American Mineralogist, Volume 81, pages 658-667, 1996 Thermochemistry and phaseequilibria in calcium zeolites InrN,l KrsELEvArr Ar.nxANon-l Navnorsxyrr IcoR A. Bu,rrsryr2 ANDBonrs A. FunsnNro2 rDepartment of Geological and GeophysicalSciences, Princeton University, Princeton, New Jersey08544, U.S.A. 2lnstitute of Mineralogy and Petrography,Academy of Sciences,Novosibirsk, 630090, Russia ABSTRACT Thermodynamic properties of the natural calcium zeolites laumontite, leonhardite, de- hydrated leonhardite (metaleonhardite),wairakite, and yugawaralitewere studied by cal- orimetry in lead borate solvenIat9T5 K. Enthalpiesof formation from the elementsat 298 K are as follows: -1251.0 + 8.5 kJlmol for laumontite,CaAlrSioO,r.4HrO; -7107.3 -r 5.6 kJ/mol for leonhardite,CaAlrSi4O,r.3.5HrO; -5964.3 + 5.1 kJ/mol for metaleon- hardite,CaAlrSioO,r; -6646.7 + 6.3 kJ/mol for wairakite,CaAlrSi4O,r.2H,O; and -9051.3 + 10.4 kJ/mol for yugawaralite,CaAlrSiuO16.4H2O. The value for leonharditeis in good agreementwith early values from acid calorimetry (Barany 196l) but not with revised values from Hemingway and Robie (1977). The enthalpy of dehydration of leonhardite is 140.2 !6.7 kJlmol, and the loss of one mole of HrO is associatedwith an endothermic effectof about 40 kJ. Standardentropies, S!nr, of wairakite [400.7 J/(mol'K)] and yuga- waralite [609.8 J/(mol'K)] were derived from our new enthalpydata combined with re- versedP-Zphase equilibria (Liou 197l;Zengand Liou 1982).The upperlimit of wairakite stability, the univariant curve for equilibrium of wairakite with anorthite, quartz, and fluid, was calculatedfrom thesevalues of enthalpy and entropy. Good agreementbetween thermodynamic calculations and reversed phase equilibria supports the reliability of the new thermodynamic data. lNrnonucrroN lorimetric measurementof enthalpiesof formation of cal- cium zeolites and related minerals is desirable. Calcium zeolitesare common in diagenetic,sedimen- No calorimetric data exist for the enthalpiesand en- tary, hydrothermally altered, and low-grade metamor- tropies of laumontite, wairakite, and yugawaralite.The phic environments.The major zeolitesencountered are enthalpy of formation of leonhardite was obtained by laumontite (CaAlrSioO,2.4HzO), leonhardite (Ca- Barany(1961) using hydrofluoric-acid calorimetry and Alrsi4orr.3.5HrO),which is the parriallydehydrated form later revised by Hemingway and Robie (1977). Low- of laumontite, wairakite (CaAlrSioO,r.2HzO),which is temperature heat capacity and entropy of leonhardite the Ca analog of analcime, and yugawaralite (Ca- were measuredby King and Weller (1961). All pub- AlrSi6O,6).Laumontite phase equilibria are especially lished thermodynamic data for wairakite, laumontite, importantforP-Zboundariesofzeolitemetamorphicfa- and yugawaralite were estimated or calculated from cies(laumontite facies). Laumontite and wairakiteare the phaseequilibrium studies and are not consistent.For index mineralsof the zeolitefacies. instance,the standardentropy values for wairakite giv- P-TstabilityrelationsinthesystemCaAlrSirOr-SiOr- en by Helgesonet al. (1978) and by Glushko (1981) HrO have beeninvestigated both experimentallyand by differ by about 80 J/(mol'K). thermodynamic calculations(Coombs et al. 1959; Ko- Severalstudies (Kiseleva and Ogorodova 1983; Cir- izumi and Roy 1960;Liou 1970,l97l;Thompson 1970; coneand Navrotsky1992; Smelik etal. 1994;Navrotsky Zeng and Liou 1982;Senderov 1980, 1988;Ivanov and etal. 1994)have shownthat oxide melt solutioncalorim- Gurevich 1975). Despite extensiveexperimental work, etry can determinethe enthalpiesof formation of hydrous some of the zeolite phaserelations have not been satis- minerals.The presentwork usesdrop-solution calorim- factorilydeterminedbecauseoflowratesofreactionsun- etry in molten 2PbO'BrO3to measurethe enthalpiesof der low-temperature conditions and becauseof the for- formation of natural laumontite, metaleonhardite(dehy- mation of metastable phases in experiments of short drated laumontite), leonhardite,wairakite, and yugawaral- duration. Many experiments report only synthesis and ite. The enthalpy of dehydration in the laumontite-leon- not reversals;hence, estimations of free energiesare un- hardite-metaleonharditeseries was studied by transposed certain. Thermodynamic data obtained from experimen- temperature-drop calorimetry. Stability relations of wair- tal equilibria, estimated from P-T data in contemporary akite, yugawaralite, and leonhardite and the upper tem- geothermal systems, and calculated from various ther- perature limit for wairakite stability were calculated on modynamic data setsshow wide scatter.Thus, direct ca- the basisof thesethermochemical data. 0003-004x/96l0506-0658$05.00 658 KISELEVA ET AL.: THERMOCHEMISTRY OF CALCIUM ZEOLITES 659 Trau 1. Chemicalanalyses and formulasof zeolitesstudied TABLE2. Latticeparameters of zeolites Leonhardite Wairakite Yugawaralite a (A) b (A) c (A) Bf) y(A.) Ref. Chemicalanalysis (wtc/o ) Leonhardite sio, 51.72 55.20 61.78 14.747(5) 13.067(3) 7.532(4) 1119(3) 1346(1) 1 Tio, 0.01 0.00 0.01 't4.770 13.056 7.595 112.8 2 A12o3 2193 23.08 17.01 14.77(2) 13.09 (2) 7.58 (2) 11 2.0 (1) MnO 009 0.00 0.00 Wairakite Mgo 0.04 0.01 0.01 CaO 12.14 12.48 o2R 13.700(15) 13.666(9) 13.559(10) 90.44(8) 882.9(8) 1 Naro 0.05 0.01 0.02 13.692(3) 13.643(3) 13.s60(3) 90.50(10) 4 KrO 013 0.02 0.01 Yugawaralite P.o. 0.11 (s) 14.007(6) 10.049(9) 11 1 .20 (4) 882.9(8) 1 13.67 11.98 6.728 H.O- 8.44 6.729 14.008 10.050 111.18 Total 99.98 99.23 100.25 Cations and HrO molecules per unit cell Note: Fot unit cells on the basis of 48 (leonhardite),96 (wairakite),and (yugawaralite)O atoms in the framework- Values in parenthesesrep- 15.s6 (1 6)-' 32.17(32) 12.08(12) 32 resent confidenceinterval (for 95% probability).References are as follows: Ti 0.00 0.00 0.00 1 : thiswork, 2 : Pipping(1 966), 3 : Yamazakiet al.(1991), 4 : Tak6uchi AI 7 s8 (8) 1s.86(1 6) 3.92(4) et al.(1979), and 5: Eberleinet al.(1971). Mn 0.02 0.00 0.00 Mg 0.02 0.01 0.00 4.02(4) 7.79(81 1.e6(2) Na 003 0.06 0.00 K 0.05 0.01 0.00 P 0.03 Latticeparameters of samplesstudied are shownin Ta- HrO 14.07(14) 16.41(16) 7.80(8) ble 2. They agreewell with previous studies(Gottardi and 'HrO contentsalso verifiedby thermogravimetricanalysis. Galli 1985). .'Values in parenthesesrepresent ideal molar ratios. CllonrPrnrnY The enthalpiesof formation and dehydrationwere de- Znor-ttn CHARACTERrzATroN termined usinga Tian-Calvethigh-temperature heat-flux We chosewell-crystallized natural samplesof laumon- microcalorimeter describedin detail by Navrotsky (1977). tite, wairakite, and yugawaralite.The laumontite is from Drop-solution calorimetric methods (Chai and Navrot- Nidym River (left tributary of Nizhnyaya Tunguska Riv- sky 1993; Navrotsky er al. 1994) were chosento avoid er), Siberia. White to pinkish nontransparent crystals up decompositionof zeolitesat the calorimetertemperature to l0 mm in length were separatedfrom hydrothermal prior to dissolution.The sampleswere dropped from room veins in Triassicbasalts. temperatureinto molten 2PbO'BrO3at 975 K. Most ca- Wairakite from Bondai-Atami, Koriyama City, Japan lorimetric experimentswere performed using pressed pel- (MineralogicalMuseum of the RussianAcademy of Sci- lets about 3 mm in diameter,0.5-l mm in height, and ences,catalogr,o.81622), was provided by A.A. Godo- 10-25 mg in weight. A few experimentsused piecesof vikov. It had been separatedunder a microscopefrom an single crystals. The heat of drop solution was a sum of aggregateof white semitransparentcrystals with some ad- the heat of solution in the melt plus the heat content mixture of quartz and calcite.Clean, transparent, color- (H8,' - Hln)- less tabular crystals were used. Heats ofdehydration and heat contentsat 975 K were Yugawaralite from Obora Toi-cho, Shizuoko Prefec- obtained using transposedtemperature-drop calorimetry ture,Japan (Mineralogical Museum of the RussianAcad- (sampledropped into an empty platinum crucibleequil- emy of Sciences,catalog no. 87616), was provided by ibrated in the calorimeter). The heat effectcontained two A.A. Godovikov. Transparentand semitransparentcol- contributions,the enthalpyofdehydration at 975 K and orlessplaty crystalsup to l-2 mm in dimension from the heat content of the mineral (H\r' - 11!*). When the the centralparts of the small veins (10-15 mm thick) in solid decompositionproducts were droppedinto the cal- hydrothermally altered brecciated volcanic rock were orimeter, the heat effectcontained only the heat content chosen. (H8,, - .F{rr) of the dehydrated zeolite. Leonhardite was chemically analyzed by X-ray fluo- We were concernedthat when the small samplesused rescenceusing a Carl Zeiss VRA 20R instrument. Wair- in this study were dropped into the calorimeter, the heat akite and yugawaralite were analyzed by electron micro- picked up during the drop, before the sample reachedthe probe (Camebax MICRO). Table I shows the analytical region of the thermopile, might be a different fraction of daIa. the total heat effectthan seenfor more massiveplatinum- High-temperature thermal behavior was studied using encapsulatedsamples, for 30-50 mg pellets,or for plat- a TA 3000 (Mettler) thermoanalyzerconsisting of a ther- inum calibration piecesweighing about 200 mg. Rather mogravimeter(293-1213 K), a scanningcalorimeter (103- than using such large platinum pieces for calibration in 873 K), and a differentialdilatometer (173-1273 K). The the usual procedure (Navrotsky 1977), we chose to cali- thermograms are similar to those published by Gottardi