 and ν3 stretching modes are decreased by ∼300 cm−1

due to changes in H-bond lengths and force constant values.Samples of 2.9–3.4 molal solution were also analysed (as shownwith mirabilite in Fig. 1), which shows that the water ν2 is about43 cm−1 higher than pure water. Peaks corresponding to theν1 and ν3 stretching modes of mirabilite are observed throughpeak deconvolution at 3092 (common to mirabilite and solution),3242, 3343, 3381 and 3506 cm−1, which is up to ∼400 cm−1

less than ν1 and up to 300 cm−1 less than ν3 of water. The ν1

and ν3 values of solution are 3092, 3208, 3257, 3299 and 3448,which is quite different from pure water but similar to mirabilite.Using the empirical regression function of Libowitzky,[38] the O–H

stretching wavenumbers can be calculated. While the equationsthey present are derived from infrared data, the difference betweeninfrared and Raman spectra generally no more than a few tensof wavenumbers, which is adequate for confirming the generalidentity of the peaks described here. Using known O–H· · ·Odistances[31] and parameters from Table 3, of reference 38 a widerange of stretching wavenumbers were calculated for mirabilitefrom 3316 to 3553 cm−1 with a notable cluster of values in therange 3377 to 3431 cm−1, which is close to two of the observedpeaks and helps in confirming their identity.

Myneni[39] states that, as water structures itself around freesulphate ions in solution, the sulphate symmetry can be removedfrom tetrahedral because of differences in water solvation and H-bonding, but that it is generally regarded as close to Td geometry.Water in the solvation shell of a sulphate ion exhibits stretchingmodes above 3000 cm−1 and a bending mode around 1640 cm−1.This is very close to the solution value presented here of 1638 cm−1,suggesting that the O–H bonds probed belong to the sulphatesolvation shell. As Myneni also notes, increasing the concentrationof a sodium sulphate solution leads to a shift in ν1 towards thevalue of the solid. Simply looking at the spectra presented inFig. 1 suggests that the sulphate and water environments in solidmirabilite and those in solution are not very different. This isreinforced by the mirabilite structure as four out of ten watermolecules per formula unit have hydrogen atoms with doubleoccupancy.

Mirabilite – overtones and combinations

A broad peak is observed at 2162 cm−1 in the mirabilite spectrum,as shown in Fig. 1(C), and a similar but much less intense peakis observed in 3.4 molal solution. This peak could perhaps beattributed to a combination of sulphate ν1 + ν3

40, but an O–Hstretch is also possible[40] and analysis of a deuterated crystalwould be required to distinguish between these possibilities.

Heptahydrate – internal SO4 modes

Unlike mirabilite, not all degeneracy is lifted by site symmetry.Table 2 shows that ν3 and ν4 retain degeneracy in the form

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400 600 800 1000 12000

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Figure 1. Mirabilite and solution Raman spectra. (A) Region corresponding to the internal sulphate modes, (B) internal water modes – bottom plot ismirabilite minus solution and (C) magnification of possible combination modes. The solid line is mirabilite and the dotted line is c. 3 molal sodiumsulphate solution. The inset figure in (A) is an enlargement of part of that figure.

of doubly degenerate E modes. ν1 has one Raman-active A1g

mode at 987.6 cm−1, and ν2 has two modes (A1g and B2g) at466.4 and 482.7 cm−1. A peak at 452 cm−1 corresponds to thesmall amount of solution present in the bottle. A doublet isclear in the ν4 region with peaks at 592.5 and 644.6 cm−1, asshown in Fig. 2(A), which should correspond to B1g and Eg modesfrom factor group analysis. Site splitting for ν3 also producesa B1g and an Eg mode, and a doublet is observed with peaksat 1134.9 and 1106.6 cm−1. The sulphate oxygen atoms cannotoccupy multiple sites and the wavenumber difference betweensplit peaks from 17 to 52 cm−1 suggests that the SO4 ions in

the lattice are distorted.[37] Krishnamurthy and Soots[37] attributethis to only two out of four oxygen atoms in gypsum’s sulphategroup H-bonding to neighbouring atoms. In heptahydrate, thesulphate oxygen atoms form four H-bonds to neighbouring waterbut the water molecules differ in terms of disorder from eachother. Two out of the four bonds listed are to an O–H group,which occupies more than one site. While this is similar instyle to mirabilite, the mirabilite sulphate group has mobilitythrough the multiple occupancy of its oxygen atoms, which isnot the case for heptahydrate. Both sulphate groups probablyexperience disorder through intermolecular H-bonds but in

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Figure 2. Heptahydrate and solution Raman spectra. (A) The region corresponding to the internal sulphate modes. (B) The internal water modes and (C)a magnification of possible combination modes. The solid line is heptahydrate and the dotted line is c. 3 molal sodium sulphate solution. The inset figurein (A) is an enlargement of part of that figure.

mirabilite the effect could be averaged out. The fundamentalvibration at 987.6 cm−1 is only about 2 cm−1 lower than thatin mirabilite and neither are far from the solution value of981 cm−1.

Heptahydrate – internal H2O modes

Water can occupy three positions of different site symmetry inheptahydrate – D2d , C2 and Cv

s , leading to three separate sets ofvibrations for water, using the correlation method.[33 – 34] The ν2

bending region should contain 11 modes – 3A1g, 2B1g and 2Eg

but only one weak peak at about 1711 cm−1 is detectable, usingeither laser. This is similar to mirabilite and it is known that water

is a poor Raman scatterer; more suited to detection by infraredspectroscopy.[41] Four ν1 and ν3 modes are observed at 3337,3384, 3418 and 3466 cm−1. Using known O–H· · ·O distances[15]

and parameters from Table 3,[38] stretching wavenumbers werecalculated at 3392, 3418, 3486, 3505, 3555 and 3576 cm−1, which isreasonably in sympathy with the observed wavenumbers. Solution(2.9–3.4 molal) has peaks in the region 3100–3300 cm−1, which arenoticeable in the heptahydrate sample and correspond to the smallamount of solution covering the crystal. In contrast to mirabilite,heptahydrate has a more definitely structured water region witha relatively sharp band at 3466 cm−1 as shown in Fig. 2(B), whichis probably because its water molecules have less freedom.

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Heptahydrate – overtones and combinations

Heptahydrate has a notable band at 2100–2200 cm−1, as shownin Fig. 2(C), which is sharper and of higher intensity than theequivalent in mirabilite or solution. Two peaks were separatedwith wavenumbers of 2157 and 2181 cm−1, which could again bea combination of ν1 + ν3

[41] but an O–H stretch is also possible.[42]

As with mirabilite, analysis of a deuterated crystal is required todistinguish between both possibilities.

Discussion

Mirabilite and the heptahydrate are very different structurally andyet share some common features. Heptahydrate is notable amonghigher sulphate hydrates for its high crystallographic symmetry.Many sulphate hydrates tend to belong to low symmetry crystalsystems with only a very few belonging to crystal classes withsymmetry higher than monoclinic, such as retgersite (tetragonal),epsomite (orthorhombic) or loweite (trigonal). Water moleculesin both structures experience some freedom through oxygenatoms occupying more than one site simultaneously, but thisis more pronounced in mirabilite, which correspondingly hasa Raman spectrum similar to that of the free ion in solution.A high-pressure (5.4 GPa) octahydrate (Na2SO4·8H2O) has beenidentified[15] and, as one might expect, sulphate and water haveno multiple occupancy and sodium is coordinated directly tosulphate and all water molecules. There is no Raman or infraredspectrum yet for the octahydrate but factor group analysis predicts183 optical modes:

�translational = 63Au, �rotational = 54Ag, �SO4,Int. = 9Ag

+ 9Au, �H2O,Int. = 24Ag + 24Au.

All degeneracy is relieved and peak splitting will probably beclearly defined as it is in thenardite, which also has sulphatecoordinated directly to sodium.

Acknowledgements

We would like to thank the Engineering and Physical SciencesResearch Council (EPSRC) for funding (A. H.) and equipmentprovision, the University of Edinburgh for an undergraduatesummer bursary (R. I. M.) and Prof. Chris Hall for valuablediscussions.

References

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