FT-RAMAN STUDY OF THREE SYNTHETIC SOLID

SOLUTIONS FORMED BY ORTHORHOMBIC SULFA TES:

CELESTITE-BARYTES, BARYTES-ANGLESITE AND

CELESTITE-ANGLESITE

Key words: FT-Raman, solid solutions, celestite, barytes, &nglesilc

1.M. Alia"', H.G.M. Edwardsb, S. L6pez-Andr�s', P. Gonzalez-Martfn".

F J. Garcla-Navarro" and H.R. Mansourb

• E. U.f. T.A. - Dpfa, de Quimica Fisica. Uni�rrsjdod de Cos/illo-Lo Manc/IU, J 3071 Cludad Real, Spain � Chem/slry and FonlUic Sdencu, Uniwrsiry of Bradford, Bradford BD7 I DP. Uniled Kingdom • DpfO. de Crislolografia y Miner-a/ogia, Fae. de Geo{ogia, Universidad Campiu/r:rlst, 2807 J Madrid, Spain

ABSTRACT

In the present work. three different solid solutions whose end-members are •

the orthorhombic sulrales celestite (SrSOd. barylc5 (Ba804) and anglesitt

(PbSO�) are studied using FT-Raman speelroscopy. SuI rate lllion symmetric

internal modes have bull examilled in detail by means of band-shape analysis and

component fitting procedures_ The symmetric stretching mode ",(AI) changes its

wavenumber position linearly with the cationic composition of the samples which

further confinns the ideal character of the solid solutions studied. The

• .... ulhoe" for corrnponckn«. E-mait: jmltiro@qirl-cr.lICtm.c:s

corresponding full-width at half-height is strongly increased in the central

components of the different solid solutions which can be understood as an effect

of the positionaJ disorder induced by random cationic substitution. Similar results

are observed in the symmetric bending mode, VJ(E). The study of the low

frequency spectral region pennits one to differentiate traslational modes of the

sulfate anion, which changes its wavenumber position when the alkali-earth metal

ion changes from rotational modes. This permits the tentative band assigrunent of

the anglesite rotational Raman bands al 134 and 152 cm-I, which were previously

not assigned.

INTRODUCTION

The study of those solid solutions fonned by sparingly soluble orthorhombic

sui fates (namely 8aSO. barytes, srSO. celestite, and PbSO. anglesite) has

received considerable attention in recent ye�I�. Geochemical, mineralogical,

industrial and environmental reasons justify this interest (see S� and references

Iherein).

Although mechanistic\ Idnelie' and structural' studies dealing with the three

binary solid solutions fonned in the system BaSO.-SrSO.-PbSO. have been

published, the vibrational dynamics of such solid solutions has not been studied,

to the best of our knowledge. This lack of data contrasts with the numerous

available references on Raman and infrared studies of the corresponding simple

sulfatesl9•

The present work reports the analysis of the three binary solid solutions

fonned in the system BaSO.-SrSO.-PbSO •. The samples obtained by precipitation

are studied by means of powder X-ray diffraction and FT-Raman spectroscopy.

The experimental results allow the calculation of the unit cell dimensions of the

samples and 10 study the ideality of the solid solutions obtained. The FT-Raman

spectra are analyzed in detail, including band-fitting of the low frequency spectral

region.

EXPERlMENTAL Samples were prepared by the method of Komicker et al.', adding dropwise

an aqueous solution with the appropriate quantities of the cations (as nitntes, 0.25

M) to 6 M sulfuric acid a' 60-65 "C with vigorous stirring. The stoichiometry of

the precipitates was detC'nnined by dissolution of ca. 0.1 g (accurately weighed to

±<I.I mg) in lOO mL of 0.02 M NaJEDTA at pH 10.8. The ucess of chdating

agent was subsequently detennined with Cal+. Triplicate analyses were run for

each sample. The observed stoichiometries of the cations �re always within ±2% of their nominal value.

X.ray diffraction was carried out using a Philips PW 1710 automatic

diffractometer with Cu K... radiation (wavelength .. 1.54056 A), automatic

divergence slit and graphite monochromator. Scanning rate (from 29 - 5· to 70")

was 0.5· min" using Si as an external standard.

FT·Raman spectra were excited al 1064 nm using an Nd:YAG laser and a

Broker IFS66 optical bench with a FRA 106 Raman JKcessory. Laser power was

selal ta. lOO mW and 1000 scans were accumulated with a resolution of 2 cm".

Powdered samples were lightly pressed in the Broker powder holder and mounted

with I gO" scattering geometry. The mathematical (curve fitting) treatment of the

spectn was carried out using the commercial software GRAMSJJ2- (Galactic

Industries). Smoothing procedures or baseline correction routines were nol

applied in this work. All the figures in the paper are prodoced from integrated

inte�ty·nonnalized spectra.

RESULTS

Figure I shows the evolution of the unil--cell paramelers against the chemical

composition of the samples. Well·fitted linear plots are obtained in all the cases (see Table 1), which dcmonstrales that the solid solutions studied obey Vegard's

law. This observation can be understood as a first index of ideality. Table 2

sumnw-jzes the unit--cell dimensions in the end·members of the solid solutions

(pure phases). As can be observed, these are very close 10 the last reported

• • . · . . • . • I";0o;;0o<I • I"'9"l

• i t · t ·

I •

I •

3 3 • •

• •

• • " .. .. • " .. .. ..

• -•

Il'I>,Sr,""SO� � .. � ......

)' -•

• t I 3

· •

r .a

I ! la

--•

,. • •

• • " .. .. • a

• • • • • -..... _-

FIG. J. Evolution oflhe unit·ccll parameters against the chemical composition of

the samples.

TABLEt

Linear Fitting of the Unit Cell Parameters (a, b, c: A ; V: A') against the Mole i Fraction x. s.e. -standard error; s.t.e. - standard error of the estimate. I ,

Sr.Ba(I·.tSO. " Paramet l/1tercept s.e. Slope s.e. s.e.t.

" a 8.8991 0.0396 -0.4871 0.0669 -0.9245 0.0702 b 5.4642 0.0060 -0.0958 0.0101 -0.9534 0.0106 , 7.1505 0.0045 -0.2856 0.0075 -0.9969 0.0079 V 346.68 0.32 -38.608 0.538 -0.9991 0.56

U.,Pb(I."SO. " Poramel 1/1/treept S.e. Slap< s.e. S.t.t.

" a 8.4664 0.0043 0.431\ 0.0073 0.9987 0.0076 b 5.3970 0.0055 0.065\ 0.0093 0.9192 0.0098 , 6.9542 0.0025 0.2051 0.0042 0.9981 0.0044 V 317.66 0.25 30.156 0.424 0.9991 0.44

Sr,P!>tl .• ,sO. " Paramet Intercept S.t. Slap< s.e. s.t.e.

" a 8.4720 0.0050 -0.102& O.OOSS -0.9708 0.0089 b 5.)942 0.0044 -0.0380 0.0075 -0.8596 0.0079 , 6.9584 0.0017 -0.0886 0.0029 -0.9952 0.0031 V 317.98 0.32 -10.032 0.536 -0.9874 0.5626

TABLE 2

Unit-c.ell Parameters of the End-members. All data in Angstrom (A).

B.SO. SrSO. PbSO. Obstrvtd Reffl I) Observed Ref[J J) Observed Re!{J J]

a 8.909 8.879 8.390 8.354 8.475 8.472 • 5.449 5.454 5.351 5.346 5.376 5.397 , 7.156 7.154 6.874 6.867 6.962 6.955 V 347.4 346.4 308.6 306.7 317.2 318.0

structure refmement" which is remarkable taking into accoWl! the IlIture of the

precipitation of the solid solutions studied here. Figure 2 shows the FT ·Raman spectral region com::sponding to the sulfate

ion symmetric stretching V)(AI) fundamental in several samples of the solid

solutions. These are fairly symmetric bands whose Raman wavenumbers change

linearly with the chemical composition of the samples (Figure 3, left).

The behavior of the corresponding half·widths (measured IS the full·width at

half·height, FWHH) (Figure 3, right) deserves some comments. The solid

solutions which contain lead (BaxPbl .• S04 and Sr.Pbl .• SO.) have a bimodal

tendency with a maximum near the central points being observed, similar to that

reported in other solid solutions't.Il. However, the system barytes/celestite

(Ba.Sr, .• SO.) shifts from this behavior and the FWI1I1 changes linearly between

both extremes. This observation could result from the geochtmical similarity

between barium and strontium ions; furthennore, their influence on the sulfate

anion vibrational dynamics is very close, as can be deduced from the reported

stretching and bending force constants of sulfate in barytes, celestite, and

anglesitel4.

Figure 4 shows the symmetric bending mode V}(E) in samples of the different

solid solutions. As in the ",(AI) fundamental, the difference behYeen the

celestilelbarytes solid solution and those which contain lead is evident. In Figure

S, left. the evolution or the band group center of mass is plotted against the

chemical composition. As can be observed. this parameter does not change along

the range or substitution in the celestite/baryles solid solution. 110wever, the shift

between the components or this doubly degenerate rWldamental is very sensitive

to the changes in the chemical composition. as shown in Figure S, right. As

ahady reported for other solid solutions'2.11,the shift between the components of

a degenerate fundamental tends to decrease in the central members of a solid

solution as an effect of the local disorder. Thus, the observed shift decreases

abruptly in the first members of the series, increasing later, because anglesite

shows the greatest shift between the components (11.4 cm-I)_ A similar, although

attenuated, effect can be observed in the celestite/barytes solid solution.

••

•• Sr,a·c • ..rS°.j

•• .-, .. ; �

" . " ••

,�

.. , .. ,� .. .. - * ,. * m � m

'" IS;.Pbc':�O� '" "

, � ! •• '-, I'·

,. ,�

, .. .. • • .. - - � * m M , . ••

'" 1B.·p�(�.;a ."

, " •

-

,�

'L--��"'; _ '1120 ."'. 1lI0II MO .. '" .. MII fOIl RI --,_.

FIG. 2. SulCate anion symmetric .stretching ",(A,) spectral region in the end­

members and several samples of the solid solutions. The roolc fractions, x, of the

corresponding cation are 003, O.S and 0.7.

••

••

� ... ,

I" ••

",

.. r----�� 1 .. ,· ....... °1 ••

� ...

i� ••

•• • I......-=""�---..:::

. .. .. <1 . .. ... _ ... •• --, ....

FIG. 4. Sulfatc anior. symmetr;e bending �£) spectral region in the end­

members and several samples orthc solid solutions. The mole fractions. x, of the

cornspoooing cation are 0.3, O.S and 0.7.

..

". • • • • • •

• • • ".

• , • E '5O i •

• · • • •

'" • E , • • • • • > ... • • •

• • ...

• Sr-Ba • • ... • Sr-Pb •

... Ba-Pb i

-• '" '" .. .. " .

% 01 the second cation

"

• Sr-Ba

• Sr-Pb • •

" •

... Ba-Pb •

• • • .. •

, • E • - . •

• • • � • • • •

• • •

• •

• • •

, •

• • " .. " .. ".

% ollh ... tond c:aUon

G. 5. (Top) Wavenumber of the sulfate &rOOD symmetric bending band against chemK:aJ cofq)Osition of the samples. (Bottom) Band full-widlh at half-heighl.

-

-

om l

r� 0"

-

om ,. o�

om

�.-• I "-,IS

ON

0-�

.-o.

0-

� ... • 1-·

_.

_.

_M -

tce .. ·w.csrSOJI

le

�I�" m

IBary1"IOaSOJI

m

[An8, .. lCa IPbSO.J1

'. '.

I, I , I \ "Iil

I �\ i j , \! 1 ,

: "._ ", ! t ' I . '/. ' ' \ ' • I, \ / i " :\1' , . J \'" --" ' <. ' !. ;

," '.

� i

" ! 1 rI

1J� 1, f , ,

\ I.i\

! ,

.. 0 --,-�

,.

"

"

"

FIG. 6. Band-fitted klw frequency spe<:tnlJ region down to the experimental limit

of 70 cm" in the end-membm (single p�) of the solid solutions stud;ed.

Components represented in broken line correspond to rotational (R) modes.

'e 0

� < • • • � «

250

Pb Ba 5, 200 •

• 0 • • •

0 • 0 150 0 • •

• • • 0 • • 0 • • • • •

100 0 • 0 • •

5O L--�--�-___ � ____ ---' 0.120 0.125 0.130 0135 0.140 0.145 0.150

h.!l·U2

FIG. 7. Wavenumber of the low ficquency fitted components against the square

root of the inverse cationic mass. Open symbols correspond to rotational (R)

mod ..

Figure 6 sho� the low frequency spectral region down 10 the experimental

limit of 70 cm" in the cnd-members (single phases) of the solid solul;ons studied.

EJclemal modes can be conveniently divided into translations en and rotations

(R). The fonner imply mainly the cation/anion interactions whereas the rotations

arc characteristic libralions of the anion. "I he R modes appear at wavcnumbers

which arc nearly independent oflhe cation present in the structure (see Figure 7).

lbe decrease in the number of components observed in barytes and, more clearly.

in anglesite is due to the experimental limit (70 cm") of the FT-Raman

spectrometer which precludes the observation or the lower wavenumber spectrum. As the T modes dt(rease their frequency when the mass of the cation

increases, observed in Figures 6 and 7, !;Cl/era] externa! modes of the samples

studied, mainly in angles;te, must appear below the limit of observation. In any

case, all these external modes are very weak in the FT-Raman spectraI3.U.l6. The

cationic substitution promotes the broadening of the T components which further

hinders the rigorous analysis of this spectral region. However, the spectral study

of those solid solutions which contain lead has enabled us to assign as rotations

the anglesite Raman bands at 134 and 152 cm·l, which were previously nOI

assigned", since their wavenwnber position and half-widths remain very

approJ[imateiy constant despite the cationic substitution.

REFERENCES

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2, Putniss. A .. Fem3ndez-Olaz.. L. and Prieto. M. Experimentally produced oscillatory zoning in Ihe (Ba,Sr)S04 solid solution. Nature, 1992; 358:743-745

3. Takahashi, S., Seki. M. and Setoyama, K. Fonnation of SrSO •. 'hH10 in an SrSO.-1120 system and its solid solution in a CaSO.-SrSO.-H20 system. Bull. Chem. Soc. Jpn .• 1993; 66:2219-2224

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;. He. S .. Oddo.l.E. and Tomson. M.B. The nucleation kinetics of Barium Sulfate in NaCI solutions up 10 6 m and 90·C. J. Colloid Jlflerjoce Scf., 1995; 174:319-326

6. He, S .. Oddo, lE. and Tomson, M.B. The nucleation kinetics of Strontium Sulfate in NaCI solutions up to 6 m and 9O"C with or without inhibitors. 1. Colloid Interjoce Sei .. 1995; 114;321-335

7. Cohen, AJ. and (Jordon, L. Co-precipitation in some binary sulfate systems. Ta/onto, 1961; 1:195-211

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9. Bostrom, K., Frazer, J. and Blankenburg. 1. Subsolidus phase relations and lattice constants in the system BaSO.-SrSO�-PbSO •. Ark. Mineral. Geo/., 1961; 4:411-485

10, Ross. S.O. Sulphates and other oxy-anions of group VI. in Fanner, V.C. (Ed.) ThtllPljrared Spectra af Mintlrais. Mineralogical Society, London. 1974:423-444

11. Jacobsen, S.D., Smyth, J.R., Swope, R.J. and Downs, R. T, Rigid-body character of the SOc groups in celestine, anglesite and barite. Can, Mineral., 1998; 36:1053-1060

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n. Alia, J.M., Edwards, H.G.M., Femandez, A. and Prieto, M. FT-Raman spectroscopie study of Ba(SO.).(crO.)I .. solid solution, J. Raman Specfrosc ..

1999; 30,105-117

14. Miyake, M., Minato, I., Morikawa, H. and iwai 5.-1. Crystal structures and sulphate force constants of barite, celestite, and anglesite. Am. Mi�ral .. 1978; 63:506-510

IS. Oawson, P .• Hargreave. MM. and Wilkinson, G.R. Polarized i.r. reflection, absorption alld la�r Rarn311 studies on a single crystal of 8aSO •. S�Cfrochim. Acta Part A, 1977; 33:83-93

16. Morandat, J., Lorenzelli. V. alld Paquet, H. Spectres d'absorption dans rinfrarouge ]oinlain de quelques sulfates metalliques. Essai d'inlerprelation. C. R. Acod. Sei. Poris, 1966; 263B:697-700