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American Mineralogist, Volume 61, pages 26-28, 1976

Stabilitv of svnthetic andradite at almosphericpressure

YosHIro SuwA, Yozo Teltal, ANDSgtceHnnu Nere, Synthetic Crystal Research Laboratory, Faculty of Engineering' Nagoya (/niuersity, Chikusa-ku, Nagoya 464, Japan

Abstract

Compositionallypure andraditeCarFedSiOo), with an averagecell edgea : 12.059A was obtainedby dehydrationofhydroandradite at I 100' to I 150'C.At I l60oCit decomposedto pseudowollastoniteand hematite. A 100percent yield of andraditewith a somewhatsmaller celledge a : 12.051Awas obtained from devitrificationof the glassat I150'C and from statereaction between wollastonite and hematiteat 1100'Cto I150"C,but only aftersix 22- hour cyclesof repetitivegrinding and heating.This andraditedecomposed at 1165'C to pseudowollastoniteand hematite.A changein cell edgea prior to decompositionwas not observedfor eitherandradite. The andraditewith cell edgea: 12.051A maycontain excess Sio+,and consequentlyvacancies and ferrous , because ofcontamination by SiO,from an agatemortar during the repetitivegrinding.

lntroduction ing and the grinding were repeatedtwice. The glass's 1.793 + 0.007 suggeststhat it con- Andradite synthesizedfrom various starting mate- tainsseveral percent FeO as discussedby Huckenholz rials under various conditions has been known to and Yoder (1971). have somewhat different physical properties (Flint, Pseudowollastonitewas prepared by heating a McMurdie, and Wells, 1941;Coes, 1955;Christophe- stoichiometricmixture of CaCOgand SiOzat 1500"C Michel-Levy, 1956; Swanson et al, 196O;Ito and for 2 hours; wollastonite,by heatingpseudowollasto- Frondel. 1967: Naka, Harata, and Noda, 1968; nite at 1000'C for about 1800hours. Huckenholz. 1969).Huckenholz and Yoder (1971) Hydroandradite (accompanied by amorphous discussedits lattice constant and thermal behavior SiOr) was prepared as follows: the glass and 30 wt relative to the oxidation state of iron. percentHzO were set in a wafer and heated Presentwriters haveobtained a 100percent yield of at 500'C under a pressureof l0 kbar for 20 hours by andradite by appropriate heatingof hydroandradite, usinga squeezer-typehigh-pressure apparatus (Dach- of a glassof andraditecomposition, and of a mixture ille and Roy, 1962; Naka, Takenaka, and Noda, of wollastoniteand hematite,all in air at one atmos- 1966). From its cell edge, 12.088 + 0.003 A, the pheric pressure. Slight differences in the hydroandradite's composition was deduced to be decomposition temperatures and cell edges of the Ca3Fe,(SiO') ,.r(OH) o.n,which is on the join andra- resultant andradites perhaps indicate excessSio+ in dite-hydroandradite(Flint et al, l94l). This deduc- some of theseandradites during preparation. tion was confirmed by a thermogravimetricanalysis up to I 100'C. Thereforethe bulk compositionof this Experimental hydroandraditeshould be accompaniedby 0.1 molar Raw materials include reagentgrade CaCO' and fraction of amorphous silica. Fe2O3,and amorphous SiO2, this latter being pre- The glass and later the andradite were synthesized pared by hydrolyzing siliconethoxide and heatingthe in a SiC furnacewith temperatureaccuracy of t 5'C. resulting SiO, gel at 1200"C for 2 hours. Glass with Quenching experimentswere carried out in a plati- an andradite bulk composition was preparedby cal- num wound furnacewith temperatureaccuracy of + cining the raw materials at 1100'C for 2 hours and 2'C. The products were identified by X-ray powder melting in a platinum crucible at 1400'C for I hour. diffraction technique and by oil immersion micro- The melt was quenchedinto water and the lumps of scopy. The cell edge of the was determined the glasswere ground in an mortar. The melt- from the 640 and the642 reflections,referenced to an 26 STABILITY OF SYNTHETIC ANDRADITE

internal standardof silicon.The amount of andradite 100 formed was estimatedfrom the relative intensity ra- g--O-O- tios in peak height of the 400 reflectionof andradite \ "80 'l2.056 to the 310 reflectionof wollastonitein the specimens a" = A treated below 1050'C, or to the 105 and the II2 reflections of pseudowollastonitein the specimens o60 treatedabove I100'C. X-ray measurementswere car- - ao= tz.osgi, looo'clo h ried out using a Rigaku Denki diffractometer em- e ploying a filtered CoKa radiation and a scintillation <4U counter. Refractiveindex of andraditewas measured

by immersion technique. P ca

Results and discussion Andradite as the only phase was obtained from hydroandradite,from the glass,and from the mixture 0 80 100 t20 i40 160 of wollastoniteand hematite. The average cell edge 12.059+ 0.003 A of the Frc r. variati"" ., *,:,,i"'ili,,t ;:"dradite devitrified andradite obtained by heating hydroandradite be- from the glassat I 150'C with intermediategrinding for every 22 tween I l00bC and I 150'C for 20 hours agreeswith hours heatingas a function of heatingtime. Startingmaterial used was the devitrified glass at l000oC for l0 hours. Lattice constants those of anhydrous andradite synthesizedunder hy- of andradites formed are indicated. drothermal conditions,a : 12.059A (Swansonel a/, : 1960)and a 12.062A lNat

Tnsr-r I Resultsfor the QuenchingExperiments by superheatingimpure (Ainslie, MacKenzie, + and Turnbull, 1960). q+.r+ihd nah-ar- tame Proouccs l4aterials ature Acknowledgment

Q-01 An d!adi te * lr50 20 h An (a=12.osgi) The authors expresstheir sincerethanks to Dr. Michio Inagaki Q-02 1r60 20 n An+Pw+He for (a=12,osgi) of SyntheticCrystal ResearchLaboratory, Nagoya University, " 0-03 Andradite** 1160 day tu1 (a=12.05IA) his valuablesuggestions and critical reading of the manuscript. 0-04 IITO h An+trPw+trHe Q-05 rl?5 h Pw+He 0-0? h Pw+He References (1960) 0-08 1190 h Pw+He+trL ATNSLTE,N G., J D. MncKENzte, eNo D. TunNnul-l 1200 man Pw+He+L Melting kineticsof quartz and cristobalite J Phys. Chem' 65, 1200 h Pw+He Q-12 L220 h Pw+He+L l7 18-1724. L (1955) High-pressureminerals J Am Ceram. Soc 3E' o-t a Pw+He 1220 h Pw+He Cors, 298 Andradite** 1250 h PwtHe+ 1250 day Pw + He Cunrsropur-MIcHEr--LEvv, M. (1956) R6production artibiciele 0-2 0 1285 day Pw+L des grenats calciques: grossulaire et andradite. Bull. Soc f'' PwfHe day Pw + He Cristallogr 79, 124-128. F., nr,ro R. Ro\ (1962) Modern High Pressure Tech- Andradite** 1295 2 h Pw + He + Dacsrrr-s, o-? I t295 I day Pw+L nique Brfiterworth lnc., P 163. Fr-rxr,E. P, H. F McMunoll, nNn L S Wlr-r-s(1941) Hydro-

* : obtained flom hydroandladite. thermal and X-rali studiesof the garnet-hydrogarnetseries and ** : obtained fron wollastonite and hematite. the relationshipof the seriesto hydration products of portland t Abbreviations : An, Ardradite; Pw, Pseudowollastonite; He, Hematite; L, Liquid; tr, trace. cement -/ Res Natl. Bur Stand, 26' 13-33 HucKENHoLZ,H G (1969)Synthesis and stabilityofTi-andra- dite Am J Sci, Schairer Vol 267-A,209-280 and hematite. The somewhat higher decomposition -! ANDH S. Yooln Jn. (1971)Andradite stabilityrelations in temperatureof the andraditeobtained from wollasto- the CaSiOr-FerO. join up to 30 kbar' Neues Jahrb. Mineral nite and hematite may be explained by considering Abh 114,247-280. lro, J eNo C. FnoNorl (1967)Synthetic zirconium and with , the stability of a of andradite . Am Mineral. 52' 773-781 excessSi4+, vacancies, and ferrous iron. A number of Nnrn, S , M Hnnnrn, ,qxoT Noon (1968)Formation of garnets defect garnets have been reported by van Hook of {Ca3f [Fe,-"Al'] (Sis)Ol, series Kogyo Kagaku Zasshi, 11, (1962)and Ronningerand Mill ( 1973).All ronicsites I l8-123 (in Japanese). -. nNo T Nonn (1966)Effect of pressureson in the garnet structure could conceivably have va- O. T,qreNnrn, the formation of compounds in the systemYzOs-AlzOt Kogyo cancles. Kagaku Zasshi, 69, I I l2-l I l6 (in Japanese) In the andraditeliom wollastoniteand hematite(4 RoNNINGTn,G, rNtr B V MtLL (t973) Vanadateswith a defect : 12.051A) quenchedafter holding for short period garnet structure. Kristallografia, l8' 303-307. at ll90'to 1250'C,a non-equilibriumglass phase SwaxsoN,H C,M I Coor,T Isancs,,tNoEH EvnNs(1960) powder pattern. Natl. Bur Stand was observed. while in the mixture of Slandard X-ray diffraction Circ 539,22-25 quenched pseudowollastoniteand hematite at the VAN HooK. H J (1962) Phase relations in the ternary system samecondition no glassphase was found. Formation Fe,O.-FeO-YFeOs J Am Ceram. Soc 45 162-165 of the glassphase might be attributed to an assumed melting of the solid solution of the andradite.The Manuscript receiued,Nouember 1|,,1974; accepted similar non-equilibriummelting has been observed for publication, JulY 3l, 1975