On the Nature of Synthetic Blue Diopside Crystals: the Stabilization Of

American Mineralogist, Volume 62, pages 522-527, 1977

On thenature of syntheticblue diopside crystals: the stabilization of tetravalentchromium

HBNnvD. ScHnnrssn Department of Chemistry, Virginia Mililary Inslitute Le xing t on, Virginia 24 45 0

Abstract

Chromium-richdiopside crystals, when synthesized in air at high temperaturefrom silicate melts,are often colored a characteristicblue, instead ofgreen as expected from thepresence of Cr(III). Previousexplanations for the anomalouscolor, in particularthat syntheticblue diopsidestabilized Cr(II) in thecrystal structure, are rebutted. The blue color ofthe synthetic diopsideis shownto be associatedwith the stabilizationof a smallportion of thetotal Cr as C(IV) within the diopsidestructure. Cr(IV) in detectableamounts is not presentinitially in the silicatemelts, but is producedby the reduction.ofCr(VI)during the crystallization.In comparisonto Cr(VI), Cr(IV) can more convenientlybe incorporatedinto the diopside structureaccording to two proposedmechanisms: the alleviation of thecharge compensation necessaryin theCr(VI)-Si(lV) substitution by thealternate Cr(IV)-Si(IV) substitution, or the lesseningof therequired charge compensation by thefavorable Cr(lV)-Mg(II) substitution.

Introduction Nevertheless,the hypothesis that the blue color is imparted by Cr(II) is, at best, speculative,but has The terrestrial distribution of chromium among beenaccepted as valid (Burns and Burns, 1975)in the igneous rock phasesis readily explained by the pre- absenceof another more compellingexplanation. ponderanceof Cr(III) in most terrestrial processes. Even on the basis of presentknowledge, the pres- Recently the importance of Cr(II) has been asserted enceof C(II) in syntheticblue diopsideis difficult to in the petrogenesisof the moon (e.g., Taylor, 19751' rationalize. Blue diopside is experimentallycrystal- Schreiberand Haskin, 1976)and in the mineralogyof lized in air from a silicate melt in which the the Earth's upper mantle (e.g., Meyer, 1975;Burns, Cr(VI)-Cr(III) redox equilibrium has been estab- 1975).However, the characteristicsof Cr(lI) in geo- lished(e.g., Bamford, 1962;Nath et al., 1965;Schrei- chemical systems have not been rigorously in- ber and Haskin, 1976). Extremely reducing condi- vestigated,although recent studies (e.g., Schreiber tions are necessaryto produce appreciablequantities and Haskin, 1976)have establishedsome basicprop- of Cr(II) in silicateliquids (e.g.,Frohberg and Rich- ertiesof Cr(II) in silicates. ter, 1968;Paul, 1974; Schreiberand Haskin, 1976). Synthetic blue diopsides have been suggestedto Likewise, very reducing conditions are required to contain Cr(II) becauseof their blue color (Dickey el synthesize blue Cr(II)-orthosilicate (Healy and al., l97l; Mao et al., 19721'Ikeda and Yagi, 1972). Schottmiller, 1964; Scheetz and White, 1972), even Most terrestrialCr-rich diopsideshave a greencolor, though a blue forsterite which contains Cr(III) at indicative of Cr(III). Since synthetic blue diopsides Mg-lattice siteshas also beenproduced in air (Finch are easilycrystallized in air at high temperaturefrom and Clark, 1971). Cr-doped silicate liquids (Dickey et al., l97l;lkeda The occurrenceof Cr(III) in tetrahedrallattice sites and Yagi, 1972),this mineral was thought to afford or the presenceof lattice defects has also been postu- convenientdirect measurementson the propertiesof lated as the causeof the color of syntheticblue diop- Cr(II). Mao et al. (1972) compared the crystal-field side(Dickey et al.,l97l; Ikedaand Yagi, 1972).Since absorption spectra of natural green and synthetic Cr(III) readily substitutesinto the favorable octahe- blue diopsides(Fig. 1), and Burns (1975)calculated dral Ml lattice site of diopside (e.g., Burns, 1970), the stabilization energy of Cr(II) in minerals on the Cr(lII) would not be expectedto behavedifferently in basis of the spectral features of the blue diopside. the experimentallyproduced blue diopsides. s22 SCHREIBER: SYNTHETIC BL(JE DIOPSIDE

This paper will advance substantialexperimental BtUE-lGREENTl-yErtoW evidenceto dispel all previous explanations for the lNDfGOr | .//TORANGE lvroter;t;t; 4\llnzol coroR scArr color of blue diopside. The characteristiccolor will WAVE[ENGTH,nm instead be attributed to the stabilization of a portion 400 500 600 looo 1500 of the total Cr as Cr(IV) in diopside crystalssynthe- sized under mildly oxidizing conditions. Although chromium existsin all valencestates from 0 to +6 in inorganic compounds with a corresponding wide rangeof colors (e.g.,Poole, 1964;Cotton and Wilkin- son, 1966),the presenceof the Cr(III)-Cr(VI) redox -ro pair in terrestrial systemsand of the Cr(II)-Cr(III) E redox pair in lunar basalts dominates the geochem- zs /: q t I ical distribution of chromium (Burns and Burns, t, I I i, t\e 1975). In addition Cr(O) has been found in highly g \/ o0 GREENNATURAT CHROME DIOPSIDE reduced meteorites (Olsen et al., 1973). Cr(V) has e60 z beenidentified as a trace component in oxidized syn- o thetic silicate melts (e.g., Es0 Gariflyanov, 1963;Schrei- e ber and Haskin, 1976) and in geologicallysignificant o 340 crystals (e.9., Banks et al., 1967; Greenblatt et ql., 1967;Grisafe and Hummel,1970a, b). The proposed 30 existenceof Cr(IV) in syntheticblue diopsidewill add the occurrenceof yet another Cr valencestate to the list of chromium speciesin minerals.Other claimsfor BTUESYNTHETIC CHROME the stabilization of Cr(IV) in glasses(Lunkin et al., o oo0 1968) and in crystals (Johansen, 1972) have pre- 25 000 20 000 15000 lo viously been advanced. WAVENUMBER.cm-l ADAPTEDFROM MAO et ol.(1972) Experimental methods Fig. l. Crystal field absorption spectra of green natural Homogeneous,bulk syntheticglasses with a diop- Cr-diopside and blue synthetic Cr-diopside as measured in the side composition or a composition defined by the three polarization directions by Mao et al. (1972). The peak at 10000cm-1 in the natural diopsideis due to the presenceofiron. acronym FAD (approximately 24Vo forsterite, 6Vo anorthite, and 70Vodiopside by weight) were pre- pared from high-purity component oxides by re- coexistedwith forsterite and quenchedliquid. Sam- peated fusions, quenchings,and grindings. To each ples were also preparedby isothermal devitrification was also added0.5 to 2 weight percentCrrO, as trace of the initial glasses. component. Sample syntheseswere performed in l- Indirect chemicalredox titrations were usedto de- atm, high-temperaturefurnaces with sample atmo- termine the fraction of each chromium valencestate spherescontrolled by CO: CO2gas mixtures. Small Pt in the synthetic glassesand crystals.The procedure capsulesholding about 200 mg of the initial glass was a modification of the basic designof Close and powder served as sample containers.Temperatures Tillman (1969)and is describedin detail by Schreiber were measuredby a PtlPt90Rh10 thermocoupleim- (1976). Unfortunately, the exact valencestate of Cr mediatelyadjacent to the charge;oxygen partial pres- can only be inferred by this method. If the Cr redox sureswere determined from fixed CO:CO, gas-flow pair is known, however, the titration in conjunction ratios calibrated by a ceramic electrode assembly with a total Cr analysisprovides a quantitative esti- (Schreiberand Haskin, 1976). mate of the concentrationof eachCr valencespecies. Experimental silicate glassesof diopside or FAD Electron paramagneticresonance (EPR) spectraof composition were synthesizedby melting at the de- the sampleswere recorded at X-band and ambient sired conditions above the liquidus. Synthetic diop- temperature under constant sample configuration; side crystalswere grown at the desiredoxygen partial 20.0 + 0.1 mg aliquots of the finely powderedglasses pressureby remelting the initial powder above the or crystalswere usedas samplesin matched3 mm ID liquidus followed by isothermal crystallization at Suprasil quartz tubes. The EPR measurementswere about 1350'C. For compositionFAD, the diopside usedfor quantitative estimatesof the Cr(III) content SCHREIBER SYNTHETIC BLUE DIOPSIDE of the glasses(Schreiber and Haskin, 1976), for iden- Not surprisingly,redox titrationson the synthetic tificationof otherCr valencestates in theglasses and diopsidesindicate that greendiopside grown at an crystals(O'Reilly and Maclver,1962),and for deter- oxygenpartial pressureof lO-e atm containsonly minationof structuralsites in which the Cr ionsare Cr(III), whereasblue diopside crystallized in air con- located. tainsCr of a highervalence state in additionto the Cr(III). Resultsand discussion The experimentalEPR spectraof syntheticgreen Experimentsin whichdiopside crystals were grown and syntheticblue diopsidesof comparabletotal Cr from silicateliquids as a functionof the oxygenpar- contentare shownin Figure2A and B. The Cr(III) tial pressureclearly indicated the inadequacies ofpre- resonancefine structure at the approximatemagnetic viousexplanations concerning the nature of synthetic field of 1500gauss [B"rr - 5] is identical for both blue diopside.For both melt compositions,blue crystals,indicative of Cr(III) in the Ml octahedral diopsidecrystallized only under an oxygenpartial lattice site in each crystal of different color. The pressureapproximating that of air. If the synthetic strong, symmetricpeak at H - 3400gauss [g"11 : diopsidecrystallized at an oxygenpartial pressure of 1.981which appears in thisparticular green diopside's l0 3 to l0-8 atm at 1350'C,it alwayshad a green spectrumcan be attributed to an insignificantamount color. Devitrificationexperiments gave analogous re- of Cr(III)-Cr(III) exchangecoupled pairs (e.9., Four- sults. nier et al., l97l) associatedwith the higher Cr(III) Theseexperiments show that the color of blue concentrationin that diopside.When synthesized un- diopsidemust be relatedto the redoxstate of Cr and der conditionswhich dictatedlower Cr(III) concen- not to simplechanges in the substitutedlattice sites, trations,the greendiopside would producean EPR sincethe incorporationof Cr(III) into a particular spectrumwithout the broad Cr(III)-Cr(III) reso- diopsidelattice site is probablynot dependentupon nanceand which wasvirtually identical to the blue the oxygenpartial pressureduring synthesis.The diopside'sspectrum. premisefor the stabilizationof Cr(II) within blue The EPR spectraof representativeCr-doped sili- diopsideis, however,directly contrary to the experi- catemelts (glasses) are shownin Figure2C through mentalresults, since the bluecolor is accentuatedat F; assignmentsof theCr resonancesin thespectra are the high oxygenpartial pressuresand not the low. accordingto the work of Schreiber(1976). An EPR Consequently,the only plausibleexplanation for the spectrumof a silicatemelt from whichblue diopside bluecolor is thepresence of a Cr valencestate greater would crystallizeis givenin Figure2C and illustrates than Cr(III). the presenceof traceamounts of Cr(V) ions in the The silicateliquid from which blue diopsidecrys- meltby thestrong asymmetric resonance at H - 3400 tallizesis yellowish-greenand containsabout l0-20 gauss,in additionto the presenceof Cr(III) ionsby percent (the actual fraction dependsupon temper- the peakat H - 1400gauss. The strongCr(V) band atureand composition) of thetotal Cr asCr(VI) with occursonly in the EPR spectraof glassessynthesized the remainderas Cr(III). At an oxygenpartial pres- in air or moreoxidizing atmospheres, the same condi- sureof about l0-3 atm, the quenchedmelt contains tions underwhich blue diopsideis produced.How- mostly Cr(III) and is dark green,whereas at lower ever,the EPR spectrumof syntheticblue diopside oxygenpartial pressures the glassesbecome progres- doesnot show any peak attributableto Cr(V); thus sivelymore blueishwith the formationof an appre- the blue diopsidecrystals must effectivelyexclude the ciablefraction of Cr(II) in the silicateliquid. How- tracesof Cr(V) presentin the initial liquid.An EPR ever,even from a silicatemelt whoseCr contentis spectrumof a silicatemelt from whichgreen diopside proportionately30 percentCr(II) and 70 percent precipitatesis givenin Figure2D. The broad,intense C(f If ) at an oxygenpartial pressure of l0-Eatm, the band is once again assignedto spin-spin exchange crystallizeddiopside is still green.The distribution couplingof Cr(III) pairswhich occuronly abovea coefficientof Cr(III) betweendiopside and melt is certain critical Cr(III) concentration(this critical about3 timesthat of Cr(II), sothat diopsideconcen- concentrationis variableand quite sensitiveto bulk tratesCr(III) relativeto Cr(II) (Schreiberand Has- melt compositionand temperature).Green diopside kin, 1976).Cr-rich diopsidehas also been produced can alsobe crystallizedfrom a silicatemelt contain- experimentallyunder a variety of mildly reducing ing lessCr(III), whoseEPR spectrumwould appear conditions,always resulting in greendiopside (e.9., asillustrated in Figure2F. The EPR spectralpattern Seward,l97l; Ikeda and Yagi, 19721'Duke, 1976), of greendiopside crystals with respectto the absence SCHREIBER: SYNTHETIC BLUE DIOPSIDE 525

or presenceof the Cr(III)-Cr(III) resonanceroughly reflectsthe pattern of the correspondingglass and, thus,just the absoluteCr(III) concentration.Even though the Cr(V) and Cr(III)-Cr(III) resonances dominatethe EPR spectrawhen they occur,studies (e.g.,Schreiber, 1976) have shown that the relative concentrationsof thesespecies are insignificant with respectto Cr(VI), isolatedCr(III), and Cr(II) in the melts.The artifactof a resonanceindicated in Figure 2F at H - 3400gauss is alwaysobserved in Cr-doped E glasses,and is possiblycaused by verysmall amounts of Cr(V) or by Cr(III) in axially-distortedoctahedral F coordination(Paul and Upreti, 1975). g c Theproperties of Cr(VI) in silicatemelts have been xtE previouslyinvestigated; Cr(VI) existsin tetrahedral !t! coordinationas discretechromate, CrO?-, groupsand impartsa yellowcolor to glasseseven at low concentration,as a consequenceofthe absorptionedge of an intensenear-UV absorptionband (e.g., Bamford, 1962;Nath et al., 1965).SinceCr(VI) is presentin the silicateliquid from which blue diopsidecrystallizes, Cr(VI) is alsoexpected in the crystals.However, if Cr(VI) incorporatesas chromate or dichromate groups within the diopsidecrystals, the crystals 'o,lo,oorrlloo /tooo 5ooo shouldbe coloredyellow-green, similar to the glass. Fig. 2. EPRspectra of representativeCr-doped diopside It is inconceivablethat the presenceof Cr(VI) with its crystals and glasses.All EPR spectra weie measured at X-band (r, = associatedintense near-UV absorptionwould shift 9.53 GHz). The derivative scaleplotted in they direction is relative theCr(III) transmissionband to a shorterwavelength for each individual spectrum; the intensities compared among in the progressionfrom greento bluediopside (Fig. spectra do not indicate relative concentrations from one sample to 1);instead the shift would be to a longerwavelength. another. The resonancesat H - 1400gauss are assignedto isolated Despite the inferred absenceof measurable Cr(lll) ions in near-octahedralcoordination; the sharp, intense, asymmetric band at H - gauss amounts 3400 is attributed to a small of Cr(IV) in silicateglasses for all synthesis amount of Cr(V) ions in near-octahedral coordination; and the conditionsstudied to date(Schreiber, 1976), the con- broad, intense, symmetric peak at H - 3400 gauss is caused by a clusionthat the anomalouscolor of blue diopsideis negligible concentration of C(Ill)-Cr(III) exchange-coupledion dueto the stabilizationof Cr(IV) in the crystalstruc- pairs. A: synthetic green diopside crystallized from a melt at P(Oz) : l0 3 atm with ture isjustified by comparisonto work of others relatively high Cr content. B: synthetic blue and diopside crystallized from an identical melt in is implied air. C: typical melt from experimentalevidence. from which blue diopside would precipitate in air. D: typical melt Lunkinet al. (1968)claim to havestabilized Cr(IV) from which green diopside would precipitate at P(Or) = l0 3 atm in an aluminosilicateglass under weakly oxidizing with relatively high Cr content. E: atypical melt, synthesizedin air conditions.Johansen (1972) argued for the existence and of high Cr content, demonstrating the Cr(V) and Cr(lII)-Cr(III) of Cr(lV) in blueCarSiOu crystals under appropriate resonancesare distinct. F: typical melt from which green diopside would precipitate, P(O,) = l0-3 atm or lower with synthesisconditions, on the basisof EPR andreflect- relativelylow Cr content. ancespectra. A reinterpretationof thechemical anal- ysesand opticalspectra of blue CarSiOucrystals re- portedby Boikovaet al. (1966)is alsoconsistent with structures.Hoskins and Soffer(1964) also present the stabilizationof Cr(IV) within this crystallattice. evidencefor the existenceof Cr(IV) in crystalsof Both Lunkin et al. (1968)and Johansen(1972) in- a-AlzOsgrown underconditions which supposedly dicatethat their resultingoptical spectraare con- favoredthe stabilizationof Cr(IV) in that lattice. sistentwith the presenceof Cr(IV) in the silicates. However,none of theseprevious authors have indi- Indeed,a comparisonof thesespectra with that of viduallymade a definitivecase for their arguments. syntheticblue diopsideshows a surprisingsimilarity In the productionof blue diopsideby devitrifica- of spectralfeatures (Fig. 3) for the different host tion of Cr-richglass in air, Dickeyet al. (1971)no- 526 SCHREIBER: SYNTHETIC BLUE DIOPSIDE

WAVEIENGTH.nm stitution.Alkaline-earth Cr(IV) chromates,MzCrOn, 500 700 900 stableblue-black compounds (Cotton 400 450 600 800 10001500 are relatively and Wilkinson, 1966),and PbCrO, can be synthe- sizedat high pressure(DeVries and Roth, 1968). Likewise,this convenientsubstitution-charge com- = pensationmechanism is consistentwith the produc- z f of blue forsterite(Finch and Clark, l97l) by t.a+". co3sio5 i tion U JoHANsEN ! processessimilar to that for blue diopside. ts (1972l AlternativelyC(IV) may substitutein the octahe- .r' J U ...... '-'.,,....;'J dral Mg latticesites of diopside.The expectedhigh = octahedralsite preferenceenergy of the Cr(IV) ion z (e.g.,Burns, 1970) might favor this explanation.Al- u /t' (J though a charge-compensationmechanism must be E at---t-/ SYNTHETIC U introducedto account for the chargedisparity be- o I tweenCr(IV) and Mg(II), thisparticular substitution z would requirecation vacancieswhich are energet- o ically more favorablethan the siliconvacancies in- r \ & troduced by the Cr(VI)-Si(IV) substitution.The o ionic radiusof Cr(IV) in octahedralcoordination is ao somewhatsmaller than that of Mg(II). However,if the octahedralsite substitution of Cr(IV) is favored overthe previousmechanism, it is difficultto explain 25 000 20 000 15000 l0 000 why Cr(VI) is not insteadreduced to the morecom- this WAVENUMBER..m-l patibleand stableCr(III) at site. Fig. 3. Comparisonof the opticalabsorption spectra of Cr(IV) A choicebetween the two proposedmechanisms in glass,Cr(lV) in CasSiOu,and syntheticblue diopside. Cr(IV) cannotbe madeon the basisof the presentexperi- shouldshow three absorption bands; the transitionat 9500cm-' mentation.Regardless of which mechanismis ac- may be the result of Fe contamination,while the peak at 22000 tually correct,the stabilizationof Cr(IV) in the blue cm-' is associatedwith C(III). diopsidestructure is easilyrationalized. Only a portion of theCr in the syntheticblue diopside is Cr(IV); ticed that the resulting charge was very vesicular, as if most of the Cr in the crystalis still Cr(III) as in- a gasphase had beenliberated in the crystallization dicatedby EPR, opticalabsorption, and redoxtitra- process.This observationwas confirmed in the pres- tion. Probablyno more than the amountof Cr ini- ent study.Whereas Dickey et al. (1971)interpreted tially presentas Cr(VI) in theliquid (about l0-20%o of thisobservation as the releaseof oxygenin the reduc- the total Cr) is transformedinto Cr(IV). However, tion of C(III) to Cr(II) duringthe crystallization, the this proportion of Cr(IV) is sufficientto shift the vesicularsamples probably suggest the evolutionof Cr(IIl) transmissionband and createthe observed oxygen as a result of the reduction of Cr(VI) to bluehue. Cr(tV) in the crystallizationof blue diopside. AlthoughCrO?- groups exist harmoniously within SummarY a diopsideliquid, a mechanismfor chargecompensa- Diopside,when crystallized from a meltcontaining tion must operatefor Cr(VI) as CrO?- or CrrO?- Cr(VI) in additionto Cr(III), will causethe preferen- to substitutefor SLO6-in diopsidecrystals. In the tial reduction of Cr(VI) to Cr(IV) to afford ready synthetic systems investigated,the difference in substitutionof Cr(IV) into the crystallattice. The chargescan be alleviatedby the unfavorablemecha- diopsidecrystals cannot incorporate Cr(VI) directly nismof formingsilicon lattice vacancies (e.9., McIn- into the structurein the syntheticsystems because of tire, 1963).The substitutionof CrOl- or CrrOS-units charge compensation limitations. Experimental [i.e.,Cr(IV)] into diopsidewould not requirecharge methodsof preparation,chemical and instrumental compensation.Although the ionic radius of Cr(IV) in analyses,and comparisonsto other studiesare all tetrahedralcoordination is about 1/z timesthat of consistentwith this assignment.The stabilizationof Si(IV) (Whittakerand Muntus,1970), the sizeis not Cr in the uncommon*4 valencestate in bluediop- expectedto be large enough to prevent the sub- side crystalsillustrates the powerful role that the SCHREIBER: SYNTHETIC BLIJE DI}PSIDE 527 crystallizing latticemay exert upon the distribution of stitutionsin the apatitestructure. Part A: Crystalchemistry. ,/. traceelements. 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