Infrared and Raman study of alunite—jarosite compounds

Specrrochimm Arm, Vol. Prmted m Great Britain.

42A.

No.

6, pp.

729-734,

0584-8539!86

1986.

Pergamon

Infrared and Raman study of alunite-jarosite CARLOS J. SERNA,* C. PARADA CoRTrNAt

and Jo&

$3 00 +

0.00

Journals

Ltd.

compounds

V. GARCIA RAMOS$

*Institute de Ft’sico-Qufmica Mineral, C.S.I.C., Serrano, 115 dpdo, 28006 Madrid, Spain, t Departamento de Qufmica Inorganica, Facultad de Qufmicas, Universidad Complutense, 28040 Madrid, Spain and $Instituto de Optica, C.S.I.C., Serrano, 121, 28006 Madrid, Spain (Receiued 22 April 1985; in final form 19 November

1985; accepted

16 December

1985)

Abstract-Infrared and Raman spectra of alunite and jarosite compounds, AR,(SO,),(OH),, where R is either Fe’+ or A13+ and A is a monovalent cation, have been obtained between 4000 and 4Ocm-‘. The observed number and position of the i.r. and Raman bands have been analysed under the centrosymmetric D:,, space group. In addition, some relationships between the crystallochemical characteristics of these compounds and their vibrational properties are presented.

INTRODUCTION

closed tube at about 150°C for periods ranging from 7 to 15 days. Under these conditions, alunite and Na alunite were formed by spontaneous hydrolysis of mixed solutions of A12(S01)s.18H20 (6N) with KIS04 (0.2 N) or Na2S04 (0.2 N), respectively. Hydronium alunite was obtained, however, from-mixed solutions of Alr(SO,,)s. 18Hz0 (2 N) and LiOH (2 N). Jarosite and Na iarosite compounds were synthe&ed,‘in an analogous way; from mixed 2 N solutions of Fe2(SO& and the corresponding hydroxide. Hydronium and ammonium jarosites were obtained by mixing 2 N solutions of LiOH or (NI-L&SO, with a ferric sulphate solution (2 N). The products thus obtained were washed with distilled water and dried on a porous plate under vacuum. X-ray diffraction patterns and chemical analyses were performed in order to assess sample purity. A least squares computer program was used to obtain the cell dimensions (Table 1). Infrared spectra were recorded on a Perkin-Elmer model 580B using the KBr pellet technique. Partially deuterated samples were obtained in a similar manner to that described before [12]. Far i.r. spectra were recorded on a Digilab model 14 Fourier transform spectrophotometer using polyethylene pellets. Raman spectra were recorded with a Jobin Yvon Ramanor U-1000 double monochromator spectrometer using the 5145 A Ar+ exciting line. Much better spectra were obtained for alunite compounds. In fact, NH,, jarosite gave almost unresolved Raman lines.

The ahmite-jarosite family can be represented by the formula ARJ(SOq)2(0H)6r where A refers to a large cation in 1Zfold coordination (Na+, K+, H,O’) and R is either Fe3+ or A13+ in octahedral coordination. For the jarosite group (R = Fe3+), members with A = NH:, Rb+, Ag+, Tl+, 1/2Pb2+ and 1/2HgZ+ have been also synthesized or found in nature [l]. In addition, several synthetic hydroxychromates of the formula AR3(Cr04)2(0H)6 (A = Na+, K+, NH:; R = A13+, Fe3+) have been shown to be isostructural with alunite [2]. The crystal structure of these compounds is still the subject of some controversy. HENDRICKS [3] proposed the space group R3m = C:, for alunite in agreement with its observed pyroelectric properties. However, the X-ray determination of the crystal structure of alunite [4] and later refinement of the structure of a synthetic jarosite and a natural alunite [S], down to R values of 0.037 and ,0.024, respectively, were carried out on the RTrn = D$d space group. Finally, the positive detection in alunite of optical second harmonic generation has again suggested the non-centrosymmetric R3m group [6]. The i.r. spectra of several members of the alunite-jarosite family have been studied, but much of the information required for a satisfactory analysis has not been obtained, partly because the study has been limited to a portion of the i.r. spectral region as well as to errors in the theoretical analysis [7-lo]. This paper presents complete i.r. and Raman spectra of synthetic alunite and jarosite compounds which can be better understood under the RTm space group. In addition, some relationships between the crystallochemical characteristics of these compounds and their vibrational features are also presented.

RESULTS AND DISCUSSION Crystallochemical

characteristics

and crystal structure

A three-dimensional view of the alunite-jarosite structure is shown in Fig. 1. The structure can be of layers, formed by regarded as a series tetrahedral-octahedral-tetrahedral (TGT) sheets,

paraTable 1. Crystallographic meters (A) of the samples Jarosites a c

EXPERIMENTAL

Na H3O

A wide variety

of experimental conditions have been reported for the synthesis of members of the alunite-jarosite family [ 1,111. In our case, all the compounds were grown in a

K NH, 729

7.33 7.35 7.32 7.33

16.70 16.99 17.12 17.37

Alunites a c 7.01 7.04 7.02

16.75 17.13 17.22

CARLosJ. &RNAet al.

b

Fig. 1. Structure of AR#O&(OH)a

jointed by the monovalent cations (A). In the octahedral sheets, the aluminium or iron atoms are linked to each other through hydroxyl groups. The value of the a parameter is directly related to the R-OH bond length. Thus, a is about 7A for the alunites (Al-OH = 1.879A in alunite) while it lengthens to about 7.3 A in the jarosites (F&H = 1.975A in jarosite). On the other hand, the c parameter varies with the nature of the monovalent cation leaving the a parameter almost unchanged (Table 1). Thus, for identification purposes X-ray data can be suIRcient for pure members of the alunite-jarosite family. However, for mineral samples, in which solid solutions are commonly present, chemical analyses are also needed. One striking difference between both groups of compounds is the thickness of the TUT layer, larger in jarosite (7.64A) than in alunite (7.48 A). However, this difference does not have an effect on the c parameter since it is compensated by a greater interpenetration of the adjacent layers in jarosite. Finally, it should be mentioned that the

viewed along the

a

and c axes [4].

OH-OS distances are about the same (- 2.92 A) for both groups of minerals [S]. Factor group analyses (K N 0) applied to determine the distribution of vibrations among symmetry species for ARB(SO.&(OHk, in the two proposed space groups (CJ. and I&) are shown in Table 2. It can be seen that the spectral characteristics of these groups are quite different. For the centrosymmetric group (D:,) factor group analysis gives 24 vibrations active in i.r. and 19 vibrations active in the Raman spectra with maxima. The non-centrodifferent frequency symmetric group (CL) gives 43 vibrations active both in i.r. and Raman spectra with the same frequency maxima The observed number and position of the i.r. and Raman bands (Tables 3 and 4) are better explained under the D& space group. It should be noted that vibrational spectroscopy when used to distinguish between possible crystallographic space groups for a compound may lead to erroneous conclusions since vibrational and crystallographic groups are not always identical [13]. It is

Table 2. Factor group analysis for AR#+O&(OH),

Lattice modes Total modes

Acoustic Internal modes modes OH SO.

Librational SO. OH

SOA

Translational OH R

A

Activity

C Iv

Al A2 E

19 I 26

z -%Y

6

2

6

2

3

1

3

1

3 3

1 1

i.r., R Inactive i.r., R

D3,

A IfI All E# A I” -42”

E”

8 3 11 4 11 15

z X,Y

R Inactive R Inactive i.r. i.r.

Alunite-jarosite

Table 3. Infrared

and Raman frequencies

K

Na

(cn-‘)

in alunites

R

i.r.

R

i.r.

R

3510 3490 1225 1165 1085 1030 685 630 602

3508 3481

3490 3450 1225 1170 1100 1030 670 630 600

3488 3452

3490 3465 1225 1170 1095 1030 665 630 600

3492

voH

1198 1172 1091 1038 655

vs(S-0) ho, “~(S-0) v1 (S-o) vq(s-0) v4 (S-0)

1081 1030 655

515 491 389 353

535 535 520 485 440 360 338 292

166

515 490 430 360 340 292

YOH

222 182 140 87

!

&Al 516 491 I 394 vz(S-o) 355 ]

243

245

241 226 185 126

517 485 403 351

Assignments

“OH

586

567 528 528 510 492 430 365 335 292

1166 1092 1030 654

223 198 159 125 84

167

Infrared and Raman spectra Infrared and Raman spectra of alunites and jarosites are shown in Figs 2-5, in which internal vibrations due to sulphates are striped and those due to hydroxyls are indicated. It can be seen that the i.r. and Raman spectra of both groups of compounds, alunites and jarosites, are rather different and vibrational spectroscopy can be conventionally used to differentiate one from the other. This is to be expected since isostructural compounds of aluminium and iron give quite different i.r. and Raman spectra [14], due to the different force constants of Al-O and Fe-O bonds. In addition, striking differences are also present between alunites and jarosites for the sulphate and hydroxyl internal vibrations which will be discussed later. The effect of the monovalent cations within each group is, however, not so clear. Only the stretching hydroxyl vibrations and the absorption maxima in the far i.r. region show differences between the members of each group, alunite or jarosite (Tables 3 and 4). For the study of the different vibrations and their assignments we will consider separately vibrational modes due to molecular units (sulphates and hydroxyls) from the rest of the lattice.

H,O

i.r.

731

compounds

O-Al t

1~1

Molecular vibrations well known that a crystallographic space group may be of lower symmetry than the corresponding vibrational one, which may well be the case here. However, these compounds will be treated as belonging to the centrosymmetric D& group.

Table 4. Infrared K i.r.

and Raman

Na R

3390 3365

Although the rhombohedric alunite-jarosite cell contains six hydroxyl groups, only two stretching vibrations are predicted under the Did space group (Table 2). They are clearly observed in alunites (Figs 2 and 3) but not so well in some jarosites (Figs 4 and 5).

frequencies

(cn-‘) NI-L

Ha0

i.r.

R

ix.

3399 3368

3380

3365

R

i.r. 3420

1161 1112

448

442

1015 625 560

1185 1095 1027 1012 675 630 570 512 480

1160 1114

1200 1090

1163 1099

1014

1012

1011

627 571

630 570 515 475

619 563 530

448

445 406 365 303 295

350 335 260

350 330 260

230 208

214 208

163

162

228

142 66

%4(.4)47:6-C

66

R

Assignments “OH “OH

3340 3220 1655 1185 1110 1028 1012 680 632 570 515 482

in jarosites

1428 I 1195 1078 1006 1000 655 630 560 505 472 1

NH, y3 (S-O) “3 (so) 6 “p(?-o, v4 (S-0) vq (S-0) YOH

@Fe

450

“2 (s-0)

350 300 255

345 320 265

O-Fe

229 205 196 182 160 136 107

233 204

382

158 110

732

CARLOS

4000

J. SERNA&~.

1600

3200

I200

800

400

loo

cm-l

Fig. 2. Infrared spectra (40-4000em-‘) of alunites.

loo

200

400

Eoo

a00

loo0

mo

3300

3so

CW

Fig 3. Raman spectra of (a) Na alunite and (b) Na jarosite.

Their frequency maxima are characteristic of each member of the alunites and of the jarosites. Since the OH-O(SOJ) bond distance is almost the same in jarosites and ahmites [5], with a value of about 2.92 A indicating the absence of hydrogen bonding [ 151, their frequency difference (- lOOcm-‘) should be due to electrostatic interactions with the monovalent cations. This effect is stronger in jarosites than alunites due to greater interpenetration between their layers which would shift vOH to lower values. In addition, the inductive influence of the R cations (Fe and Al) would also cause vOHto be observed at lower frequencies in jarosites than in alunites [14].

Other vibrational modes due to hydroxyl groups (60, and yoH) which overlap with S-O and R-O vibrations can clearly be observed in deuterated samples (Fig. 5). Thus, the a,, mode can easily be confused with vl(S-O) in jarosites and with v&-O) in alunites (Figs 2 and 4) and yoH , observed at around 600 cm- ’ in alunites and 570 cm- 1 in jarosites (Table 2), overlaps with R-O type modes in each group of minerals. In the spectral region between 400 and 13OO~m-~, bands due to sulphate anions are found, which are striped in Figs 2-5. In these compounds, the sulphate anions have Co. site symmetry, with the apical oxygen lying on the c3 axis (Fig. 1). In agreement with the

Alunitejarosite

compounds

733

“30

No

4000

800

I200

3200

too

400

cm-1 Fig. 4. Infrared

i 4000

I

3500

I

3ccO

I

spectra

(4&4MO cm- ‘) of jarosites.

AI

I

I

1200

2500

I

I

81

1

I

800

I

I

I...

400

I

100

cm-l Fig. 5. Infrared

spectra

of (a) jarosite

centrosymmetric D:,, space group six internal vibrations are predicted in the i.r. and Raman spectra (Table 2) and can be readily observed (Figs 2-5). Site group to factor group analysis yields the assignments given in Tables 3 and 4 in which HERZBERG’S notation [16] has been followed. It should be noted that the assignment of the vq vibrational mode in alunites (63&690 cm- ‘) and the v2 vibrational mode for both alunites and jarosites at around 430-450 cm-’ could also include R-O type modes. In agreement with crystallographic determinations [s], the i.r. spectra show the distortion of the sulphate tetrahedra to be greater in alunite than in jarosite. Thus, the frequency difference between the vj vibrational modes is around 130 cm- 1 in alunites and around 95 cm- 1 in jarosites, although it must be a complex function of the A and R cations [S].

and (b) partially

deuterated

jarosite.

Molecular absorptions due to HJO+ in alunites and jarosites have been considered previously [8,17], but it remains uncertain whether a distinct absorption due to HsO+ exists. The detection of absorption bands characteristic of the oxonium ion in sulphates is difficult since the more intense H30+ band (v2) occurs between 950-l 175 cm - i, a region where SO vibrations also occur. An interesting case is presented by NH, jarosite whose alunite analogue has not been obtained so far. Absorption bands attributed to ammonium are observed at 3340, 3220, 1655 (shoulder) and 1428 cm-’ (Table 4). The absorption bands at 3340 and 1428 cm-’ compare well with those reported for NH.+ montmorillonite (3280 and 1430cm-‘) and assigned to vj and v.+,respectively [ 183. The other two absorptions at 3220 and 1655 cm-’ could be attributed to the vl and v2 symmetric modes which become

134

CARLOSJ. S~RNAet al.

i.r. active for perturbed NW ions. However, the space group D& cannot accommodate the ammonium cation since its symmetry site (Ds,,) is not a subgroup of the undistorted ammonium symmetry (T’.).A comparison of the symmetry sites of Dsl and T1 indicates that ammonium could have Csv symmetry in the jarosite, in which the activation of the v1 and v2 vibrational modes can be understood. Thus, the space group of NH, jarosite could be different from that of the other members of the alunite-jarosite family. A similar case has been found for NH4 dawsonite [19]. Another explanation could be similar to the order-disorder effects observed in ammonium salts [20]. If the structure is not completely ordered, i.e. with all the ammonium molecules oriented by the symmetry operations of the space group, but partially or totally disordered (the ammonium molecules rotate freely) changes in the symmetry of the crystal may well take place. Lattice vibrations According to the factor group analysis (Table 2), 13 vibrations are expected in the i.r. for those compounds: 10 associated with R-O type vibrations, two due to translational motions of the interlayer monovalent cations and a librational mode due to sulphates. However, the Raman effect is less rich in bands, eight vibrations are predicted of which five will be of the RCl type and three due to librational modes from the sulphates. In general, the number of observed i.r. and Raman bands are in agreement with the theoretical calculations. Thus, nine to 10 i.r. absorptions and six to seven Raman bands are observed, depending on the sample, which can be assigned to lattice vibrations (Tables 3 and 4). Vibrational modes due to R-0 must be the fundamental type of motion between -250 cm- ’ in alunites and between 550-250 cm-’ in jarosites. This is supported by the i.r. and Raman spectra of many compounds in which aluminium and iron are in an octahedral coordination with oxygen [14]. As expected, the Al-O or Fe-O vibrations are rather insensitive to the nature of the interlayer (T-O-T) cations. The far i.r. region (250-40 cm-‘) in which librational modes due to sulphates and translational vibrations associated with the interlayer cations are found is where the individual alunites and jarosites can really be differentiated. However, assignments of ab-

sorption bands to specilic type of cations could not be clearly justified. In zeolites, for example, vibrations associated with monovalent cations can vary from 200 to 40 cm-’ depending not only on the type of zeolite but also on the cation positions in the framework [21]. Acknowledgements-The authors would lie to thank Dr. M. HANDKE and C. PALU~ZKIEWICZfrom the Regional

Laboratory of Physico-chemical Analyses and Structural Research,Craeow, Poland for recording the far i.r. spectra. REFERENCES [1] J. E. D~TRIZACand S. KAIMAN,Can. Miner. 14, 151 (1967).

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