THD AMERICAN MINERAI-OGIST, VOL. 53, NOVEMBER-DECEMBER' 1968

SYNTHESIS AND STUDY OF YTTRIALITE

Jurv Irol aNo Hanolo JottNsor't,l{ational Bureau of Stanilards, Washington, D. C. 20234.

ABSTRACT

The rare earth pyrosilicates, Rg3+SizOz 1Br+:Sc,In,REl where RE:Lu,Yb,Tm,Er,

Ho,Y,Dy, and Gd) were synthesized at temperatures from 900-1600"c in air. A solid-

solution series exists between SczSirOz (thortveitite) and B-YzSizOz at 1300'C in air and at

700oC under 2.5 kbar. The compositional range from YzSiOs-YzSLOz-YzSiaOg was

investigated at temperatures from 900 to 1500'c. "High-yttrialite" was identified as

a-(Y,RE)gSizOz from the results of recrystallization of the natural minerals and by direct

synthesis from sodium-free gels both in air and under water pressure. The phase described

as ,,low yttrialite" is a compiex silicate of approximate formula R(Y,RE)5Si6or where R

includes ions of great variety, such as H+, Na+, Fe2+, Mg2+, Mn2+, p":+, A13+, Tha+ and

Zra+.

INrnooucrror.t

This investigation was undertaken in order to give reasonable inter-

pretations to the conflicting experimental results given in the l iterature

and to establish yttrialite as a well defined mineral.

Yttrialite, metamict yttrium silicate, YzSizOz, was first founcl a't

Llano, Texas (Hidden and Mackintosh, 1889). Several other localit ies

were reported subsequently (Hata, 1938; Nishimura and Ueda, 1954;

Protosenko, 1962). Despite its earlv discovery, crystallographic char-

acterization of yttrialite has been delayed because of its occurrence in

the metamict state. The formula for yttrialite has been established as

(Y,RE3+,Th4+,M2+)2Si2Oz by chemical analysis.Nishimura and Ueda (1954) reported that the yttrialite from Ko-

menono is isostructural with thortveitite, Sc2SLOz, monoclinic C2/m;

Omori and Hasegawa (1953) indicated that the yttrialite from Suisho-

yama has a crystal structure similar to p1'rochlore, isometric' Fd3m;

Protosenko (1962) gave x-ray powder data obtained from heated

yttrialite from the USSR aL 92O and 1000oC. Faria (1964) reported the

results of heating of yttrialite from several localit ies and described two

phases, y(low) and a(high). The former is identical with the phase given

as yttrialite by Protosenko.Warshaw and Roy (1964) synthesized a(low) and B(high) YzSizOz

with a transition temperature at 1240'C. The former is identical with

a high temperature phase given by Lima de Faria, but the relation to

l,ttr ialite was not mentioned. In his recent review of rare earth sil icates

Sidorenko (1966) statecl that "yttrialite is not isostructural with

1 Present address: Department of Geological Sciences, Harvard University, Cambridge,

Mass. 02138.

1940

YTTRIALITE 194l

thortveitite because of the l imited isomorphism between Sc and ypyrosil icates and the different behavior during heatingtt. More recentlv,Bondar and Toropov (L967) reported that a synthetic compoundidentical with a mineral described as yttrialite by protosenko can onlybe produced by the substitution of 13 percent of ThOz in a startingmixture of YzSizOz, and suggested that thorium nay be an essentialcomponent of yttrialite.

A new experimental study was carried out using yttrialite specimensfrom the original locality of Baringer Hill, Llano, Texas, and directsynthesis from precipitated hydrous silicates prepared by a new methodusing a cation exchange resin. The work was later extended to thestudy of polymorphism of rare-earth pyrosilicates with special emphasison the lower-temperature region from 900 to 1500oC.

ExpBnrupNrer Pnocroune

For the synthesis of rare-earth silicates (below 1200"c), preparation of reactiveamorphous silicates containing stoichiometric amounts of the elements is needed, becauseat low temperature certain mixtures of oxides do not react (Toropov, Bondar, Sidorenkoand Kololeva, 1965) within a reasonable time of heating. Furthermore, preparation ofalkali-free precipitated hydrous silicates is necessary for the accurate determination ofphase transition temperatures oI rare-earth silicates, because small amounts of sodium(less than o.l/) can lower the transition temperature as much as 100oc. For this reason,starting materials prepared by the neutralization or hydrolysis of reagents containingalkalies (Luth and rngamells, 1965; Roy, 1956) could not be used without removing thealkalies by the tedious process of electrodialysis. rn this respect, a method given by croftsand Marshall (1967) using hydrolysis of pure silicon tetrachloride and aluminum chloridefor aluminosilicates seemed promising, but there are two inherent difficulties in thetechnique: (1) volatile liquid must be injected through a diaphragm with a syringe, and(2) not all of the desired chlorides are readily available.

we adopted a relatively simple resin method originally developed for analytical pro-cedure for the separation of pure silicic acid from cations (Shehyn,7957). Relatively pureand almost alkali-free hydrosilicates are precipitated at pH 9 with ammonia, from solu-tions containing pure silicic acid and rare-earth chlorides. comparison of purities ofhydrated yttrium silicates prepared from both alkaii-free and alkali-containing solutionsis given in Table 1. This shows the satisfactory removal of most ions, particularly sodium,to less than 1 ppm by the resin method. The precipitation from sodium silicate solutionsleft about 100 times more residual sodium. Both sodium values are much less than thosereported in the gels (sodium content 0.1/s) prepared by Luth and rngamells (1965) forsupposedly sodium-free composition. This resin method may have wide applicability forsilicate syntheses; the detailed procedure is therefore described in the following section.

Preparalion of the pure silicic acdd. solutiozs. Approximately 150 ml of H form resin (Dow X50, w-x12 200 mesh) was transferred into a beaker containing 200 ml of dilute sodiumsilicate solution (20 miliimole of c.P. NazSioB'9H2o), and stirred by a magnetic stirrerior 2 hours. The pH of the resulting solution was betu,een 2.5 and 3.0. practically all the:ations were removed and most of the coz absorbed in the sodium silicate was evolved.-{.fter the resin had settled, the soiution was passed through a column, 2,, OD and 10, in

1942 JUN ITO AND HAROLD JOHNSON

Tarllr 1. Spocrnocnapnrc Anar-vsrs or rur HvotrosrlrcATEs Usr:l ton rrrp

Svnlmses. Al+alvsr. G. BourNr, JARREL Asn Co., WalluArt, Mass. (1967)

Sample prepared using resin

purified silicic acid

B, Al, Ca, Fe, Cu, Be,

Na, Cr, As, Sn, Pb

Range of concentrationSample prepared using

sodium silicate solution

10%0.01-0.001%0.001 0.0001less than-O.0001

Y, SiNo, Ti, MnB, Mg, AIBe, Ca, Cr, Cu, Sn, Pb,Ag

length, containing 1" of the same H-form resin to complete the cation separation. Approxi-

mately 400 ml of distilled water was used for the total washing process of the resin. The

eluent was transferred to a volumetric flask (1 l.) and standardized by the gravimetric

method This silicate acid solution can be stored safely in a plastic bottle for several weeks

without sign of gelatinization.

Preparation oJ the omorphous hydrous rare-eorth s,ilicara.t. A slightly acid solution containing

stoichiometric amounts of silicic acid and rare-earth (including yttrium) chlorides was

heated to boiling and conc. ammonia was added gradually to bring the pH to 9. The solu-

tion was boiled for five minutes, then cooled to room temperatule. The precipitates were

then decanted, centrifuged and completely dried in an oven at 110'C. No washing was

required, because the solution was essentially free from alkali. The final materials contained

approximately 30 percent H2O, but were nonhygroscopic. They remained reactive for a

period of more than one year when kept in a tightly covered vial. Chemical analyses showed

that the precipitates contained Si and rare earth ions in the desired ratio. Slight Si defi-

ciency was common, and a small adjustment of starting ratios was required for accurate

work.

Synthesis. Heating was carried out in a furnace made of Pt-20 percent Rh winding at

temperatures ranging 900-1350'c. Platinum foil was used as a container. An induction

heater was used to obtain temperatures above 1400"C. The products were air quenched

and examined by X-ray powder diffraction.

The results of the experiments with the original specimens of yttrialite from L1ano,

Texas, and of Lhalenite from Suishoyama, Japan, are given in Table 2' Comparison of the

X-ray powder data of heated yttrialite and the synthetic a-Y:SizOz and 0-Y:SirOz of the

present study and of the previously reported works are made in Table 3''Ihe results of partial phase-syntheses of Mr3+SizOz (M3+:Y'Lu,Yb,Tm,Er,Ho,Dy,

Gd,In,Sc, I{o5oDy56,Ho75Dyr), YzSiOr and YzSirOs in air (900-1600'C) and under hydro-

thermal conditions (550 650'C, 2 kb) are deposited with the National Auxiliary Publica-

tions Service, American Society for Information Services.l Partly indexed X-ray powder

data of the following identified compounds were also submil-ted to the ASTM powder data

file: Z-YzSLOz, y-YzSirOz, "y-ErrSi:Oz, 6-DyzSi:Oz, 6-YrSirO?, Y:SiOs (Xr) and YzSiO; (Xz).

l Order NAPS Document 00100 from ASIS National Auxiliary Publications Service,

c/o CCM Information Sciences, Inc.,22 West 34th St, New York, N.Y' 10001;remitting

in advance $1.00 for microfiche or $3.00 for photocopies.

I/TTRIALITE

Ttr*,n 2. Rrsulrs ol rnn HnarrNc ExprnrurNrs ol Yrrrralrrn rnouLLANo, TExAs, aNt TnllnNtm lnou Sursnoy,tMA, JAIAN

1943

Minerals andLocality

YttrialiteLlano County,Texas

Temp. of heating(c)

9001,0001,20o1 ,3501,4001,400

450500620680

Duration(hr.)

242424

?

r/21/224242424

24621 /''

1 / 7

242+

1 bar1 bar1 bar1 bar1 bar1 bar2 k b2kb1 k b

2 . 5 k b

Phase identified

amorphous

va

a

a

a & melta (v. weak)q

d

d

P & thalenite (weak)"v^v

thalenitethalenite

Thalenite

Suishoyama,

Japan

9001,0507,4651,600

520630

NonBNcraruno

Yttrialile. Our heating experiments on the original yttrialite samplesgave the same two crystalline phases as reported by Lima de Faria(1964) (Table 3) the low form, y, stable at 900-1000oC and the highform, a, stable between 1200 and 1450'C. The former phase wa's sug-gested by Protosenko (1962) to be ytrrialite. Lima de Faria also assumedthat the lower y-phase may be an original crystall ine yttrialite, becauseit develops first on heating. Ilowever, in the compositional range of theYzOs-SiOz system studied, we found that a single y-phase is stable onlyat a chemical composition richer in SiOz than that of yttrialite YzSizOzat temperatures between 900 and 1050oC in air, with the possibleformula: Ha.Yo-"SioOn (n11). This phase was often found as a meta-stable or intermittent phase in the lower temperature equil ibrium rangefor the d-phase of composition (Y,Ho,Er)zSLOz synthesized from gel attemperatures below 1100'C. The presence of small amounts of sodiumseems to stabil ize this phase even up to 1200'C in air. Ferrous or ferriciron also stabil izes this phase up to 650oC under 2 kbar pressure. Underboth conditions, the compounds of this range containing only yttriumand silicate ions are converted to the higher a- or B-phases. Recrystal-lized "low yttrialite" (y-phase), therefore, may be a complex silicate

barbarbarbarkbkb

IgM JUN ITO AND HAROLD JOHNSON

with approximate chemical formula R(Y,RE)bsioOzr where R includesII+, Na+, Mg2+, Mn2+, Fe'*, psr+, AI3+, Th4+ (Bondar and Toropov,t967 ) and Zra+.

The a-phase formed from natural yttrialite above 1200oC is iso-

morphous with a-YrsizOz and a-(Gd,Tb,Dy)rSLOz (both low tem-perature forms), found by Warshaw and Roy (1964). IIowever, the

a-phase obtained from natural mineral displays no transitions to higher

temperature phases such as those described in the following section for

synthetic compounds ranging from Tm to Gd including Y. The natural

specimens melt at approximately 1450'C and the d-phase still persists

at that temperature. This is due to the fact that the greater complexity

of the chemical composition of the natural mineral lowers the eutectic

melting point considerably, so that the high-temperature phase does not

appear. Hydrothermally recrystallized natural specimens gave only the

a-phase at temperatures from 450-720"C under 2 kbar of HzO pressure'

From the results described above, we can state that a-phase, cY-

YzSizOz, may exist in a wider range of stability than y-phase, and the

original crystalline yttrialite was most likely a-YzSi2O7. Hydrothermallysynthesized a-YzSizOz at 650oC and 1 kbar showed under the microscopehexagonal prisms with flat {0001} faces. This compound probably is of

hexagonal synmetry, but indexing of the powder data was not achieved.

Very few X-ray diffraction Iines were reported for natural unheated

vttrialites by Protosenko (1962), Nishimura and Ueda (1954) and Omori

and Hasegawa (1953); the l imited data recorded in the l iterature are

both inconsistent and inadequate for identifying the species.

Thalenile. Another yttrium pyrosilicate known in nature is thalenite.

The attempted synthesis of thalenite, YzSi:Oz was not successful' The

natural specimen of thalenite from Suishoyama described by Nagashima

and Kato (1966), was heated hydrothermally and in air. Under hydro-

thermal conditions from 450-700oC and 2-3 kbar, thalenite remained

unchanged, and gave an X-ray powder pattern of the typical thalenite'

In air at 1050oC, thalenite converted to 0-YzSiuOr, but a few strong

lines of thalenite still remained after 62 hours at that temperature. At

1400'C it was further converted to the 7-phase (see following section),

and it remained unchanged up to 1600'C. No higher phase was obtained.

The g- and 7-phases have been identified recently as phase II and I

respectivel,v in two-phase thalenite found at 0sterby, Sweden (Vatelieva,

Krivokoneva and Pyatenko, 1967).The above experimental results indicate that thalenite is definitely

a low-temperature phase and most probably hydrous, as was shown by

Proton Magnetic Resonance studies (Nagashima and Kato, 1966). We

suspect that the mineral described as thalenite maY not be in its original

YTTRIALITE 1945

crystalline form. It may have been alterecl to a hydrous phase duringthe later stage of crystallization. The X-ray powder data given for thesynthetic thalenite by Lazarev, Tenisheva and Bondar (1965) showedsome similarity to the data of the natural specimen (Neuman el ol.,1957; Adams et a1.,1962),but their conditions for the synthesis (1300"Cin air) may be questioned on the basis of the present experiments. Amineral described as thalenite from Annercid Vaaler, Norway, bySabina and Traill (1960), subsequently identified by Warshaw andRoy (196a) as 0-YzSiroz (isomorphous with thortveitite; Strunz, 1957)should be given a new name. If an yttrialite described by Nishimuraand Ueda (1954) also is tnrly isostructural with thortveitite, it mustbe the same mineral. Rowlandite (Frondel, 1960) and Iimoriite (Naga-shima and Nagashima, 1960) are difierent yttria-earth silicates and havenot been identif ied in this study.

On the polymorphism of (Y,Ln,In,Sc)zSizOz.In addition to the character-ization of the mineral yttrialite, our study was extended to the poly-nrorphism of rare-earth pyrosil icates, (Y,Ln,In,Sc)zSLOz (Ln:Lu,Yb,Tm,Er,Ho,Dy, and Gd), because some discrepancies exist in the resultsand interpretation of the polymorphism of rare-earth pyrosilicates inthe l iterature.

Warshaw and Roy (1964) reported that lower temperature a-YzSirOzis structurally similar to the lower polymorph of the so-called inter-mediate rare-earth pyrosil icates, a-(Gd,Tb,Dy)rSLOt. At 1240oC, thed-phase YzSizOz was converted into a higher polymorph 0-YrSirOz, iso-structural with so-called smaller rare-earth pyrosilicates (Lu,Yb)zSizOzwhich shows only one phase up to the melting point.

Toropov and Bondar (1960) reported phase equil ibria studies of thesystems Ln:Or-SiO:. They found only one type of LnzSizOz at tem-peratures above 1600"C for each rare-earth oxide-SiOz system. TheirX-ray powder data of YzSi:Oz are completely unrelated to those of a-or 0-YzSizOz, but show isomorphism with B-TbzSi2O7 reported byWarshaw and Roy (1964).

Lazarev, Tenisheva and Bondar (1965) obtained one compoundYzSizOz which was spectroscopically isostructural with SczSizOz and(Lu,Yb)zSirOT by sintering mixed oxides at temperatures between1300-1400'C. They also reported a high temperature polymorph ofY:SLOz prepared by arc fusion at over 1450oC. It has the same infraredspectrum as (Sm,Gd,Dy):SLOz. This phase is apparently the samecompound as that given by Toropov and Bondar (1960). Vatelieva,Bondar and Sidorenko (1965) reported one more polymorph of YzSizOz,grown in KF flux by slow cooling from 1400-1200"C; they obtainedgood X-ray data"

t946 TUN ITO AND IIAROLD JOHNSON

p-chose

o

o

o

. 8 5 . 8 6 , 8 7[ u Y b T m

1 : . 8 e . 9 0 . 9 1t ' lo Dy

,91G dE r Y

. 9 2 . 9 3

( A ) r o N r c R A D r u s

Frc. 1. Schematic presentation of polymo4lhism of Y2Si2O7 in terms of the trivalentionic radii of rare-earth ions (Tempieton and Dauben, lg54), In (0.81 A by Pauling,1960) and Sc (0.76 A by Frondel and Ito, 1967). Black circles:a-YzSi2O7; open circles:0-YzSizOz; black squares:"y-YzSi:Oz; open squares:6-YzSizOz; black and white circlesand squares two phases.

We found YzSizOz displays at least three first-order poll'morphic tran-s i r ions. Theseinc lud.o izzs lc r to"qB,B 144s"c t l0 '9" , r5 j l9 + l9 j o .

The additional lowest temperature Z phase of YzSirOz was observedbelow 1030"C, but its composition was not ascertained. Warshaw andRov (1964) studied the a and B phases only. The crystals described byVatelieva, Bondar and Sidorenko (1965) are the T phase. The compoundsynthesized by Toropov and Bondar (1960) and also the high tem-perature phase obtainedby l,azarev, Tainisheva and Bondar (1965) arethe 6 phase. The low temperature phase given by the latter authorsprobably is the B phase because of its similarit.y to ScrSizOz. Our resultsagree reasonably well with the transition temperatures found by War-shaw and Roy (a 1?{S B) and by Lazarev, Tenisheva and Bondar(B 4ES r).

HozSizOz also gives the same four-phase polymorphic transitions asYuSi:Oz, but the stabil itv ranges and the transition temperatures areslightly different: ' rrl! g=-]j$

B, B 1330"C + 10'q r, r 4-I3 's€ O.

ErzSLOr shows three polymorphs, namely a 1050'C t 25'Cr 6, 81400'C t 10'9 t.TmzSizOz has only two phases, cv 1050"C + 19"C> B. Actually, the a-phaseof ErzSizOz and TmzSizOT are accompanied by the y-phase (Protosenko,

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YTTRIALITD

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1948 JUN ITO AND HAROLD TOHNSON

F , o ,

o6 7 0

6 5 0

r o N r c n o o , u s ( i )

Fro. 2. Unit-cell dimensions of B-YrSizOz and its analogues Plotted against ionic radii

of trivalent rare-eartlr, Sc and In ions (Templeton and Dauben, 1954; Pauling, 1960;

Frondel and Ito, 1967).

t962;Lima de Faria, 1964).Lu and Yb analogues do not display poly-

morphism. F-LnrSirOz is the only phase in the temperature range studiedhere. The so-called intermediate (Gd,Tb,Dy)zSirOz shows dimorphisma to D, which corresponds to (a) and (p) TbrSirO? reported by Warshawand Roy (1964).d-DyzSizOz is the phase found by Toropov and Rondar(1961). Among the four phase transitions, only a reverse reaction lrom

6 to 7 takes place within reasonable time of heating at slightly below

the transition temperatures.The polymorphism described above is shown schematically in Figure

1. The experimental results in this diagram were plotted in terms of

YTTRIALITE

I 5 0

6 9 0

6 7 0

o 2 5S c 2 5 i , O ,

T h o r t v c i t i r c M o l c

d - Y 2 s i 2 o 7

I'rc. 3. Unit-cell dimensions for the solid solution series of SccSirO-B-YzSrrOz.

the temperature and the radius of trivalent rare-earth (Templeton andDauben, 1954), In (Pauling, 1960) and Sc (Frondel and lto, 1967) ions.From the results, it is clear that the phase transitions are controlled byionic radii as well as temperature. The gradual changes of the tem-perature of the phase transition observed in this diagram are not con-sistent with a simple classification of rare-earth ions into, for example,small, intermediate, etc., based on the polymorphism of these com-pounds.

We found that Sc:Si.zOz (thortveitite), In2Si2Oz, (Lu,Yb)zSi:Oz and

B-(Er,Ho,Y)zSizOz are isostructural. X-ray powder data for B-YzSizOz(Table 3) were indexed according to the space group for thortveititeC2/m monoclinic, and the unit-cell dimensions of all other analoguesare tabulated in Table 4. The gradual change of the unit-cell dimen-sions in terms of trivalent ionic radii of cations is illustrated in Figure 2.

b. I 7 0

o

5 0

1950 TUN ITO AND HAROLD JOIINSON

Tanr,r 4. UNrr CBrl DrurNsroms ol d-Y:SizOr AND OrHER R,r.nr EanrnANALocUES AND oF THE Sor,ro SolurloN Srrrns ol Sc:SizOz-0-YzSizOz

Formula

6 . 9 06. 896. 856 . 8 16 . 7 86 . 8 26.636 . 5 26 . 7 96.696 . 6 16 . 5 4

8 . 9 88 . 9 88 . 9 48 . 8 88 . 8 48 . 9 08 . 6 38 . 5 18 . 8 08 . 6 78 . 5 18 . 5 2

4 . 7 24 . 7 14 . 7 1

4.694 . 6 84 . 7 21 . 6 74 . 6 6+ . o l

r0r.71 0 1 . 81 0 1 . 6102.01 0 1 . 9101 .9102 .8t 0 2 . 7TOT.71 0 1 . 6102.4t02.3

" Unit-cell dimensions by Dawson, Harrison, Gatlagher and Horne (1966).

Furthermore, complete solid solubil ity was established betweenScrSizOz and 0-YzSLOz at 1300oC in air and at 700oC under 2.5 kb HrOpressure. Unit-cell dimensions of this series are also given in Table 4and Figure 3.

Hydrothermally crystall ized p-YzSi2O7 prepared from sodium-freegel, yield fine transparent crystals up to 0.5 mm in size with well de-veloped faces. The single crystals of Z-YzSizOz were grown in a flux ofLi:rWzOz at a" l:5 charge/flux ratio by slow cooling from 1250-900"C.Flat crystals of almost square section up to 1X 1X0.3 cm were obtainedin a fine mass of 6-YzSizOz. Eu and Tb activation of the four differentmodifications of Y2SLOz yields mildly fluorescent crystals.

In addition, we studied YzSiOs using the same experimental techniquesof synthesis from gel. We found only one phase (X2-phase) at tempera-tures above 1190oC. This is the same compound described bv Toropovand Bondar (1961), and Harris and Finch (1965). At temperaturesaround 10500C we obtained one low temperature phase (X1-phase, pos-sibly hydrous), but its stoichiometry is uncertain. These Xr- and Xz-phases are different from that described by Warshaw and Roy (1964).

Acrrqowr,roclrnNrs

The authors thank Mr. H. S. Peiser for his continuing interest and encouragement.Thanks are due to Mr. S. F. Holley for his assistance in some high temperature experi-ments. Thanks are also due to Professor Brian Mason at U. S. National Museum, andProfessor Clifiord Frondel of Harvard University in supplying the specimens of yttrialitefrom the original locality, and Dr. Akira Kato at the U. S. National Museum for the

B-YrSirOTHozSizOzErsSi2OzYbzSizOzLuzSizOzTmzSi:OzIngSizOzSczSLOz

0-(YzsSczs)zSuO;(Y5oScbo)sSi2O?(YraScrs)zSi:Oz

Tirortveitite'

I/TTRIALITE 1951

thalenite specimen. This research was supported in part by a grant from the AdvancedResearch Projects Agency under Order 373-62.

RrlnnnNcrs

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Manuscript receiteal, Moy 3, 1968; accepted for pu.bl,ication, August 17, 1968.