Phase Transformations in Silica-Alumina- Magnesia Mixtures As Examined by Continuous X-Ray Diffraction: I

$Phase Transformations in Silica-Alumina- Magnesia Mixtures As Examined by Continuous X-Ray Diffraction: I$ THE AMERICAN MINERALOGIST, VOL. 47, NOVEMBER-DECEMBER, 1962 PHASE TRANSFORMATIONS IN SILICA-ALUMINA- MAGNESIA MIXTURES AS EXAMINED BY CONTINUOUS X-RAY DIFFRACTION: I. TALC-KAOLINITE COMPOSITIONS R. B. Gnen,lF. M. Wnnr, aNp R. E. Gnru, Uniaersityof Illinois, (Jrbana, I llinois. Ansmacr The phase transformations of natural and synthetic oxide mixtures of talc-kaolinite compositlons were examined by continuous r-ray difiraction while heated to 1450' C. Dif- fraciion intensity data show the enhanced development of protoenstatite within the sheet structure of talc, thus indicating that the growth of this mineral on firing is largely con- trolled by the structure of the talc. The formation temperatule of cordierite, which was used as a measure of the reactivity of various starting materials, indicates that alumina in the form of mullite, rather than as corundum, is most efiective in lowering the development temperature of this mineral. Additional cordierite formation, through recrystalliza- tion on cooling indicates the presence of an intermediate licluid phase in the formation of this mineral. INrnotucrlow Generalsta,tement . Continuous high-temperature o-ray diffraction methods make it pos- sible to observehigh-temperature reactions as they occur (Wahl et aI., 1961). Previously, high-temperatrte n-ray diffraction has been used mainly for selected observations at some desired temperature and not for continuousobservations of a sampleas it is heated.The techniqueof continuous scanning was used in the present investigation for the study of high-temperaturereactions in talc, kaolinite, talc:kaolinite mixtures, and mixtures of oxides with compositions corresponding to talc and kaolinite. Infl,uence of starting materials Reactionrates between solids are known to be influencedby the nature of the starting materials. Significantly different results have been obtained by using different starting materials which have the same bulk composition.Thus, Fyfe (i960) has discussedthe reactivity of oxidesas opposedto glasses,and Roy (1956)has reported on the reactivity of co- precipitatedgels. Reaction rates in the systemsSiOz and AhOrSiOz with different starting materialshave beeninvestigated by wahl et al. (l96la, 1961b)by meansof continuoushigh-temperature r-ray difiraction. I Present address: General Dynamics Corporation, Fort Worth, Texas. t273 1274 R. B. GRAF, F, M. WAHL AND R, E. GRITI Co ntinwo us hi gh- tem p eratur e x-r ay d,if r action In the technique,as usedin the presentstudy, the samplewas packed on a small platinum holder which was then placed in a small platinum- wound furnace.This furnacewas mounted on a water-cooledbase which, in turn, was mounted on a Philips recording r-ray diffractometer.The desired20 values were repeatedly scanned while the samplewas heatedat a constant rate. The intensities of the various diagnostic diffraction peakswere plotted againstthe temperaturesat which they occurred.The curvesobtained in this manner depict changesin the amounts (not abso- lute quantities)of crystallinematerials as a function of temperature.Be- causethe samesample was usedthroughout the run, differencesin pack- ing or preferred orientation were minimized and smaller changesin difiraction intensity could be detected. Purpose of the inaestigation A primary objectiveof the investigationwas to determinethe reaction rates of various starting materialswhich had the samebulk composition within the nIgO-AlrOe-SiOzsystem. Cordierite2 was formed from most of thesemixtures, and the formation temperatureof cordieritewas usedas a measureof the reactivity of the starting materials.The transformationsof kaolinite to mullite and cristobalite and of talc to protoenstatitesand cristobalite,were brieffy investigated. Expnnrunwtar, PnocpounB Furnace and sample holder The furnaceused in this investigationwas similar to that describedby Grim and Kulbicki (1957). In the construction of the present furnace, however,the resistancewinding (PL-137a Rh) was coiledbefore it was wrappedon the alundum core. Five feet of .012-inchwire wereused and coiled in such a manner that four evenly spacedwraps could be made on the alundum centerpiece. The baseplate was groovedso as to insure reproducible aiignment of the platinum sample holders, which were 1X8X 14 mm with a depression0.5 X6X 10 mm. T emperature control and,measur etnent The power suppliedto the furnace was controlledby two variacs,and a constant heating rate of 5.6' C. per minute was obtained. When the temperature of the furnace had reachedthe desiredmaximum (usually '?All of the cordierite which was synthesized in this study was apparently indialite, as discussedby Sorretl (1960) 3 The enstatite in this investigation is apparently the protoenstatite for which a com- plete structure analysis was made by Smith (1959). PHASE TRANSFORMATION 1275 about 1475oC.), the power was turned off and the furnacewas allowedto cool.Because the furnacehas a small heat capacity,a rapid and uniform coolingrate was obtained. Temperatureswere measuredby using Pt-Pt l07o Rh thermocoupleswith a cold junction and a Minneapolis-Honey- well Model 2732 potentiometer. The accuracy of the thermocouples was checked several times during the investigation by using the inversion of BaSOaat 1149" C. The agreementwas usually within about 5o C. and never greaterthan 10" C. At least two runs were made for each sample.In the first run, 20 values oI 2" to 50' (the maximum range with this furnace) were repeatedly scannedas the sampleswere heated. In the secondrun, only the diagnostic difiraction peaks for each phase were measured, usually at temperature intervals of about 25" C. AII of the data were obtained using nickel-filteredcopper radiation generatedat 45 kv and 18 ma. Starting materials The following materials were used as reactants: MgO, light powder, Allied Dye and Chemical Corporationl "silicic acid," Mallinckrodt Chemical Works; talc, Baker Chemical Company; kaolin, Mclntyre, Georgia; and synthetic gibbsite, AI(OH)3, Merck and Company. The kaolin was shown by x-ray diffraction to be substantially pure well- crystallized kaolinite. A small amount of mica was present and analysesof similar kaolinsindicate that oneor two per cent K2O may be present.The "silicic acid" and synthetic gibbsite were of reagent grade. When the MgO was fired in a continuous run, it was at first characterizedby very broad difiraction peaks which correspondedto those of periclase.These diffraction peaksbecame sharper as the firing was continued. The mixtures were made according to mole per cent calculations in ratiosof talc to kaoliniteas followsl 4:1,3:2, l:1,2:3, and 1:4. Oneof group of mixes was composedof natural talc and kaolinite in those proportions. Another group was composedof kaolinite and MgO-"silicic acid" which were mixed in proportions corresponding to talc. A third group consistedof talc and syntheticgibbsite-"silicic acid" corresponding to the composition of kaolinite. The mixtures of oxides correspondingto the natural mineralswill be referredto as TC and KC, meaningrespec- tively talc compositionand kaolin composition.Thus, 4K: TC refersto a mixture in which MgO and "silicic acid" corresponding to the composition of talc are mixed with four times as much naturally occurring kaolinite (mole per cent). In this investigation,talc and kaolinite were comparedto the mixtures of oxidesas if they were anhydrous.However, the water content was consideredin the weighing and mixing of the, samples. 1276 R. B. GRAF, F. M. WAHL AND R. E. GRIIT I I F a z lal F z CrisL(lll) P-En.(l2l) 800 900 rooo lloo 1200 1500 14000c Frc. 1 lntensity of difiraction by high-temperature phases in heated talc. The materials which were used in the various mixes were dried, weighed and mixed in the desired proportions, mixed further as a paste with distilled water, dried at 100' C. for about 8 hours, and then re- ground. An attempt was made to pack the sameamount into the sample holder each time. ExpBnnrBNrAL RESULTS When talc wasfired in a continuousrun (Fig. 1), it beganto decompose at about 830" C. and protoenstatitewas formed immediatqly. When the temperature reached approximately 1220o C., cristobalitea also was formed. The most prominent feature of the formation of protoenstatite was the enhancedintensity of the (310) as compared to the (l2t) rc- flection.Such intensity relationshipshave beenused by Kulbicki (1959) to explain the structural relationships of the transformations of sepiolite, attapulgite, and saponite to enstatite. He consideredthe formation of enstatite to be favored by the fibrous nature of sepiolite and attapulgite, and stated that enstatite would form with less intensity and rapidity from a mineral with a layer-type structure.In the presentinvestigation, however, protoenstatiteformed rapidly and in considerableabundance from talc. Using the (310) and (121) reflectionsit is possibleto compare the intensitiesof reflectionsfrom planesparallel to the c axis (310) with those from planes which intersect the c axis (l2l). The intensity differ- a The cristobalite formed at elevated temperatures is beta cristobalite in every case. PHASE TMNSFORMATION 1277 encesindicate that protoenstatite is preferentially grown with the c axis in the plane of the silica sheets of the original talc. The sheets must therefore be partitioned into chains to afford the protoenstatite nuclea- tions, with consequent expulsion of one-fourth of the silica, which later forms cristobalite. Therefore, it is considered that the protoenstatite has grown perferentially within the

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