American Mineralogist, Volume 71, pages 937-945, 1986
Phasetransitions in leucite (KAlSirOd), orthorhombic KAlSiOo, and their iron analogues(KFeSirOu, KFeSiOa)
Rnnncc.q. A. LaNcn. LtN S. E. Cnnilucrrlnl Departmentof Geologyand Geophysics,University of California,Berkeley, Berkeley, CalLfomia94720 JoNlrrrlN F. SrnnnrNs Earth SciencesDivision, LawrenceBerkeley Laboratory, Berkeley, CaLifornla 94720
Ansrnlcr The heat capacitiesof natural and synthetic samplesof leucite (KAlSirO6), orthorhombic KAlSiO4, and their iron analogues,KFeSirOu and KFeSiOo,have been measuredbetween 400 and 1000 K by differential scanning calorimetry. For the various phase transitions that occur, temperaturesoftransition and associatedchanges in enthalpy and entropy have beendetermiled. The tetragonakubic transition in leucite spansl22to 176deg, depending upon the sample studied, and is characteiaed by two peaks on a C, heating curve. Heat treatment of a natural leucite for one week at 1673 K lowers the transitionby 24 deg. Two distinct peaks in the C,gurve, enhancedby heating, suggestthat an intermediate phase exists that is stable over 17 deg. A spacegroup of l4r/acdhas been assigrredto this phase becauseit can be related to the low-temperature tetragonal leucite phase(I4r/a) by mer- ohedric twinning and the high-temperaturecubic leucitephase(Ia3d) by pseudomerohedric twinning, both of which are found in leucite. In contrast, KFeSirOufleucite structure) has a single, sharp C. peak at a lower transition temperature. A natural leucite consisting of 88 wto/oKAlSirO6 and 12 wto/oKAISi:O, has an X-ray pattern that indicates tetragonal symmetry, yet appearsto be without twinning when viewed optically. The C"data indicate a very small transition effectwhich may, however, be spreadout in temperatureabove the limit of the DSC (1000 K). The orthorhombic polymorph of KAlSiOo undergoesone transition at 695 and one at 8 l7 K, whereasits iron analogue,orthorhombic KFeSiOo,has a single transition at729.5 K. The enthalpy and entropy data indicate that the single transition in orthorhombic KFeSiOo is approximately equivalent to the sum of the two transitions in KAlSiO..
IurnooucrroN Leucite is a characteristicmineral of K-rich, SiOr-poor Many tectosilicate minerals, such as tridymite (SiOr), lavas. At high temperaturesit has cubic symmetry, but pure-Na nepheline (NaAlSiO4), leucite (KAlSirOu), and during cooling it reversibly changesto a tetragonal form. orthorhombic KAlSiOo, have high-order phase changes The transition involves a collapseof the (Si,Al)-O frame- that are affectedby composition, ordering state,and crys- work about the K cations and two types of complex, re- tal structure. To understand transition mechanisms and peated twinning on the {110} crystallographic planes: to calculate high-temperature phase equilibria, the ener- merohedric and pseudomerohedric(Mazzi et al., 1976). geticsof thesetransitions must be known. Currently, lim- Thermal-expansion data (Taylor and Henderson, 1968; ited information exists on the thermodynamic properties Hirao et al.,1976) in conjunction with heat-contentmea- of the various feldspathoid minerals. The best-studied surements(Pankratz, 1968) indicate that the tetragonal- minerals in terms of calorimetric data are nepheline [(I! cubic transition is continuous, displacive, and secondor- Na)AlSiOol and tridymite (Henderson and Thompson, der. DTA (differential thermal analysis) heating curves for 1980;Thompson and Wennemer,1979), which are both natural and synthetic leucites(Faust, 1963) show that the characterized by multiple transitions below 700 K. On transition is characterized by two endothermic peaks with the potassicside of the nepheline-kalsilitejoin, enthalpy a positive hysteresis(i.e., the transition occurs at lower and heat-capacitydata exist for only one phase, kalio- temperaturesduring cooling than during heating). How- philite (Pankratz, 1968),although severalpolymorphs are ever, when Fe3*substitutes for AI, KFeSirOuhas a single, known (Smithand Tuttle, 1957;Cook et al., 1977;Abbott, sharp DTA peak at a lower temperature. 1984). Heat capacities for synthetic leucite have been Two hexagonalpolymorphs of KAlSiOo also occur in measured@ankratz, 1968),but data on natural samples, K-rich, volcanic rocks: kalsilite and, less commonly, ka- KFeSirOu (leucite structure), and SiOr-rich leucite are liophilite (Bannister,I 93 I ; Holmes, I 942).Pankratz ( I 968) lacking. conducteddrop calorimetry experimentson a synthesized 0003-004x/86/0708-0937$02.00 937 938 LANGE ET AL.: LEUCITE, ORTHORHOMBIC KAlSiOo, AND Fe ANALOGUES
Table 1. Key to specimennumbers Table 2. Chemical analyses of specimens
89-21 Natural leucitefrom leucite-trachyte,Mt Cimini, Samote 89-2lr 89-21-At 90-052 1174t FL lr sL22 ol2 a-KF2 Roman Distnct, Italy si02 549 552 562 484 566 168 323 Alzor 225 230 221 203 2J0 349 89-21-,{ Mt. Cimini leuciteheated at 1673 K for one week Fe2O3 03 0l 03 2l 323 0l 0l 421 Na2O 0l 0l 08 90-05 Natural leucitefrom leucite-phonolite,Rocca Monfina, Kro 20,9 200 202 2Ll l89 202 219 251 Roman District, Italy Toul 984 985 992 997 996 993 997 995
2t663 21900 24109 21601 15648 1E694 t774 Natural leucitefrom wyomingite,I-eucite Hills, sfr"t 2t1 97 2t645 Wyoming,U.S.A. rElectron microprobe daE (E %)
zwet SL.I Syntheticleucite (KAlSizOo) chemical dau (w %)
lglw - four for Ol and d-G FL.I Synthetic KFeSizOe gm fomula weithr based on six oxygens for leuciles and oxygens
SL.2 Syntheticsolid solution of 86 wt % (KAlSi2O6) l4 w1 % (I(AlSi3O8)
Ol Syntheticonhorhombic KAlSiO4 Syntheticleucite (KAlSirOu; specimenSL.1) was preparedhy- drothermally from a gel at 1033 K and I kbar for 37 h by W. S. a-KF SyntheticorthorhombicKFeSiOa MacKenzie.KFeSirOu (eucite structure,specimen FL.1) was syn- thesizedby mixing together appropriate amounts ofpotassium p-KF Syntherichexagonal KFeSiOr carbonate, ferric oxide, and dehydrated silicic acid. The mixture was melted at 1573 K and allowed to crystallize for 4 d at 1273 K, then remelted at 1673 K and crystallized for 3 d at 1273 K. material identified by X-ray data as kaliophilite. A tran- Electron-microprobe and wet-chemistry data (Table 2) confrrm sition near 810 K with an enthalpy of transition of 576 that the resulting crystalline material consists of the single phase Jlmol was described as first order. The kaliophilite was KFeSirOu and that all of the iron is in the ferric state. of leucite (specimenSL.2) was prepared synthesizedby Kelley et al. (1953) ar 1723 K and I atm. A silica-rich sample as a gel (Hamilton and Henderson, 1968). The dehydrated gel However, based on phase-equilibrium studies by Tuttle was loaded into a Pt crucible and heated for 3 d at 1373 IC A (1958) (1977), and smith and cook et al. suchconditions portion of the sample was then removed while the rest was heated of synthesis should produce the orthorhombic form of for another 5 d at L773 K. Electron-microprobeand wet-chern- KAlSiO4. Perhaps, like nephelines synthesized in the istry data on the crystalline product (Table l) indicate that it is NaAlSiO"-KAlSiOo system (Henderson and Thompson, a leucite solid solution with approximately 14 wto/oofthe KAlSiOt 1980), the structure of the various KAlSiOo polymorphs component. is critically dependenton the method of synthesis. The high-temperature KAlSiOo orthorhombic phase (denoted To study the effectsofcation size and ordering on the as Ol after Smith and Tuttle, 1957) was synthesizedby mixing phasetransitions in leucite and KAlSiO4, heat capacities together potassium carbonate, aluminum oxide, and dehydrated acid. The sample crystallized for 2 d at 1573 K and for 3 between400 and 1000K for natural and syntheticsamples silicic d at 1773IC To synthesizehexagonal p-KFeSiOo (isomorphous of KAlSirO. and KAlSiOo and for their iron analogues, with kalsilite: Bentzen. 1983) and orthorhombic KFeSiO. (a- KFeSirOuand KFeSiOo,have been measuredusing a dif- KFeSiOo;Bentzen, I 983), potassiumcarbonate, ferric oxide, and ferential scanningcalorimeter (DSC). For the various phase dehydrated silicic acid were mixed and melted. Half of this melt transitions that occur, temperaturesoftransition and as- crystallized for 2 d at 1573 K and 3 d at 1373 K, forming or- sociatedchanges in enthalpy and entropy have been de- thorhombic a-KFeSiOo (denoted as a-KF), while the other half termined. Various structural interpretations of the nature crystallized at ll03 K for 5 d, forming hexagonal 0-KFeSiOn of the tetragonal-cubic transformation in leucite, KFe- (denoted as B-KF). X-ray data for each of the KAlSiO4 and SirO., and SiOr-rich leucite, as well as the two transitions KFeSiOo sampleswere compared to data presented by Smith and (1957) (1983). in orthorhombic KAlSiOo vs. the single transition in or- Tuttle and Bentzen thorhombic KFeSiOo,are discussed. DSC procedures ExpnnrvrBNrAr METHoDS Heat-capacity data were collected on each of the specimens described above with a Perkin-Elmer DSC-2 calorimeter. De- Compositionand synthesisof specimens tailed DSC proceduresare given by Stebbinset al. (1982). Each The specimenlocalities of the threenatural leucites used in specimen was heated and cooled at a rate of 20 deg/min through- this studyare givenin Table l. A portion of the Mt. Cimini out the temperature range of 400 to 1000 K. The entire ternper- leucite(specimen 89-21) was heat treated for oneweek at 1673 ature range was split into five separate intervals, each spanning K andlabeled 89-21-A. Chemical analyses of thespecimens (Ta- approximately I 50 degand overlappingby 20 deg.Each specimen ble 2) were performedby both electron-microprobeand wet- was nrn three times over each interval alternating with runs on chemicaltechniques. The stoichiometryof the wyomingiteleu- NBS-720 sapphire, but only the last two runs were recorded. cite (specimen1774) corresponds to a solid solutionof 88 wto/o Data points were collected every 0.5 deg. Plots ofheat capacity KAlSirO6and 12 wto/oKAlSi3Os, in additionto 2.1 wt9oFerO.. as a function of temperaturefor eachsample (Figs. 1, 2) between Althoughthe wyomingiteleucite shows no twinning optically,its 400 and 1000 K not only record phase transitions with their X-ray diffraction patternmatches that of the tetragonalform. associatedtemperature interval, but also demonstrate the repro- LANGE ET AL.: LEUCITE, ORTHORHOMBIC KAlSiO", AND Fe ANALOGUES 939
a FL.1 t 401
\l-- Yr ') o- O
400 500 t: 700 800 s00 1000 femperature, K Fig. 2. Heat-capacitydata showingphase transitions in (a) Temoerature.K Ol and O) a-KF (orthorhombicKFeSiOn). Arrows point to the Fig. l. Heat-capacity data showing phase transitions in five beginning,the peak,and the end of eachtransition. leucite samples:(a) synthetic KFeSLO. (specimenFL.l); O) syn- thetic KAlSirOu (specimen SL.l); (c) natural leucite from Mt. Cimini, Italy (specimen89-21); (d) heat-treatedMt. Cimini leu- deg. In contrast to leucite, the efect ofFe3* substitution cite (specimen89-21-A); (e)leucite from a wyomingite lava (spec- in Ol is to make the transition broader rather than nar- imen 1774).Arrows indicate points on the Cp curve associated rower. The secondtransition observedfor O I corresponds witl the beginning of the transition, the primary peak, the sec- to the single transition observed by Pankratz (1968) for ondary peak, and the end ofthe transition (Table 3). the polymorph he identified as kaliophilite. The stable, low-temperature polymorph, hexagonal p-KFeSiOo (P-KD, undergoesno phasetransitions below 1000 K. Table 3 lists transition temperature intervals and tem- ducibility of the runs. C" measurementscollected several months peratures associated with the heat-capacity peaks upon apart on the samespecimen were identical within less l0lo than leucite, KAlSiOo, precision. heating and cooling for each of the and KFeSiOo samples.Figures I and 2 illustrate which tem- Hn,lr-c^q.p,LcrrY DATA The tetragonakubic transition in leucite takes place over a broad temperatureinterval spanning82 to 176 deg, depending upon the sample studied. Figure 1 shows data for five samples:(a) synthetic KFeSirOu (FL.l), (b) syn- thetic leucite (SL.1); (c) a natural leucite from Mt. Cimini of the Roman volcanic province, Italy (89-21), (d) heat- Tempsature+ treated Mt. Cimini leucite (one week at 1673 K 89-21- A), and (e) leucite from a wyomingite from Leucite Hills, b Wyoming (l 774).The abrupt phasechange in the KFeSirOu contrasts sharply with the broad, double-peakedtransi- I co T tions of the Al analogues.Comparison of the Mt. Cimini T leucite (c) to its heat-treated analogue (d) shows that the heat treatment lowered the transition temperature by 24 degand enhancedthe separationoftwo peakson the heat- I emperaure + capacity curyes. The wyomingite leucite (e) begins to transform at 900 K but continues above the limit of the Fig. 3. (a) A schematicillustrationofthe calculationofAll*" DSC at 1000K. Thesedata are from heatingexperiments, usingEquation 3. The maximum point in the C"curve is assigned whereas C. plots from cooling runs are identical with to a temperature 7,. The points oftangency between the transition peak respectto the shapeofthe transition peaksbut show minor and the extrapolated C. equations below and above the uansition define the beginning and end transition temperatures, hysteresis(Table 3). T, and T r. Two integrations are performed from T, to T, and T, Figure 2 is a plot of heat capacity vs. temperature for to fr to calculate AIl*". O) A schematic illustration of the qal- 01 and orthorhombic KFeSiOo(a-KF). Ol undergoesa culation ofAll* using Equation 4 in the text. A straight line is rapid transition at 695 K and another rapid one at 817 interpolated between T, and T, and defines the vibrational C. K, whereasa-KF has a single transition spreadover 180 which is subtracted during a single integration. 940 LANGE ET AL.: LEUCITE, ORTHORHOMBIC KAlSiOo, AND Fe ANALOGUES
Table 3. Transition temperatures
Sample T1 T5 Ttz T" Tcz T2 Hysteresls AT No. (K) (K) (K) (K) (K) (K) (K) (K)
89-21 760.0 8640 877.0 855.0 868.0 920.0 9.0 160.0 89-21-A 756.0 840.0 857.0 8330 850.0 878.5 7.0 t22.5 9G.05 763.5 66) ) 906.5 876.5 898.0 92'70 9.0 163.0 SL.I 800.0 9r8.0 946.0 904.0 932.0 9't6.0 r4.0 r76.0 FL.I 757.0 823.5 807.5 839.5 r 6.0 825 SL.2 751.0 E41.5 858.0 817.0 8420 900.5 24.5 149.5 o1' 660.0 695.0 687.0 7400 80 80.0 ol2 780.0 817.0 812.0 835.0 50 550 a-KF 6500 7295 720.0 830.0 95 r80.0
lFirst Tr - temperatureat which the transilion begrns transition at 695 K 2Second Th - temperatureat peakofCo curve upon heating transilionat El7 K T62 - temperatureat secondarypeak upon heating T" -- temperatureat peak of Co cuwe upon cooling Tc2 - temperatureat secondarypeak upon cooling T2 - temperatureat which the transition ends Hysteresis- differencein temperaturebetween Th and Tc AT - temperatureinterval over which the transition occurs
peratureswere chosen.The two temperaturesassociated continuous temperature interval. Integration of this heat- with the onset of a transition and its completion were capacrty data over any temperature interval provides the determined by finding the points of tangency between the total changeof enthalpy of the material: transition peak and the heat-capacitycurves both below and above the transition (Fig. 3a). To determine what : effect the rate of heating has on thesedata, sampleswere * T,: CPdT' (l) heated at 5 deglmin and 40 deg/min. Neither produced any significant difference in the temperatures of transition Integration of heat capacity divided by temperature pro- from samples heated at 20 deg/min. However, samples vides the changeofentropy: heated at 0.3 deg/min have transition temperaturesthat are approximately 3 to 4 deg lower than samples heated ^r: cp/rdr. (2) at the faster rates. J, Heat-capacity data were fitted using standard regression purposes, phase-equilibrium calcu- procedures.Data points corresponding to temperatures For some such as it may be sufficient to know below the transition were fitted to a three-term Maier- lations at high temperatures, mineral above its transi- Kelley equation of the form Co: a + bT + cT-2. Tbe the enthalpy and entropy of a by choosing a limited number of data points correspondingto temper- tions. Calculations are made convenient f,, usually the peak in Cr, atures above the transition, up to 1000 K (DSC upper single transition temperature and deriving AII and A,Sof the transition by subtracting limit), were best fitted to the linear equation Cr: a * (C"-,J of the low-tem- bT. Table 4 compiles the fit parametersof d, b, and c for the extrapolated heat capacity perature phase the high-temperature each specimen. Calculated molar thermodynamic prop- below ?",and that of phase I erties (enthalpy, entropy, heat capacity, and free energy) above 7,. Equation becomes between300 and 1000 K basedon fit C, equations from the experimentaldata are available upon request' for each : - of the ten sampleslisted in Table l. Appendix Tables 1- AH*" I,'i ,r** cp..**o)dr 4 tabulateexperimental heat capacitiesfor FL. l, synthetic frz leucite(SL.l), natural leucite(89-21), and Ol. + | (cr.* - cP..**) dT. (3) Jr, ENrH,rlpy AND ENTRopy oF TRANsrrroN Differential scanningcalorimetry allows direct and highly A similar calculation is made for A,S*.. Here, I, and, T, precise measurements of heat capacity to be made over a are the temperatures between which transition behavior is observedand are chosen as discussedabove. Calcula- ' To obtain a copy of the thermodynamicdata, order Docu- l0 deg in the assigrment of mentAM-86-309 from the Business Office, Mineralogical Society tions show that a changeof of America,1625 I Street,N.W., Suite414, Washington,D.C. the beginning or end transition temperature effects our 20006.Please remit $5.00in advancefor the microfiche. calculated values of A.EIand AS by approxirnately 4o/o.An LANGE ET AL.: LEUCITE,ORTHORHOMBIC KAlSiOo, AND Fe ANALOGUES 941
Table 4. Fitted heat-capacity parameters Table 5. Changes in enthalpy and entropy at transitions
tAH 'AS rAH Sample T.ry(K) bxrd c X l0-J SEE, sample "AS No o/ctu) (J/sfi, K) a /ctv\ (J/sfwK) t9-21 41G760 1964 7 515 -55l7 48% 89-21 920-999 224 E | 063 42% E9-21 2,t62 33ll 3,3E3 39t2 89-21-A 2,629 3 109 3,006 3 6t4 -28 t9-21-A 4tO-756 1612 ll5 28 23% 9G.05 2,851 3 220 3,719 4 275 t9-21-A 90G999 230 2 6416 l4% sLl 3,El3 4 225 FLI 2,049 2 4t6 29t9 9045 41G763 t96 1 1 -56 256 & t0% sL2 2,010 2812 2,905 3 486 90-05 9,rG999 226| I o52 39% ol 5 597 0 791 ol 6 427 0 528 t114 4tGt72 201 | 4 808 -66 t2 l9% c-KF I 2E0 sLl 4lGE00 t62 5 ll 13 -23 35 36% rAH FLI 41G756 173I l0 65 -24 E7 23% - changern enthalpy using melhod shown in Figure 3a [Eq (3) in rcxr.] ']AS FLI 848-999 242 6 I 906 26% - changein entropy using method shown in Figure 3a. rAH - chang€in enthalpy using merhod shown in Figure lb. IEq. (4) in text-l sL2 4lG75l lE4 3 9 080 48 20 2tjlo 4AS - changein entropy using method shown In FiSur€3b. sL2 9rG999 1440 25% 5Fisr tmnsition ar 695 K osdond ttusitron at 817 K ol 41G.630 l9 06 2l 15 62 70 ll% gfu - gam fomula w€ight basd on slx oxygensfor leucitesand four oxygensfor ol and d-K-F - 'The c KF 4l G6,10 2491 2095 1t 04 42% frnsition in SL I extendsro the upper limlt ofthe DSC and a fit equation for Cp above the a-KF t5G999 t47 7 4 000 .t007 l8% lransitionisnotpossible ThrsprecludestheuseofEqution(3)tocalculateAHrd3 ForrheOl and d-K-F sp€cimens,extrapolarion ofCD above and below the tBnsitions producesan essentially ,-KF 4to-999 r5E0 2 83E .37l8 46% stralghl line and thus only F4uation (4) is usedlo calculateAHhns and Asnzns. lFit pmmet€rs to rhe Maier-Kelleyquation: Cp = a + b + cT-2 (J / gfi, K)
2s E E - sundard eror ofesrimateofCD gfu - gmm fomula weightbasd on six oxytensfor leucitesand four oxygensfor Ol, d-KF and P.Kf especiallywithin the uncertainty of the integrations.Ad- ditionally, the data show that the sum of the enthalpy changesfor the two transitions in Ol is closeto the value illustration of this method is shown in Figure 3a, and calculated for the single transition in orthorhombic results are included in Table 5. KFeSiOo(a-KF). This method, however, ignores the continuous nature of the transition that is spread over a broad range of DrscussroN temperature. The heat absorbed during this reversible Leucite transition between tetragonal leucite and cubic leucite The double-peaked nature of the heat-capacity curve cannot be assignedto a singletemperature. Therefore, we during the transition in the natural and synthetic leucites present an alternative method for calculating the A.EIand suggeststwo structural events during the transformation AS of a transition that is still approximate and somewhat from tetragonal to cubic symmetry. The two events are arbitrary, but takes into account its continuous natlrre. either related to two types of domains within the leucite The simplest assumption,in light of our ignoranceabout crystal structure,each of which inverts to the cubic phase the vibrational C. during the transition, is to interpolate at different temperatures, or they are related to two tran- peak a straight line beneath the transition from Tr to T2 sitions in which the whole structure undergoes two re- (Cr."*'nl Fig. 3b) and use it to define the Cr, which is spective changesin symmetry (such as l4r/a lo l4'/acd subtracted during a single integration: to l3ad). The latter interpretation implies that leucite has two tetragonal phases with different space grotps (I4r/a AH^: (c.o* - cP,",n*)dr. (4) JTIf"
Table 5 lists the results of both methods. The second approach leads to values ofA.EI and AS that are consis- tently larger by l0 to 300/0.Neither method is entirely i satisfactory:the subtracted value in Equation 4, for ex- I 3,500 ample, may show a decreasein C, with temperatureover the transition interval. The difference I between the two < 3,000 methods serves best to point out the uncertainties in es- timates of Afl*. and A,S* for a transition which is spread over a temperaturerange. These results indicate that the Ail and A^Sof transition for each leucite sample is strongly dependent upon the 820 840 860 880 900 920 940 temperature of transition (Frg. 4). Thoie samples with Temperature,K higher transition temperatures have greater AIl and AS Fig. 4. The AIl*" for each specimen is plotted against its values.Also, sampleswith similar transition temperatures transition kmperature (fh, Table 3). Increasesin transition tem- have enthalpy and entropy changesthat are also similar, peratures can be correlated to greater AIl of transitions. 942 LANGE ET AL.: LEUCITE. ORTHORHOMBIC KAlSiO,. AND Fe ANALOGUES and 14,/acd).It is possiblethat different twin mechanisms proposedspace group for the intermediate phase,I4r/acd, are associatedwith the proposed cubic-tetragonal, and can be related to the low-temperature tetragonal leucite tetragonalr-tetragonal,transitions, which explainsthe two phase(14,/a) by merohedric twinning and to the high- types of twinning found in leucite (merohedric and pseu- temperature cubic leucite phase(Ia3d) by pseudomero- domerohedric; Mazzi et al., 1976). The twin planes for hedric twinning. AccordingtoMazzi et al. (1976),mer- either of these twin mechanisms may form at domain ohedric twins develop on the tetragonalplanes (l l0) and boundaries.Evidence of a domain structure at high tem- (1 l0) with twinned structureshaving exactly parallel crys- perature is demonstratedby the "memory" effect(Mazzi tallographic orientations, but with the a and b axes in- et al., 1976) found in twinned leucites. When a twinned terchanged. X-ray diffraction gives hkl dlftactions of one leucite crystal is heatedto its cubic form and then cooled, twin exactly superimposed v/rth khl diffractions of the the twinned crystal has the same proportion of twin do- other. The two individuals of the tw'in show simultaneous mains with the same previous twin boundaries (Peacor, extinctions under crossednicols, and the crystal appears 1968; Sadanagaand Ozawa, 1968;Mazzi et al., 1976). optically to be without twinning. In contrast, pseudo- The leucitesare also characterizedby a broad transition merohedrictwinning developson the (l0l) (011) (l0l) temperatureinterval that could be related to a coexistence (011) tetragonal planes and createstwins with parallel a of phases.Henderson (1981) has presentedhigh-temper- or D axeswhereas the remaining pair of axesare not par- ature X-ray data indicating that SiOr-rich natural leucites allel. Thus, the individual twins have distinct spots in X- from the Leucite Hills, Wyoming (Carmichael, 1967),and ray photographs and do not extinguish simultaneously the leucite-orthoclase solid-solution series (KAlSirOu- when viewed under crossednicols. It is this type of twin- KAlSi3O8)(MacKenzie et al., 1974) are characterizedby ning in leucitewith which petrographersare most familiar. a coexistenceoftwo phases,one tetragonaland one cubic, The single thermodynamic processinvolved with the during the tetragonakubic transition. A coexistenceof transition in KFeSirO. is in marked contrast to the be- two phasesduring a transition has been documented in havior of the aluminous samples.The failure of KFeSirOu severalother framework silicate,aluminosilicate, and alu- to show a double set of peaks and hence, as we have minate structures:the a-B transition in cristobalite (Lead- implied, an intermediate tetragonalphase, may arisefrom better and Wright, 1976), the tetragonal-{ubic transition a lack of Fe3+-Si4+disorder. Owing to the difference in in the solid-solution series(K,Cs)AlSirO6-(K,Rb)AlSirO6 their ionic size,Fe-Si disorder cannot easily take place in (Martin and kgache, 1975), and the monoclinic-hexag- KFeSirO. (Faust, 1963).In contrast, leucite is completely onal transition in (Sr,Ba)AlrOo(Henderson and Taylor, disorderedwith respectto Al and Si in both the tetragonal 1982). and cubic phases(Peacor, 1968;Mazzi et al., 1976).Fe- One explanation for a coexistenceof two phasesduring Si order in KFeSirO. may preclude the stability of an a transition involves martensitic-like behavior (Hender- intermediate tetragonal phase. son and Taylor, 1982). Henderson and Taylor have sug- The G heating curve for the wyomingite leucite (Fig. gestedthat volume discontinuity and anisotropy of shape le) lacks a peak yet does record a distinct, though small, between a transformed nucleus and its host crystal gen- increasein C, al900 K that continues to the upper limit erateselastic strain that opposesthe progressoftransfor- of the DSC (1000 K). The apparent absenceof twinning mation. Increasesin temperatureare necessaryto provide when viewed optically indicates suboptical pseudomer- sufficient energy for the transformation to proceed further. ohedric twinning since the X-ray data shows it to be te- Thus, a broad interval of temperature(during which time tragonal.Because of the difrculty of obtaining a pure min- both the transformed nuclei and the host phase coexist) eral separatefrom the wyomingite groundmass,the leucite is required to transform the whole crystal. The volume sample is diluted by as much as 300/owith other ground- difference between tetragonal leucite and cubic leucite mass constituents which affect the size of the recorded makes leucite eligible for this type of transformation. peak. The aboveexplanation supportsour contention that the tetragonal
of silicate compositions by a gelling method. Mineralogical Mazzi, F., Galt, E., and Gottardi, G. (1976) The crystal structure Magazine,36, 832-838. of tetragonalleucite. American Mineralogist, 61, 108-1 15. Henderson,C.M.B. (1981)The tetragonakubicinversion in leu- Nukui, A., Hiromoto, N., and Masaru,A. (1978)Thermal changes cite solid solutions.Progress in Experimental Petrology,5, 5 1- in monoclinic tridymite. American Mineralogist, 63, 1252- 55. 1259. Henderson,C.M.B., and Roux, J. (1977) Inversions in sub-po- Panktatz, L.B. (1968) High-temperature heat contents and en- tassic nephelines. Contributions to Mineralogy and Petrology, tropies ofdehydrated analcite,kaliophilite and leucite. Bureau 6r,279-298. of Mines Report of Investigation 7073, l-8. Henderson, C.M.B., and Taylor, D. (1982) The structural be- Peacor, D.R. (1968) A high-temperature single-crystaldiffrac- havior of the nepheline family: (1) Sr and Ba aluminates rometer study of leucite (K,Na)AlSirO6. Zeitschrift fiir Kris- (MAl,On). Mineralogical Magazine, 45, | | I-127 . tallog,raphie,127, 213-224. Henderson,C.M.B., and Thompson, A.B. (1980) The low-tem- Sadanaga,R., and Ozawa,T. (1968)Thermal transition ofleucite. perature inversion in sub-potassic nephelines. American Min- Mineralogical Journal, 5, 321-333. eralogist,65, 970-980. Smith, J.V., and Tuttle, O.F. (1957) The nepheline-kalsilitesys- Hirao, K., Soga,N., and Kunugi, M. (1976) Thermal expansion tem: I. X-ray data for the crystallinephases. American Journal and structure of leucite-type compounds. Journal of Physical of Science,255, 282-305. Chemistry,80, 1612-1616. Stebbins,J.F., Weill, D.F., and Carmichael, I.S.E. (1982) High Holmes, A. (1942) A suite of volcanic rocks from south-west temperature heat contents and heat capacities ofliquids and Uganda containing kalsilite (a polymorph of KAlSiOr). Min- glassesin the systemNaAlSi3Os-CaAlSiOr. Contributions to eralogicalMagazine, 26, 197-217. Mineralogy and Petrology, 80, 276-284. Kelley, K.K., Todd, S.S.,Orr, R.L., King, E.G., and Bonnickson, Taylor, D., and Henderson,C.M.B. (1968) The thermal expan- K.R. (1953) Thermodynamic propertiesof sodium-aluminum sion of the leucite group of minerals. American Mineralogist, and potassium-aluminum silicates.Bureau of Mines Report of 53.1476-1489. Investigation 4955, l-25. Thompson, A.B., and Wennemer,M. (1979)Heatcapacities and kadbetter, A.J., and Wright, A.F. (1976) The alpha-beta tran- inversions in tridymite, cristobatte, and tridymite-cristobalite sition in the cristobalite phasesof SiO, and AIPO.. I. X-ray mixed phases.American Mineralogist, 64, 1018-1026. studies.Philosophical Magazine, 33, 105-l 12. Tuttle, O.F., and Smith, J.V. (1958) The nepheline-kalsilitesys- MacKenzie, W.S., Richardson, D.M., and Wood, B.J. (1974) tem. II. Phase relations. American Journal of Science,256, Solid solution ofSiO, in leucite. Bulletin de la Soci6t6frangaise 57l-589. de Min6ralogie et de Cristallographie,97, 257-260. Martin, R.F., and Legache,M. (1975) Cell edgesand infrared spectra of synthetic leucites and pollucites in the system KAlSirO6-RbAlSirOu-CsAlSi,Ou.Canadian Mineralogist, 13, MeNuscmpr REcETvEDMev 6, 1985 275-281. MaNuscnrpr AccEp"rEDM.rrncn 18. 1986
App. Table l. Experimental heat capacitiesof89-21 (natural App. Table 2. Experimental heat capacitiesof SL.l (synthetic leucite) leucite)
Tenp Heat Capacity Temp Heat Capacity Temp Heat CaDacity Tenp Ileat Capacity (K) (J/efw K) (K) (r/efw K) (K) (J/efw K) (K) (J/efw K)
420,0 t97.4 845.0 274.7 420.0 I95.2 885.0 270,3 450.0 203.4 850.0 281.5 450.0 201.0 890.0 273.r 500.0 212.1 855.0 29r.5 500.0 209.5 895.0 276.5 550.0 2I8 .5 860.0 304.7 550.0 2t6.5 900.0 280.7 500.0 224.9 865.0 3r0.3 600.0 222.3 905.0 285.3
650.0 231.3 870.0 300.5 650.0 228.2 910.0 290.7 700.0 238.2 875.0 289.5 700.0 236.3 915.0 295.1 750.0 245.0 880.0 280.2 750.0 242.1 920.0 296.3 760.0 246,6 885.0 267.2 800.0 246.4 925.0 29r.1 765.0 z4I.Z 890.0 252.5 805.0 247.3 930.0 283.8
770.0 248.3 895.0 t4).c 8r0.0 248.0 935.0 279.0 775.0 249.0 900.0 z4L.) 8r5.0 248.7 940.0 275.3 780,0 250.0 905.0 238.9 820.0 249.6 945.0 27r.2 785.0 25t.r 910.0 237.4 825.0 252.2 950.0 264.6 790.0 252.3 9r5.0 236.0 830.0 955.0 255.6
795.0 251.4 920.0 234.9 835.0 254.2 960.0 248.3 800.0 252.5 950.0 z)).o 840,0 255.2 955.0 243.6 805.0 253.8 990.0 237.0 845.0 z)o.J 970.0 24t.r 8I0.0 254.9 850.0 257.7 975,0 239.9 815.0 256.6 855.0 258.8 980.0 239.2
820.0 t)o.) 860.0 260.0 990.0 237.9 825.0 260.5 865.0 261.9 830.0 263.2 870.0 263.4 835.0 266.1 875.0 265.3 840.0 269.8 880.0 267.6 LANGE ET AL.: LEUCITE, ORTHORHOMBIC KAlSiOl, AND Fe ANALOGUES 945
App. Table 3. Experimental heat capacitiesof FL.l App. Table 4. Experimental heat capacities of Ol
Temp Heat Capacity Temp Heat Capacity Temp Heat Capacity Temp Heat Capacitv (K) (J/gfw K) (K) (J/gfw K) (K) (J/gfw K) (K) (J/gfw K)
420,0 203.0 840.0 z4).) 420.0 r40.9 770.0 t73.4 450.0 208.6 845.0 245.2 450.0 I t,< a 780.0 173.7 500.0 zLo.) 850.0 244,8 500.0 151.8 785.0 r74.2 550.0 223.8 900.0 243.8 550.0 157.2 790.0 r76.2 600.0 229.7 950.0 LqJ.J 500.0 16r.9 795.0 177.4
550.0 236.0 990.0 245.4 650.0 t69.2 800.0 179.7 700.0 242.2 660.0 172.4 80s.0 184.0 750.0 249.5 665.0 169.I 810.0 191.5 /f).u 250.6 570.0 l/u.) 815.0 202.5 760,0 z)1.) o/).u IIZ.J 820.0 r95.2
765.0 680.0 I75.6 825.0 r78,5 770.0 z)5. I 685.0 180.1 830.0 L76.9 775.0 254.9 690.0 187.0 835.0 r76.6 780.0 z)o.) 695.0 t92.8 840.0 t/o.o 785.0 258.2 700.0 188.1 850.0 t76.9
790.0 260.1 705.0 18I.3 795.0 263.9 710.0 t76.9 800.0 268.0 715.0 174.6 805.0 273.9 720.0 L73.3 810.0 288.0 725,0 172.7
815.0 3I1.3 730.0 L72.L 820.0 J/f.D 735.0 172.0 825.0 386.6 740,0 171.8 830.0 252.2 750.0 L7T.7 835.0 246.2 760.0 172.3