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A Calorimetric Study of Synthetic Amphiboles Along the Tremolite

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A Calorimetric Study of Synthetic Amphiboles Along the Tremolite American Mineralogist, Volume 79, pages I I I 0- I I 2 2, I 994 A calorimetric study of synthetic amphibolesalong the tremolite-tschermakitejoin and the heats of formation of magnesiohornblendeand tschermakite EucnNr A. Snnrr.rx Departmentof Geologicaland GeophysicalSciences and PrincetonMaterials Institute, Princeton University, Princeton, NewJersey 08544, U.S.A. Dlvro M. Jpxrrns Departmentof Geologicaland EnvironmentalSciences, State University of New York at Binghamton,Binghamton, NewYork 13901.U.S.A. Ar,nx,lNon c, NlvnorsxY Departmentof Geologicaland Geophysical Sciences and PrincetonMaterials Institute, Princeton University, Princeton, NewJersev 08544. U.S.A. Arstnlcr A suite of synthetic calcic amphiboles in the tremolite-tschermakite (Tr-Ts) serieshas been studied by high-temperature drop-solution calorimetry in molten 2PbO.BrOr. The pseudobinaryjoin, which is shifted toward magnesiocummingtonite(Mc) by l0 mol0/0, extends from tremolite (Tr), Ca,rMgrrSirOrr(OH)r, to magnesiohornblende(MgHb), Ca,,MgrAlrsirorr(OH)r, and representsthe operation of one full Mg-Tschermak substi- tution, I6rMg,l4rsi= IoJdl,t+l[],into the tremolite fOrmula. Transmission electron micro- scope (TEM) examination of the amphiboles shows that all the samples contain a low concentration of (0 l0) chain multiplicity faults (CMFs), suggestingthat the increaseof Al in the amphibole does not significantly increaseor decreasethe tendency ofthese defects to form. The calorimetric measurementsindicate that the energy changeassociated with one full Mg-Tschermak substitution is quite small, on the order of -5 to -10 kJlmol. Enthalpies of formation from the elements at 298.15 K and I bar have been calculated using the new calorimetric data for the magnesiohornblende(MgHb) and tschermakite (Ts)end-members.ThevaluesofAlf forMgHbandTsare -12401.2 + 10.6and-12527.7 + 16.4 kJlmol, respectively.The entropy of MgHb, at 298.15 K and I bar, has also been calculated using four different site-mixing activity models involving the octahedral (M2) and tetrahedral (Tl) sites and the experimental phase-equilibrium data of Jenkins (1994). Two of the models, the two-site and four-site coupled models, are essentially indistin- guishableand gave the most consistentvalues for ^q"Hb,575.1 + 3.9 and 575.6 + 3.9 J/(mol. K), respectively. Irvrnooucrrox Any attempt to relate amphibole composition to a spe- cific P and ?" or to an exchangereaction in P-Z space The general form of the Tschermak cation exchange requires accurate thermodynamic data for certain end- can be expressedas r61M,l4lsi= I6lAl,t41Al,where M nor- member amphibole compositions.For calcic amphiboles, mally refers to a divalent cation such as Fe, Mg, or Ca. these data have generally come from phase-equilibrium This exchangeis of fundamental importance in many sil- studies (e.g.,Welch and Pawley, l99l1, L6ger and Ferry, icate solid solutions, including the pyroxenes, amphi- l99l; Jenkins, 1994).Thermodynamic data for a large boles, micas, chlorites, and melilites. In amphiboles, this number of rock-forming minerals, including amphiboles, coupled substitution is of primary importance in two sol- are tabulated in various thermodynamic data bases(Ro- id-solution series,the orthorhombic ferromagnesianse- bie et al., 1979;Helgeson et al., 1978;Berman, 1988; ries and the monoclinic calcic series.Recent transmission Holland and Powell, 1990). analytical electron microscope (TEM-AEM) studies have In this paper we present results from a calorimetric study shown that this substitution dominates during exsolution of synthetic amphiboles in the tremolite-tschermakite (Tr- reactions between anthophyllite and gedrite in the ferro- Ts) series [CarMgrSirOrr(OH)r-CarMgrAloSiuOrr(OH)r]. magnesianseries (Smelik and Veblen, 1993)and between The primary goals of the study are to (l) investigate the actinolite and hornblende in the calcic series(Smelik et energychange associated with the Mg-Tschermak exchange al., l99l). in calcic amphibole and (2) determine independently the 0003-004x/94/ | | r2-t I I 0$02.00 1110 SMELIK ET AL.: CALORIMETRIC STUDY OF SYNTHETIC AMPHIBOLES 1l1t Ca2Mg3Al2[Al2Si6]q2(OH)2 a. Ts f MgHb .144 AMPH I44 o 28-2 6 19-5 AMPH 28-2 o 25-l AMPH 19-5 o 29-l tr 23-10,23-5 AMPH 25-1 AMPH 29-1 TREM 23-IO,TREM 23-5 Tr tM4lCa= tM4lMg MC Tr IVIC Ca2Mg5[Sfu]Qz(oH)2. Mg7[Sis]Q2(oH)2 Fig. l. (a) Plot of tremolite-tschermakite-magnesiocummingtonite(Tr-Ts-Mc) showing starting compositions for amphibole synthesis,made at intervals of 10 molo/oalong a pseudobinaryjoin shifted toward the Mc apex by l0 mol9o (circles).The solubility limit for Al in tremolite is near the MgHb end-member.(b) Representativeelectron microprobe analysesof the syntheticamphiboles. The dashedline at 0.5 Ts representsthe operation of one full Mg-Tschermaksubstitution, i.e., the Al content of magnesiohornblende. heatsof formation for the amphibole end-membersmag- (Jenkins,1987; Graham et al., 1989;Pawley et al., 1993). nesiohornblende (MgHb), CarMgoAlrSi'Orr(OH)r, and Therefore, the amphiboles were synthesizedat l0 molo/o tschermakite, and compare theseresults with those from increments along the join Ca, , Mg, , Si, Or, (OH)r - phase-equilibrium studies. By incorporating thermo- Ca, rMgorAlrsirorr(OH)r. Electron microprobe (EMP) chemical data from Berman (1988) and Holland and analysis of the amphiboles (discussedbelow) and previ- Powell (1990) in our analysis,the results presentedhere ous work on similar compositionsby Jenkins(199a) in- should be compatible with those internally consistent data dicate that the compositional offset is preservedalong the sets. join. Approximately I wto/oqvarlz was mixed into each experiment to compensate for the loss of silica to the ambient fluid during hydrothermal treatment (Jenkins, Enpnnnrrcxur. METHoDS 1987). Syntheseswere performed by loading a stoichio- Amphibole synthesis metric oxide-carbonatemixture into a Pt capsulesealed All amphiboles were synthesizedfrom mixtures of re- at one end, heating it with a butane torch to drive of CO, agent grade CaCOr, MgO, and AlrOr; SiO, was prepared from CaCOr, and then sealingthe other end after adding from desiccatedsilicic acid. Previous experimental work 25-30 wto/oHrO. by Jenkins(1988) and Cho and Ernst (1991) has shown Synthesesbelow l0 kbar were performed in internally that the solubility limit of Al in tremolite approachesthe heated vesselsusing Ar as the pressure medium. Pres- magnesium hornblende end-member composition, sureswere measuredwith both a PPI bourdon-tube and Ca,MgAlrSi?Orr(OH)r, which is equivalent to one full Harwood manganin resistance-coilgauges, the latter be- Mg-Tschermak substitution into the tremolite formula. ing factory calibrated againsta dead-weightpiston gauge. For the synthesisof amphiboles for calorimetry, we pur- Pressuremeasurements are believedto be accurateto + 50 posely chose a join parallel to the true join of tremolite bars. Temperatures were measured with two chromel- and magnesium-hornblende,offset by l0 molo/omagne- alumel thermocouplescalibrated against the freezingpoint siocummingtonite (Mc), to obtain the purest amphibole of NaCl and are believed accurate to + I K. Syntheses yields possible. The magnesium-cummingtonitecompo- above I 0 kbar were carried out in a piston-cylinder press nent is reflected by a constant substitution of 0.2 Mg for t/z in. rn diameter using NaCl as the pressure medium. Ca in the M4 site acrossthe entire join (Fig. la). We chose The syntheseswere performed in a hot piston-out fashion to offsetthejoin on the basisoffindings ofprevious work- by pressurizingthe assemblageto about 2 kbar below the ers, who reported a 5-l0o/o enrichment of Mg in the M4 desired pressureand then using the thermal expansionof site in their attempts to synthesizeend-member tremolite the assemblageto reach the desired pressure. Pressure ttt2 SMELIK ET AL.: CALORIMETRIC STUDY OF SYNTHETIC AMPHIBOLES TABLE1. Details of amphibole synthesis Sample Composition r(K) P (kbar) r (h) Synthesis products' TREM 23.10 Ca, 6Mg5rSisOr2(OH), 113q10) s.20(70) 239 amph, trace cpx, and trace opx TREM 23-5 Ca, sMgs,SisOr2(OH), 11 1 1(17) 6.10(10) 188 amph and trace cpx AMPH 29.1 Ca, sM95oAlo4SiTsOz(OH), 11 37(5) 6.35{5) 237 ampn AMPH 25-1 Cal sMg4sAlosSiT"Oz(OH), 11 48(1 0) 6.0q5) 109 amph AMPH 19-5 Cal sM946A112Si7oO"r(OH), 1134(5) 13.0(3) 120 amph, trace opx, and trace qtz AMPH 28-2 Ca, sMgliAll 6SiTrOz(OH), 11 15(s) 12.9(3) 147 ampn AMPH 14.4 Ca, sMg4rAl2oSiT oOz(OH), 1096(s) 12.0(5) 70 amph, cor (1.6 wtTo)*, and trace opx /Vote:the error ranges for P-f conditions (shown in parentheses)take into account small P and ffluctuations during the course of the experiment. 'Amph : amphibole; cor : corundum; opx : orthopyroxene; cpx : clinopyroxene;qtz : quartz; trace : below detection limit on powder XRD (about 1.0 wto/"). ** Amount determinedby Rietveldmodal analysis. transmission to the sample was calibrated by the reaction amphibole stoichiometry was achievedwhen the analysis albite : jadeite + qvartzat 873 K to a precisionof +250 was normalized to 23 O atoms. Representative EMP bars, and it was found that virtually no correction was analysesare plotted on the Tr-Ts-Mc triangle in Figure required to obtain the acceptedvalue of 16.25 kbar (Jo- lb for comparison with the end-member compositions hanneset al., l97l;, Holland, 1980).Temperatures were (Fig. la). Despite the scatter in the probe data, most of measured with chromel-alumel thermocouples newly the synthetic compositions cluster well along the offset made for each experiment and have an estimated uncer- Tr-Ts join. The most Al-rich one, AMPH l4-4, does not tainty of t 5 K. The synthesisconditions for the amphi- match the composition of the MgHb end-member. Op- boles are summarized in Table l. tical examination of this sample indicated the presence of unreacted corundum from the original oxide mix in Amphibole characterization the experiment products (Table l). The amount of co- Optical examination. All experimental products were rundum was quantifiable by Rietveld modal analysis, as examined with the petrographic microscope using an oil discussedbelow.
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