Phys Chem Minerals (2007) 34:699–704

DOI 10.1007/s00269-007-0186-2

ORIGINAL PAPER

Thermal behaviour of �-anhydrite CaSO4 to 1,263 K

Paolo Ballirano · Elisa Melis

Received: 14 May 2007 / Accepted: 15 August 2007 / Published online: 28 September 2007© Springer-Verlag 2007

Abstract The thermal behaviour of �-anhydrite CaSO4

has been investigated to 1,263 K in-situ real-time using lab-oratory parallel-beam X-ray powder diVraction data. Thecell parameters expanded anisotropically, the c axis beingthe “softest”. This behaviour is due to the deformation ofthe CaO8 polyhedron. In fact the two longest, independent,Ca–O bond distances show a signiWcant component alongthe z direction.

Keywords Anhydrite · CaSO4 · Thermal behaviour · X-ray powder diVraction · Rietveld method

Introduction

Beta-anhydrite CaSO4 (insoluble anhydrite, space groupAmma, orthorhombic, pseudotetragonal, with a = 6.99 Å,b = 7.00 Å, c = 6.24 Å, Strunz and Nickel 2001) is one ofthe most common calcium sulphate minerals. It exhibitsa wide variety of paragenesis ranging from evaporitic tofumarolitic deposits. Its crystal structure at room tempera-ture (RT) is known in detail (Morikawa et al. 1975; Haw-thorne and Ferguson 1975; Kirfel and Will 1980). Therelevant structural motif is represented by a chain of alter-nating edge-sharing SO4 tetrahedra and CaO8 distorted tri-angular dodecahedra running along c axis (Fig. 1). The SO4

group has mm symmetry with two independent, but very

similar, S–O1 and S–O2 bond distances. It is only slightlydistorted from an ideal tetragonal disphenoid, the distortionarising from a contraction of the bond angles correspondingto the O2I–O2III edges shared with CaO8. The calcium atomis coordinated by eight oxygen atoms (four couples of inde-pendent distances ranging from 2.35 to 2.57 Å) forming adistorted triangular dodecahedron. The link is via edge-sharing along a and by corner-sharing along b. Identicalchains also occur in the structure of gypsum. Despite beinga very important feature, thermal expansion of �-anhydritehas not yet been investigated in detail. Khan (1976) wasable to predict the thermal expansion of a few non-cubicknown crystal structures, including �-anhydrite. The proce-dure used a distance least-squares (DLS) reWnement of thestructure after an empirical estimation of the expansionof the various coordination polyhedra. The estimate wascarried out using a square bond-strength s2 versus averageexpansion coeYcient �AV of cation-oxygen bond lengthplot. The derived expansion coeYcients, in the 293–548 Krange, were found to follow the �c > �b >> �a trend. Subse-quently, Evans (1979) experimentally determined the thermalexpansion of �-anhydrite up to 1,273 K from Guinier–Lennè photographs. Data were Wtted with a second-orderpolynomial function obtaining values in good agreementwith those predicted by Khan (1976). According to Evans(1979), the orthorombic unit cell becomes pseudo-tetrago-nal at 683 § 100 K (so-called a-b crossover temperature), avalue only in fair agreement with the temperature of 573 Kindicated by Khan (1976). However, Evans (1979) pointedout that the low resolution of the Guinier–Lennè data doesnot permit a diVerence between a and b up to 773 K to bedistinguished. Present work reports new data on the thermalbehaviour of �-anhydrite to 1,263 K from laboratory paral-lel-beam transmission powder diVraction data evaluated bythe Rietveld method.

P. Ballirano (&) · E. MelisDip.to Scienze della Terra, Università di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italye-mail: paolo.ballirano@uniroma1.itURL: http://cristal.geo.uniroma1.it

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Experimental methods

A sample of powdered synthetic analytical-grade gypsumCaSO4·2H2O (Analar, product 10071) was heated at1,123 K for 2 h and subsequently cooled to RT. A prelimi-nary powder diVraction spectrum on a conventional Bragg–Brentano instrument conWrmed the completeness of dehy-dration. The powder was loaded in a 0.7 mm diameterSiO2–glass capillary, and glued to a 1.2 mm diameter Al2O3

tube by means of an high-purity alumina ceramic (Resbond989). The capillary/tube assembly was subsequently alignedon a standard goniometer head. Non-ambient data werecollected on a parallel-beam Bruker AXS D8 Advanceautomated diVractometer operating in �–� geometry. Theinstrument is Wtted with two Soller slits (2.3° opening angleon the incident and radial on the diVracted beam) and aPSD VÅNTEC-1 detector set to a 6° 2� aperture. Thecapillary heating chamber is a prototype developed by MRI

and AXS Bruker and is Wtted with an MRI TCPU1 temper-ature control and power unit equipped with a 2404 Euro-therm controller. The capillary heater consists of a boredcorundum cylinder that act as thermal insulator. Incidentand diVracted beams pass through two 12 mm wide win-dows covered by aluminium foils with a maximum angularopening of ca. 70°�. The chamber slides along two guidingrods allowing the insertion of the capillary via a 2 mmdiameter hole. During measurements the capillary spins at60 rpm. The heater element is a Kanthal spiral and tempera-ture measurement is carried out by a type K (NiCr/NiAl)thermocouple. Chamber cooling is obtained via water Xow.Thermal calibration of the chamber was carried out usingMgO (periclase) (Reeber et al. 1995). The same capillarywas used for two subsequent heating cycles from 303 to1,263 K with 10 K increments. DiVraction experimentswere made through an angular range of 10–140° 2�, usingCuK� radiation. The full data set consists of 97 diVractionpatterns for each cycle. A magniWed view of the completedata set for the Wrst heating cycle is reported in Fig. 2 as apseudo-Guinier plot. DiVraction data were evaluated by theRietveld method using the GSAS crystallographic suite ofprograms (Larson and Von Dreele 2000) coupled with theEXPGUI graphical user interface (Toby 2001). Peak shapeswere modelled by means of a standard Thompson–Cox–Hastings (TCH) pseudo-Voigt function (Thompson et al.1987) modiWed to incorporate asymmetry (Finger et al.1994). ReWned parameters included tan2�-, tan�-dependentand �-independent Gaussian parameters (GU, GV andGW), 1/cos�- and tan�-dependent Lorentzian parametersLX and LY, asymmetry parameters S/H and L/H. The peakcut-oV was set to 0.2% of the peak maximum. Peak posi-tions were corrected for sample displacement from thefocusing circle. The background was Wtted with a 27-termChebyshev polynomial of the Wrst kind. Besides cell para-meters, reWned structural parameters include fractionalcoordinates and displacement parameters. S–O and Ca–Odistances were unconstrained. An absorption parameter(including the contribution from the aluminium heatingchamber windows) was reWned at 303 K and subsequently

Fig. 1 Fragment of the structure of �-anhydrite showing the CaO8distorted triangular dodecahedron. Bond distances in Å

Fig. 2 MagniWed view (20–75° 2�) of the complete data set of the Wrst heating cycle shown as a pseudo-Guinier plot

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kept Wxed for the remaining spectra. The presence oftexture was checked by means of a generalised spherical-harmonic description (Von Dreele 1997). No signiWcantimprovement of Wts was observed as a result of texture indi-ces J very close to one, as expected for a capillary mount.Fitted X-ray diVraction patterns obtained at 303 and 793 Kare reported as example in Fig. 3. Miscellaneous data of thereWnements are reported in Table 1.

Results and discussion

Cell parameters and volumes obtained for �-anhydrite fromRietveld reWnements within the 303–1,263 K range are dis-played in Fig. 4. Relative expansion of unit cell and volumeare reported in Fig. 5.

It is interesting to notice that the two heating cycles leadto cell parameters markedly diVerent. In detail the a and bcell parameters from the second run are more expandedthan those obtained from the Wrst one, but the diVerencereduces as the temperature increases. On the contrary, the cparameter is substantially unchanged, a part for a very mar-

ginal reduction for the second run at temperature smallerthan ca. 600 K. As a result the volume for the second cycleis consistently slightly larger than that of the Wrst one(Fig. 4d). The diVerence between cell parameters observedduring the two runs has been related to the presence of �0

strain that is progressively released during the heatingcycles. In fact, for the Wrst run we observe a regulardecrease of the proWle parameter LY (related to �0 strain)with increased temperature (Fig. 6). The decrease of LY isregular and almost linear until ca. 1,000 K but above thistemperature it modify the behaviour decreasing at a veryfast rate. Cooling down the sample to RT we obtain an LYvalue smaller than that reWned for anhydrite before the Wrstheating cycle. During the second run the decrease is almostperfectly linear, apparently tending to the same Wnal valueof LY of the Wrst heating cycle. This behaviour provides aclear indication that the strain is relaxed manly during theWrst cycle. ReWnement of data from further heating cycleslead to the same cell parameters obtained during the secondrun without any further modiWcation nor further reductionof LY. The presence of strain could be possibly attributedto the relatively fast cooling rate of the synthesis routeused.

Evans (1979) observed the presence of extra phases atT > 1,173 K attributed to some reaction products of anhy-drite and silica, the latter coming from the SiO2–glass capil-lary. In the present work such extra phases were notdetected.

The following discussion is based on the second heatingcycle. The data conWrm that thermal expansion of �-anhy-drite is markedly anisotropic. However there are signiWcantdiVerences with respect to prior data. In particular Evans(1979) indicates that the a cell parameter reaches a maxi-mum value at ca. 873 K and then slightly reduces. More-over the b cell parameter was reported to follow asigmoidal behaviour. On the contrary, the present data indi-cate that the a cell parameter (Fig. 4a) still increases above

Fig. 3 Fitted X-ray diVraction pattern obtained at a 303 and b 793 K.Experimental data are shown as dots, calculated as solid line. Thelower curve represents the diVerence between observed and calculatedproWles

Table 1 Miscellaneous data of the reWnements. Statistic indicators asdeWned in Young (1993)

First cycle Second cycle

2� range (°) 10–140

2� step (°) 0.022

Counting time (s) 0.15

ReWned parameters 55

Number of observed reXections 372–374

Reduced �2 1.053–1.200 1.148–1.340

Rp (%) 4.03–4.62 4.12–4.63

wRp 5.18–5.96 5.30–5.90

RBragg 3.85–6.04 1.80–3.70

DWd 1.731–1.919 1.585–1.751

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the limit indicated by Evans (1979) and that the b parameter(Fig. 4b) expands linearly above ca. 873 K. The c parame-ter (Fig. 4c) shows a behaviour in agreement with thatreported by Evans (1979).

The unit cell parameters and volume were Wtted with asecond-order polynomial of type p = a0 + a1T + a2T

2 (Fei1995) where a0 is the value of the parameter at 0 K, a1 is theWrst-order coeYcient of expansion, a2 the second-ordercoeYcient and T the temperature in K. The calculatedparameters are shown in Table 2. The coeYcients for a andb are in poor agreement with those, recalculated, of Evans(1979). On the contrary, those of the c cell parameter and

volume are in very good agreement. This is clearly due tothe diYculty to discriminate a and b due to the limited reso-lution of the Guinier–Lennè apparatus used by Evans(1979). The a-b crossover T has been evaluated to be of508 K by equalling the two functions describing the Tdependence of a and b cell parameters. This value is signiW-cantly smaller than those reported in reference. Regulartrends in fractional coordinates (and therefore in bond dis-tances) and ADP’s were also observed. The expansion ofbond distances has an anisotropic trend. The S–O bond dis-tances do not change signiWcantly during the heating cyclesshowing a regular reduction. However, applying a simple

Fig. 4 Evolution of cell parameters and volume with temperature: a a cell parameter; b b cell parameter; c c cell parameter; d volume

Fig. 5 Relative expansion of unit cell parameters and volume Fig. 6 Evolution of LY proWle parameter with temperature

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rigid body correction (Downs et al. 1992) the S–O bonddistances show only a marginal increase with increasingtemperature, as expected. On the other hand CaO8 polyhe-dron expands anisotropically with increasing temperature.Bond distance expansion has a general linear behaviour(Fig. 7). Only Ca–O2I seems to depart signiWcantly fromthis general trend (Fig. 7c). As expected, expansion isroughly proportional to the “softness” of the bond, thelongest (RT = 2.57 Å) expanding more than the shortest(RT = 2.35 Å). The relatively strong Ca–O2III bond (ca.0.36 valence units v.u. following Breese and O’KeeVe

1991) is almost parallel to [100] providing an explanationto the small expansion coeYcient of the a cell parameter.Moreover, it is the only Ca–O bond showing a signiWcantcomponent along a. Ca–O1II has the largest componentalong [010] and being the second shortest Ca–O bonddistance is responsible for the slightly greater b axis expan-sion with respect to the a axis. The longest Ca–O bonddistances (Ca–O2I and Ca–O1IV, corresponding, respec-tively, to ca. 0.23 and 0.20 v.u.) show principal componentsalong [001] justifying the largest thermal expansion of thec cell parameter.

Fig. 7 Evolution of individual Ca-O bond distances with tem-perature: a Ca–O2III; b Ca–O1II; c Ca–O2I; d Ca–O1IV

Table 2 Thermal expansion coeYcients for anhydrite as obtained for the two heating cycles

Fitted function of the type p = a0 + a1T + a2T2. For comparison purposes recalculated data from Evans (1979) are reported

a b c Volume

First cycle (present work) R2 0.9988 0.9997 0.9996 0.9999

a0 6.976(4) 6.965(4) 6.220(1) 302.3(3)

a1 5.4(1) £ 10¡5 7.1(1) £ 10¡5 6.0(3) £ 10¡5 8.19(9) £ 10¡3

a2 ¡4.2(7) £ 10¡9 1.56(7) £ 10¡8 9.4(2) £ 10¡8 5.38(5) £ 10¡6

Second cycle (present work) R2 0.9993 0.9998 0.9995 0.9999

a0 6.985(3) 6.969(3) 6.220(1) 302.4(3)

a1 4.38(8) £ 10¡5 6.69(9) £ 10¡5 6.04(3) £ 10¡5 8.52(8) £ 10¡3

a2 ¡1.9(5) £ 10¡10 1.67(5) £ 10¡8 9.4(2) £ 10¡8 5.11(5) £ 10¡6

Evans (1979) R2 0.9400 0.9843 0.9971 0.9990

a0 6.966(6) 6.982(7) 6.220(9) 302.5(4)

a1 1.3(2) £ 10¡4 5(2) £ 10¡5 5(2) £ 10¡5 1.0(1) £ 10¡2

a2 ¡6(1) £ 10¡8 2(1) £ 10¡8 1.0(2) £ 10¡7 3.2(6) £ 10¡6

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Acknowledgments We thank F. Scordari, two anonymous referees,and Editor P.C. Burnley for their constructive reviews of the paper.This work was supported by Università di Roma “La Sapienza”.

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