American Mineralogist, Volume 7I, pages 795-804, 1986 High-temperature crystal chemistry of dolomite Rrcn,Lno J. RBnoBn Department of Earth and Space Sciences,State University of New York at Stony Brook, StonyBrook, New York I1794 SrBvrN A. M,tmcur Materials Research Laboratory, Pennsylvania State University' University Park, Pennsylvania 16802 Ansrucr Structural parametershave been refined from X-ray intensity data for a stoichiometric dolomite single crystal at 24, 200,400, and 600'C. Both a rigid-body model and a con- ventional anisotropic thermal model were used for refinements,yielding nearly identical valuesfor temperaturefactors and residuals.This atteststo the suitability of the rigid-body model over the temperature range studied. Thermal expansion of a and c are similar to that of magnesitebut markedly different from that of calcite. The Ca and Mg octahedraexpand more rapidly than those in calcite and magnesite,respectively, and also differently. The Ca octahedron showsno increasein distortion with temperature as it does in calcite, and yet the Mg octahedron does, which contrasts with its behavior in magnesite.Rigid-body libration of the CO, group is inter- mediate in magnitude between that in calcite and magnesite,and neither rotation around the threefold axis nor out-of-plane tilting is dominant. Expansionof the libration-corrected C-O bond length is nearly identical to that in magnesiteand substantially less than that in calcite. No evidenceof cation disorder is found throughout the temperature range studied. INrnooucrroN magnesite.Our interest in understanding the high-tem- peraturebehavior of dolomite is quite varied. Dolomite, Over the past frfteen years, high-temperature crystal- Iike calcite, can be describedas a corner-linked structure structure investigations have been conducted for many of filled octahedra and nearly planar CO, groups. The rock-forming silicate and oxide minerals. More recently, lower symmetry of dolomite (spacegroup R3, compared interest in carbonateminerals has gained some attention. with R3c for calcite) results from the alternating Ca and Reederand Wenk (1983) refined structuresof severaldo- Mg layersand the slight rotation of the CO, groups which lomites quenched from temperaturesbetween 1050 and move the oxygensof the diad axes that exist in calcite. 1200'C, obtaining information about cation disordering. Since dolomite contains octahedra of both Ca and Mg, Owing to re-ordering during quenching, they were unable which as a first approximation are similar to those in to relate site distributions uniquely with temperature. Since calcite and magnesite,we have the opportunity to com- intensity data were collected at room temperature, no pare their relative behaviors in the same structure. Ther- evaluation of the effect of temperature on the structure mal expansion of dolomite is known to be positive for was possible.Markgraf and Reeder(1985) recentlyrefined both a and c (Bayer,l97l), similar to magnesitebut in the structures of calcite from data collected at several contrast to calcite whose expansion along a is negative. temperaturesup to 800'C and of magnesiteup to 500'C. The various factors influencing expansionin double car- We were specificallyinterested in the behavior of the CO, bonateshas not previously been investigated. group in relation to thermal expansionand, in the caseof In view of the role that thermal vibrations of the CO3 calcite, in relation to proposedanion rotational disorder. group play in the single rhombohedral carbonates,it is The characterization of the CO3 group using the rigid- desirable to understand their role in dolomite. The po- body model proved to be very useful.The most significant tential importance may be shown by two particular ex- finding was the rather different vibrational character in amples, one relating to the unusual strength-temperature calcite than in magnesite.In the former, the dominant relationship shownby dolomite. Higgs and Handin (1959) motion of the CO3 group is a low-amplitude rotary os- and more recently Barber et al. ( I 98 I ) showedthat critical cillation around the threefold axis; in magnesite,it is more resolved shear stress for all but one deformation mech- nearly a tilting out of the plane in which it lies. anism increaseswith temperaturein dolomite singlecrys- The present study of dolomite can be viewed as an tals. Calcite singlecrystals show the more typical behavior extension of our high-temperature work on calcite and of decreasingstrength with temperature.Barber et al. at- 0003{04v86/05064795$02.00 79s 796 REEDER AND MARKGRAF: DOLOMITE HIGH-TEMPERATURE CRYSTAL CHEMISTRY tributed this strength behavior in dolomite-particularly Data collection for c slip-to impedence of dislocation movement asso- A automated Picker four-circle diffractometer operating with ciated with thermal motion of the CO, group, which of graphite-monochromatedMoKa radiation (I : 0.7107 A) was course increases with temperature. At the time of their usedto collect integratedintensities at24,200,400, and 600"C. study there were no data describing thermal motions at Roughly 530 diffraction intensities(sin d/), < 0.8 1)were collected high temperatures in either calcite or dolomite, when at eachtemperature using the ot-20scan mode with a scanwidth clearly a need existed. of 2.0' + 0.7 tan d and maintaining a constant precision of o,/I :0.01, where o, is basedon counting statistics.A As in the case of calcite, the question of possible COr- standard intensity was collectedevery 20 reflectionsthroughout eachdata group rotational disorder in dolomite has been raised. set for the purposesof indicating crystal movement or decom- Although Reeder and Nakajima (1982) and Reeder and position. Wenk (1983) have discussed pertaining some aspects to Unit-cell parameters were refined at all temperatures for which such anion disorder at high temperature, there is little intensity data werecollected and also at 700.C. The least-squares evidence to support it. Changes in rigid-body motion pa- refinement used 24 reflections (0.50 < sin d/tr < 0.65), each of rameters with temperature may provide an indication of which was centered at positive and negative 2dand then averaged. premonitory behavior for such a transformation. Refinernent details Our results reported here for single-crystal, high-tem- perature X-ray diffraction experiments on dolomite help Intensities were correctedfor Lorentz and polarization effects to address these various considerations. and also absorption. For the latter, the crystal shapeand orien- tation relative to the X-ray beam were modeled using D. K. ExpnnrunNrAl METHoD Swanson'sprogram nxl. The absorption corrections were cal- culated using a modified version of L. W. Finger's program AB- Specimen sonn, yielding maximum and minimum transmission factors of 0.86 and 0.75, respectively (pr*" : 15 cm-'). No The dolomite crystal used in this study was taken from a clear absorption correction was made for the silica cleavagerhomb (measuringseveral centimeters in mean dimen- capillary. A reflection was consideredunobserved when 1 < 2or. Absorption-corrected sion) found in the magnesitedeposits at Eugui in northern Spain. data were symmetry-averaged,resulting in ro"ehly 300 independent This sample is from the same cleavagerhomb that was used in observationsfor eachtemperature. This resultedin earlier studiesby Reederand Wenk (1983) and Reederand Na- an I 8:1 ratio of independent observations to refined parameters. kajima (1982). Eugui dolomite has also been used for several Refinementswere done usingthe least-squaresprogram REFTNE other studies requiring good-quality single crystals, including the in the pnor"trnEus system(Zucker et al., 1983),which single-crystaldeformation work of Barber et al. (1981). In their is a mod- ifred version ofthe program nnNra (Finger Prince, room-temperaturerefinement, Reederand Wenk found the cat- and 1975).It incorporatesthe rigid-body treatment (as ion orderingto be ideal. Examination in the transmissionelectron implemented in nrlNu) by refining rigid-body parameters microscopehas shownthat Eugui dolomite is homogeneous,with directly. Weights were assigned proportionally to l/o2r,where o" is based on counting very low dislocation densities(cf. Barberet al., I 98 l; Reederand statistics. Wenk, 1983). Starting parametersof Reeder and Wenk (1983) were used for the initial refinement. Final refinements used isotropic The composition reported by Reederand Wenk showsthat it the ex- tinction correctionofBeckerand Coppens(1975), assuming Type is nearly ideal-Ca, *,MgrrrFeoo,oMno*r(COr)r.The crystalused I behavior and a Lorentzian mosaic angular distribution. in this study has approximate dimensions of 240 x 235 x 95 The refineableparameter G, for mosaic spread,ranged pm. The crystal was mounted on a silica-glassfiber using Cera- from 1.02(7) at 24.to 0.77(6)at 400"C.Atomic scatteringfactor mobond 503 (Aremco Products, Inc.). It was put in a 0.5-mm cures for neutral atoms and corrections for silica capillary, which wasthen evacuatedand sealed,and mount- anomalous dispersion were taken from International ed in a standardgoniometer head. Precessionand Laue photog- the Tables for X-ray Crystallography, volume IV (Ibers and Hamilton, 1974). raphy produced sharp spots and showed that it was a single crystal. In addition to the standardrefinements using anisotropic tem- perature factors, we also carried out rigid-body refinements using In situ heating the T, L, and S tensors (Schomakerand Trueblood, 1968) for the averagethermal motion