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 solid 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 iron, because ofcontamination by SiO,from an agatemortar during the repetitivegrinding. lntroduction ing and the grinding were repeatedtwice. The glass's refractive index 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 platinum 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 garnet was determined the glasswere ground in an agate 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<aet al,1968), and with those obtained in an oxidized and hydrous smaller value may indicate an excessof Sia+in the atmosphereunder I to 30 kbar total pressure,a : andradite - and consequentreduction of Fe3+ to 12.059A and 12.061A respectively(Huckenholz and Fe2+ plus formation of structural vacancies.Such Yoder. 1971). excesscould result from contamination by SiO, dur- Between 1000'C and I 150'C the finely powdered ing the repeatedgrindings (in an agate mortar) re- glassof andradite composition devitrified rapidly to quired to produce a 100 percent yield of andradite produce andradite plus metastablewollastonite or from a glassor wollastonite-hematitemixture. Huck- pseudowollastonite.Conversion of this metastable enholz and Yoder (1971),who obtained andradite phaseinto andraditewas very sluggish.At 1150"C, with a : 12.056+ 0.003 A by crystallization from the completeconversion to andraditewas obtained only glassat 1050' to ll35oc, attributed the small cell after six repetitionsof grinding after every 22 hours edge to the presenceof a small amount of ferrous of heating. Increase in amount of andradite with iron. heatingtime is shown with the lattice constantof the The quenchingexperiments summarized in Table I andraditein Figure 1. The latticeconstant, 12.059A, show that andradite obtained from hydroandradite of andradite from glassdevitrified at 1000"C for 10 beganto decomposeat I160"C to pseudowollastonite hours decreasedwith the repetition of heating and and hematite; however the cell edge of residualan- grinding,beow,.rrn! 12.051 t 0.003A after 130hours dradite remainedunchanged by the heating.In addi- of heating. tion, the andraditeobtained from the mixture of wol- A mixture of wollastonite and hematite, ground lastoniteand hematite did not break down evenafter separatelyfor 25 hours in an agatemortar, if heated 7 days of heatingat ll60oC, nor did its cell edge at ll00' to 1150"Cfor about 130 hours with inter- change. At ll70"C, it began to break down to ruption every 22 hours for regrinding,.yielded 100 pseudowollastoniteand hematite.The decomposition percent andradite (averagea : 12.051+ 0.003 A; temperatureof this latter andraditeis thus considered 1.889 + 0.007). However. a mixture of to be I165+ 5'C. Huckenholzand Yoder(1971), on pseudowollastoniteand hematite, similarly treated, the other hand, reportedthat their andraditedecom- yielded only a trace amount of andradite. posed above ll37 + 5'C to pseudowollastonite, Andradite formed from the glass and from the hematite,and a garnet(a : 12.045A); that the result- wollastonite-hematitemixture has a cell edse. 12.05I ing garnet contains a small amount of ferrous iron A , which is somewhatsmaller than 12.059X, the cell reducedfrom flerriciron; and that at 1157 + 5oC the edgeof andradite formed from hydroandradite.This garnetcompletely decomposed to pseudowollastonite 28 SUWA, TAMAI, AND NAKA Tnsr-r I Resultsfor the QuenchingExperiments by superheatingimpure quartz (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 Mineral 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 titanium with , the stability of a solid solution of andradite garnets. Am Mineral. 52' 773-781 excessSi4+, vacancies, and ferrous iron.