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