Redox equilibria, structure, and properties of Fe-bearing … · 2007. 8. 29. · American...

American Mineralogist, Volume 74, pages 58-76, 1989

Redox equilibria, structure, and properties of Fe-bearing aluminosilicate melts:Relationships among temperature, composition, and oxygen fugacity in the

system NarO-AlrOr-SiOr-Fe-O

B.ronN O. MvsnN. Dlvrn VrncoGeophysical Laboratory, 2801 Upton St., N.W., Washington, D.C. 20009, U.S.A.

AnsrucrRedox ratios of Fe and structural positions of Fe3* and Fe2* in melts in the system

NarO-AlrOr-SiOr-Fe-O have been determined with Mdssbauer spectroscopy. The value oflog(Fe2+,/Fe:*) generally is linearly correlated with logf, and, l/T (T inkelvins). The Fe2*/Fer' ratio decreases linearly with increasing All(Al + Si) and, for NBO/T > 0.4, the Fe2*/Fe3" ratio decreases with increasing NBO/T of the melt (NBO/T : nonbridging oxygensper tetrahedrally coordinated cation). The inverse correlation of Fe2+/Fe3+ with All(Al +Si) is more pronounced at higher temperatures and with/o, reduced below that of air. Thefree energy ofreduction ofFe3t to Fe2* increases (becomei less negative) with decreasingAl/(Al + Si) and NBO/T. With Fe3*/)Fe > 0.6, Fe3+ is tetrahedrally coordinated (t4rFe3+)-whereas for Fe3*/ZFe < 0.3, Fe3* occurs in octahedral coordination (l61Fe3+). In the Fe3*/)Fe : 0.6-O.3 range, tetrahedral and octahedral Fe3* coexist. Fe2* generally is a networkmodifier.

The degree of polymerization of silicate liquids depends, therefore, on Fe3*/IFe. TheNBO/T generally will increase with increasing temperature, decreasing fo2, and, decreasingAll(Al + Si). For properties that depend on degree of melt polymerization, these propertieiwill also be functions of Fe3*/2Fe. An example of such a property is melt viscosity wherethe activation energy of viscous flow decreases with increasing NBO/T. For melts in thesystem Naro-Al.or-Sior-Fe-o with 5 wto/o iron oxide added as Feror, the viscosity de-creases by more than 400/o in the Fe3*/EFe range between 1.0 and 0.0. The temperaturedependence of the viscosity is more pronounced than in Fe-free melts with the same Al,/(Al + Si), in which NBO/T does not depend on temperature.

The results are combined with published experimental data from simple synthetic andcomplex natural liquids to show that in natural magmatic liquids the proportiol ef trtps:+increases systematically as the liquids become more felsic. For example, Fe3* in anhydrousrhyolite melts is predominantly in tetrahedral coordination, whereas Fe3* in basaltic andpicritic melts principally exists in octahedral coordination. Thus, during fractional crys-tallization of basaltic liquids toward felsic melt compositions, the t+1fs:+/terFe3* ratio willlncrease.

INrnolucrtoN

Natural magmatic liquids commonly have appreciableconcentrations of iron oxide (FerO, and FeO). The Fe3*in such melts may be both a network former and a net-work modifier (Mysen, 1987). Because Fe2* generally oc-curs in octahedral coordination (e.g., Mao et al., 1973Nolet et al., 1979), reduction of network-forming Fe3* toFe2t results in changes in degree of polymerization ofsilicate melts. For example, with tetrahedrally coordinat-ed Fe3*, reduction to octahedrally coordinated Fe2" re-sults in an increase in NBO/T (i.e., melt depolymeriza-tion), whereas reduction of octahedrally coordinated Fe3*to octahedrally coordinated Fe2* is associated with a de-crease in NBO/T. Melt properties that are related to meltpolymerization-such as viscosity, density, and expan-sivity (see, for example, Takahashi, 1978; Watson,1977;

0003-004x/89/0102-O058$02.00 58

Mysen and Virgo, 1980; Dingwell and Virgo, 1987; Ding-well et al., 1988; Bottinga et al., 1982, 1983; Bockris andKojonen, 1960; Robinson, 1969; Hofmann and Magaritz,1977; Bockris and Reddy, 1970)-will, therefore, also varywith Fe3*/2Fe of the melt.

In Fe-bearing aluminosilicate systems, crystal-liquidphase equilibria and other properties also depend on theabundance ofAl3* and on the types ofcations that charge-balance Al3* and Fe3* in tetrahedral coordination (Muanand Osborn, 1965; Riebling, 1964, 1966; Navrotsky etal.,1982; Seifert et al.,1982a). It has been suggested thatthe stabilities of aluminosilicate liquidus minerals (e.g.,feldspars) depend on the proportion of related structuralunits in the melts (e.g., Burnham, 1981; Fraser et al.,1983; Mysen et al., 1982a). If so, the liquidus fields ofsuch minerals depend, therefore, on the Fe3*,/)Fe of theliquid whether or not appreciable amounts of iron oxides

MYSEN AND VIRGO: FE-BEARING ALUMINOSILICATE MELTS 59

. 0.6

oTL

: o .7C!oIL

t l

> 0.8-j.(d

O+> 0.e

atO

NBO/TFig. l. Shifts ofliquidus boundaries (expressed as degree of

polymerization of the melt with the assumption of tetrahedrallycoordinated Fe3* and octahedrally coordinated Fe'?*) indicatedin the systems CaO-FeO-SiO, and CaO-FerOr-SiO, (phase-equi-librium data from Osborn and Muan, 1960a, 1960b).

are contained in the mineral(s). Furthermore, becauselowering of Fe3*/ZFe can result in depolymorization ofthe liquid, the liquidus phase of a silicate liquid maychange to a more depolymerized mineral (e.g., pyroxeneto olivine) as the Fe3*/2Fe ratio is decreased. In otherwords, decreasing Fe3*/2Fe (with constant total iron ox-ide content) is equivalent to increasing the concentrationof network-modifuing cations (in this case Fe'z+), thuspossibly affecting liquidus phase equilibria, as also dis-cussed by Kushiro (1975) and Ryerson (1985). This prin-ciple is illustrated in Figure I (data from Osborn andMuan, 1960a, 1960b). In this figure, the degree of poly-merization (NBO/T) along liquidus boundaries of iron-free calcium silicate minerals with different degrees ofpolymerization (pseudowollastonite, rankinite, and lar-nite have NBO/Si equal to 2, 3, and 4, respectively) isshown as a function of Ca/(Ca * Fe'z*) (curve markedp.:+) and Cal(Ca + Fe3*) (curve marked Fe3t). The arrowindicates the direction of shift of these liquidus bound-aries as a function of decreasing Fe3*/ZFe for constanttotal Fe content. Clearly, from these phase-relationshipdata, the liquidus field of the least-polymerized phase ex-pands with decreasing Fe3*/2Fe. Thus, the redox ratio ofFe in magmatic liquids will influence liquid fractionationtrends even without precipitation of iron oxide(s) in thefractionation process (see Osborn, 1959, for conse-quences of iron oxide crystallization from basaltic mag-ma).

In view ofthe above considerations, it was decided toundertake a study on the relationships among redox equi-libria of Fe, melt structure, and properties in NarO-AlrOr-SiOr-Fe-O melt. In combination with data already avail-able in related alkali and alkaline-earth aluminosilicate

AI/(Al+Si) = 0.334

Fig.2. Composition of starting materials (Fe-free; designatedin text by NAS plus roman numeral; see Table l) and the prin-cipal liquidus-phase equilibria in the relevant portions of thesystem NarO-AlrO3-SiO, (Osborn and Muan, 1960c). Abbrevi-ations: NS, NarSiOr; NS2, NarSi'Or; N2S, Na"SiOo; Tr, tridy-mite; Qtz, quartz; Ne, nepheline; Ca, carnegieite.

melt systems, a basis for quantitative characterization of

the role of redox equilibria in natural magmatic systems

might be developed.

ExpnnrlrnNTAl METHODS

The Fe-free compositions in the system NarO-Al'O.-SiO, (Fig.

2) were chosen so as to encompass the range of Al/(Al + Si) andNBO/T (Table 1) commonly encountered in igneous rocks (0.15-

0.4 and 0. 1-l .0, respectively; Mysen, 1987). This compositionalrange also covers liquidus boundaries of minerals with greatly

diferent compositions and degrees of polymerization (Osborn

and Muan, 1960c; see also Fig. 2).The starting materials were produced by grinding mixtures of

spectroscopically pure Na'COr, Al.Or, and SiO, (Johnson andMatthey, Inc.) under alcohol in batches of 250 mg for approxi-mately t h. This material was decarbonated near 800 qC for 2

h, and the temperatures were slowly raised to immediately abovethat of the liquidus to produce the Fe-free base glass. Iron oxide

TABLE 1, Nominal composition of starting materials

NASI NASII NASII I NASIV NASV NASVI

1 . 01 . 41 . 8z - z2.6a n3.4

NAS.vt l

sio, 78.97 71 08Al2o3 5.63 8,42Na,O 10.39 15 50Fe,O. 5.00 5.00

55 54 4397 33 75 35 0613.89 1 8.70 21 .56 14.8925.58 32.33 39.69 45.05

5.00 5.00 5.00 5.00

39.4216.7738 82s 0 0

NAS-VII I NASIX NASX

NAS- NAS-NASXI XI I XI I I

sio,Al,03Na2OFe.O"

47.32 52.43 37.6120.12 22.30 21.8927.56 20.27 35.50

5.00 5.00 5 00

49.63 53.63 58.6914 .30 | 1 .37 7 .9731.34 30.00 28.3s

5.00 5.00 5.00

si02

N2S nr v NBO/T = 0.668

60 MYSEN AND VIRGO: Fe-BEARING ALUMINOSILICATE MELTS

Fe3*

rfc) log fo,' IS OS

Taele 2. Experimental results

Fe,.(t)

rsISOSFe,+(ll)

Composition os Fe3+/>FeNASINASIINASII-"NASII INASIVNASVNASVINASVIINASVII INASIXNASXNASXINASXIINASXII INASIVNASVIINASVII INASIXNASXII INASIVNASIXNASXII INASIVNASIV*NASIVNASIV'-NASIXNASXII INASIVNASVII INASIVNASIXNASXII INASIVNASIV--NASIVNASIV- 'NASIXNASXII INASXII I*NASIVNASIVNASIXNASXII INASIVNASIVNASIV'-NASIXNASXII INASIVNASIVNASIXNASXII I

1 5501 5501 5501 5501 5501 5501 550I ccu

1 5501 5501 5501 5501 5501 5501 4001 4001 4001 4001 4001 2501 25012501 5501 5501 5501 5501 5501 5501 4001 400125012501 2501 5501 5501 5501 5501 5501 R R N

1 5501 400125012501 2501 4001 5501 5501 5501 5501 40012501 2501 250

-0.68- u,0t'-0.68-0.68-0.68

0.680.680.68

-0.68-0.68-0.68-0.68

0.68-0.68-0.68-0.68

0.68-u.bt'-0.68-0.68-0.68- u.06

2 0 0-2.00-3.00-3.00

3.00-3.00-3.00-3.00

3.00-3.00-3.00-4.50-4.50-6.00_ O . U U

-6.00-6.00-6.00-6.00-6.00-6.00-6.00

-9.00-9.00-9.00

9.00-9.00-9.00-9.00-9.00

0 2 90.290.380.290.250.230.230.240.230.260.23o.26o.270.270.220.23U . Z C

0.230.27o.240.230.260.280.360.440.4s0.520.480210.260.280.260.290.540.600.590.830.75u.5/0.620.350.280.270.360.59

0.700.560.500.59

0.900.880.940.870.941.070.950.960.99n 0 7

0.980.910.870.870.980.930.970.980 8 60.980.990.910.820.950.60o.770.500.560.980.930.900.92n o e0.540.680.700.67U / J

0.330.380.660.850.880.730.47

0.580.400.4so.32

0.910.931 . 1 40.880.900.940.970.880.940.910.880.910.910.990.940.77o.870.820.91n 7 t

0.610.880.901 .020.971 . 1 10.941 .030 9 81 .010.800.821 .O20.961 . 1 00 9 91 1 21.071 .001 .080.960.931 . 0 11 .040.981 . 1 91 . 2 11 .211 .010.97n o 70.910.99

2.052 . 1 02.402.122 . 1 11.891.992.151.951.972 . 1 02.082.152.241.972.292.212.072.142.182.022.082.322.442.312.442 . 1 02.281.882 . 1 62.242.202.282.242.462.172 3 71.932.232.242.252.O32 . 1 12.082.222.392.452.452 . 1 12.222.252 . 1 62.24

U . Y O

0.721.080.791 . 1 10.730.87

0.87

0.850.921.050.981.080.88n 0 7

0.970.81

0.850.790.941 . 1 21 1 51 . 1 50.980.940.910.860.95

2.04

1 0 F

1 .981 .962.071 .671 .95

1 .87

1 .931 .741 .861 . 6 1'1.75

1 .561 .721 .731.95

1 .801 .891.691 .601 .711 .711 R e

1 .561 .721 .591 .71

0.7970.730o 7540.7450.7900.7990.843o.8270.7430.7790.8160.7630.7480.7610.858o.8710.8480.8620.8230.9180.8930.8950.683u . o / o0.4720.4870.3550.3400.6550.7490.8110.8150.8000.2970.2560.2000.1 840.1010.0870.136o.4750 5720.6280.4440.1880.0000.0000.0000.000012'l0.2340.3950.200

Nofe: lS and OS (isomer shift and quadrupole splitting, mm/s) relative to Feo- fo. in bars..'Spectrum obtained at 77 K.

was added (in this study, 5 wto/o FerO3 was added to all startingcompositions) as spectroscopically pure FerO, with approxi-mately 100/o 57FerO, enrichment to facilitate Mdssbauer analysis.Quenched, Fe-bearing melts were formed by suspending sintered50- to 80-mg pellets of starting materials on 0.1-mm-diameterPt loops (Presnall and Brenner, 1 974) in vertical quench furnaces(Pt- and MoSir-heated). The weight ratio of sample to pt wasapproximately 100/1. Mysen and Virgo (1978), in their study ofmelts on the composition join NaAlSirOu-NaFe3*SirO6 with sim-ilar sample size and configuration, found from time studies that30 min was sufrcient time to obtain redox ratios that did notchange with additional run duration, and 30- to 90-min exper-

imental run durations were employed in the present study. Therun durations were kept this short in order to minimize possibleFe loss to the Pt loop and alkali loss to the atmosphere. Reliableelectron-microprobe analyses of the quenched gJasses could notbe obtained because of Na volatilization under the microprobebeam. In experiments with equivalent Ca- and Mg-bearing melts,however, Mysen et al. (1985a) found 5-lOVo Fe loss under theseconditions. Similar uncertainties probably apply, therefore, tothe present samples.

The oxygen fugacity in the furnace was controlled with CO-CO, gas mixing with an yttria-doped ZrO, oxygen-fugacity sen-sor (Sato, 1972) to monitor the /o,. The oxygen fugacity was

MYSEN AND VIRGO: Fe-BEARING ALUMINOSILICATE MELTS

0 . 0 0 0 0 . 0 0 0

6 1

C

o_L

ou)-o

C(tjcU)o

E.

co

P

L

(no

C(\,C

ao

E.

C

o-t-

U)-o

c((tC

aoE.

0 . 0 0 0

0 . 0 4 5

0 . 0 9 07( c J

=E 0ao) _7( r J

0 . 0 4 5

0 . 0 9 0

7( E v

no3-JE

0 . 0 7 0

0 . 1 4 0z

( o v=E n@

oE.

- 4 . 8 - 2 . 4 0 . 0 2 . 4 4 . 8V e l o c i t g , m m / s

- 4 . O - 2 . O 0 . 0 2 . 0V e l o c i t g , m m / s

4 . O

Fig. 3. 57Fe resonant-absorption Mdssbauer spectra (at 298 K) of quenched NASIVF5 melts as a function of oxygen fugacity at1250 .c.

- 4 . 0 - 2 . 0 0 . 0 2 . 0 4 0V e l o c i t g , m m / s

precise to within 0.01-O.02 log units and accurate to better than0.1 log unit as calibrated against Ni-NiO and Fe-FeO oxide buFers (Chou, 1978; Deines et al., 1974). The temperatures weremonitored with one S-type (Pt-PtroRh,o) thermocouple (cali-brated against the melting point of Au) I cm above the sampleand another S-type thermocouple inside the oxygen-fugacity sen-sor displaced approximately 1 cm horizontally from the sample.The temperatures recorded with these thermocouples differed byless than 2 "C. The experiments were terminated by quenchingon a Pt disc standing in liquid N, or quenched directly in water.The quenching rates were on the order of 500 'Cls to tempera-tures less than 1000'C resulting in glasses free of quench crystals.These quenching rates are comparable to the most rapid quench-ing rates in the rate studies by Dyar and coworkers (e.g., Dyarand Birnie, I 984; Dyar et al., I 987) where only "subtle" changesin the M

62

c

r_

a-o

07030

C(Ucaot r n

(of

aOaa


0 0 0 0

0 0 5 5 X 2 = 6 9 5 0 0 5 0PC(oCo@

o ^ ^ - ^o r u u / u

c.FAL

a

PC(I)

cotc)

E.

0 0

5 0

0 0

- 3

0 0

0 5

1 0

0 .

0

0

c 0 0 0 0

o-t-

a

{ o o s sX 2 = 3 0 5

- 3 4 - t . 7 o 0 t 7 3 4

V e l o c i t g , m m / s

Fig. 4. 57Fe resonant-absorption M


0 . 0 0 0 0.000

63

C

Ft-

a_o

c(t'CoaoE

0 0 7 5

0

(UlU

ao)m

1 5 0

3

0

- 3

c9o-Loa

c

c(oC

lno

E.

0 0 0 0

0 .0s0 N B O ̂ ^TA l . 0 3 3 4A t - S iFaJ '- = 0 7 9

X 2= 3540 . 0 3 5

0 . 0 7 0

?

C.Fo_t-oU)

C((tC

tot

=

aotr

a

U)oE.

0

2

0

3 4 - 1 7 0 0 1 f 3 4

V e l o c i t g , m m / s, 3 . 4 - 1 . 7 0 . 0 1 . 7 3 . 4

V e l o c i t g , m m / s- 3 . 4 - 1 . 7 0 . 0 1 . 1 3 - 4

V e l o c i t g , m m / s

Fig. 5. 57Fe resonant-absorption Mrissbauer spectra of NASVIIF5 (A), NASM5 (B), and NASXIIF5 (C) composition quenchedmelts equilibrated with air at 1550 'C.

what, however, from those of sodium aluminosilicateglasses in that the high-velocity component of the Fe3tdoublet shifts from about 0.5 mm/s to values near 0.8-0.9 mm/s with decreasing Fe3*/2Fe, whereas the maxi-mum value in the sodium aluminosilicate system is near0.65 mm/s for spectra obtained at 298 K. In the 77-Kspectra, the high-velocity components of the Fe2* andFe3* doublets occur at values about 0. l-0. l5 mm/s higherthan in the 298-K spectra, thus giving rise to the changesin QS and IS values shown in Table 2.

The isomer shifts and quadrupole splittings of Fe3* inthe aluminosilicate glasses are systematic functions of theFe3*/ZFe in the Fe3*/2Fe range between 0.6 and 0.3 (Fig.6). The ISo.,- increases rapidly from 0.3 mm/s for glasseswith Fe3*/)Fe > 0.6 to values between 0.5 and 0.6 mm/s with Fe3*/>Fe < 0.3. This change is associated with adecrease of QS."rr from an average near 0.9-1.0 mm/s foroxidized samples to near 0.4 mm/s with Fe3*/2Fe < 0.3.In the intermediate Fe3t/)Fe range (0.6-0.3), the ISup.and QSo"', whether from 298-K or 77-K spectra, exhibitintermediate values (Table 2, Fig. 6). These relationshipsbetween Fe3*/)Fe and hyperfine parameters are indepen-dent of temperature, oxygen fugacity, and bulk compo-sition, although they are tied to these parameters insofaras their control of the Fe3+,/)Fe itself is concerned. Sim-ilar relationships have been reported for other systemsand other temperatures, pressures, and oxygen-fugacityvalues (Mysen et al., I 984, I 985a; Virgo and Mysen, I 985;Mysen and Virgo, 1985). Virgo and Mysen (1985) wereable to insert two Fe3* doublets in some of the spectrafrom samples in the system NarO-AlrOr-SiOr-Fe-O inthis Fe3*/)Fe range. In those fits, the velocities of thecomponent peaks of the two Fe3* doublets were similarto those for the oxidized and reduced samples, respec-tively. When fitted in this fashion, the isomer-shift andquadrupole-splitting values for the two Fe3* doublets didnot change as a function of Fe3*/2Fe, but their relativeintensities were systematic functions of Fe3*/ZFe.

Structural interpretations of the spectra

The hyperfine parameters (isomer shifts and quadru-pole splitting) ofFe2* and Fe3* are sensitive to structure.In particular, the isomer shift of Fe3* (ISo.:r) and, to alesser degree, the quadrupole splitting (QS.aJ respond to

i t '+

t ,?

1 1 5 5 0 ' Ci r


the number of oxygen ligands in and the distortion of thepolyhedra.

Ferric iron. Information from crystalline silicates aidsin the interpretation ofthe spectra ofFe-bearing glasses.In crystalline iron silicates, 298-K values of ISo.r- < 0.3mm/s (relative to Feo) typically result from tetrahedrallycoordinated Fe3t, whereas values greater than about 0.5mm/s are due to octahedrally coordinated Fe3* (Anner-sten and Olesch, 1978; Seifert et al., 1979; Mysen et al.,1980; Virgo et al., 1981, 1983a; Calas and Petiau, 1983a,1983b; Waychunas and Rossman, 1983). The isomer shiftof Fe3* in oxidized glass samples (Fe3*/)Fe > 0.6) is near0.26 mm/s and 0.35 mm/s at 298 K and 77 K, respec-tively. This temperature-dependence of the IS..:, com-pares well with that observed for tetrahedrally coordi-nated Fe3* of crystalline silicates [about 5 x l0-a mm/(s.K); see Hafner and Huckenholz, l97l; Amthauer et al.,r9771.

The assignment of Fe3* to tetrahedral coordination inoxidized glasses is also supported by results from otherspectroscopic techniques. The K-edge X-ray absorptionspectrum of NarO.2SiO, glass, containing 3 wto/o FerO,equilibrated with air aI 1200 oC, was studied by Calasand Petiau (1983a, 1983b). Those investigators conclud-ed that the minimum proportion of tetrahedrally coor-dinated Fe3t was 700/0. Furthermore, Raman spectra ofFe-bearing glasses on the join NarO-SiO, (Fox et al.,1982;Virgo et al., 1983a), formed by melting in the tempera-ture range 1250-1550 'C in equilibrium with air, indi-cated that I4lFe-O bonds existed in the samples. The as-signment by Fox et al. (1982) was further supported byluminescence spectra. From this comparison of Mdss-bauer spectra of crystalline and glassy materials, as wellas data from other spectroscopic techniques, it is con-cluded that for ISp.:- < 0.3 mm/s, Fe3* is tetrahedrallycoordinated in the glass.

The QS."r- may be considered a measure of the degreeof distortion of the Fe3*-O polyhedra. For example, fromthe trends of QS.":- in crystalline ferrisilicate mineralswith tetrahedral Fe3* (e.g., Annersten, 1976; Hafner andHuckenholz, l97l; Glasser etal.,1972), it has been foundthat the quadrupole splitting of Fe3t increases from 0.0in an ideal cubic symmetry to values near 0.5-0.6 inI(Fe3*Si3O8 and about 1.5 mm/s in CaFeSi3*SirOu, wherethe FeO, tetrahedra are highly distorted. For the presentglasses, the average room-temperature (298-K) quadru-pole splitting of t41Fe3+ is 0.93 + 0.05 mm/s (31 sampleswith IS.":- < 0.3 mm/s) suggesting, therefore, significantdistortion of the polyhedron. In comparison, in theequivalent CaO-Al,O.-SiO, and MgO-Al,O.-SiO, glasssystems, the average quadrupole splittings were l.3l +0.07 mm/s (30 samples) and 1.43 + 0.06 mm/s (6 sam-ples) (Mysen et al., 1985a). This trend of increasingQSr.r. with increasing Z/r, of lhe metal cation might bethe result ofincreasing distortion ofthe Fe3"-bearing te-trahedra as the Z/r2 of the charge-balancing cations in-creases. In reduced samples (Fe3*/)Fe < 0.3), the isomer

shifts for Fe3* exceed 0.5 mm/s for 298-K spectra withvalues about 0.1 mm/s higher for 77-K spectra (Table 2,Fig. 6). The ISo":- values from spectra of reduced samplesare somewhat greater than those observed for octahe-drally coordinated Fe3+ in crystalline silicates (Annerstenet al., 19781, Hafner and Huckenholz, 197 l; Amthauer etal., 1980; Evans and Amthauer, 1980; Nolet and Burns,1979; Huegins et al., 1977), and assignments alternativeto octahedrally coordinated Fe3* might be entertained. Itis possible, for example, that these higher IS."r valuesfrom the glassy materials compared with the IS.":- fromI61Fe3+ in crystalline materials might reflect an interme-diate oxidation state ofFe where an averaged electronicstate between that of octahedrally coordinated Fe3* andoctahedrally coordinated Fe'?* might be recorded (elec-tron hopping). From crystalline oxides and silicates, twoextreme cases of electron hopping may be considered: (l)The exchange electrons arelocalized on neighboring sitesin the lattice, or (2) the electrons are delocalized over thewhole sublattice. Delocalized hopping has been observedabove the Verwey transition in FerOo spinel (Weber andHafner, l97l; Haggstrom et al., 1978) and localized elec-tron hopping in spinels and silicates (e.g., Grandjean andGerard, 1978; Annersten and Hafner, 1973; Evans andAmthauer, 1980; Coey et al., 1982). For the glasses, lo-calized electron hopping is unlikely, however, because theintensity of the Fe3* absorption is insensitive to the tem-perature at which the spectra were recorded (Table 2).Delocalized electron hopping between octahedral Fe3* andoctahedral Fe2* locations in the glass is possible. It issuggested that this process may take place in the reducedglasses, where Fe3* is in octahedral coordination. Similarconclusions might apply to the interpretation of theMdssbauer spectra of other alkali and alkaline-earth sil-icate and aluminosilicate glasses (see also Virgo and My-sen, 1985, for a more detailed description and discussionof such processes).

In the Fe3*/)Fe range between 0.6 and 0.3, the ISu.:-increases gradually with decreasing Fe3+/ZFe. Whether thischange reflects a gradual shift ofan individual doublet orwhether there is more than one Fe3* doublet in the spec-tra ofthese glasses cannot be resolved statistically. It issuggested, however, that the observed gradual changes inISp.:- and QS..r- in the Fe3+/ZFe : 0.6-0.3 range reflect agradual coordination transformation of Fe3* from tetra-hedral to octahedral coordination.

In correlative studies utilizing M0ssbauer and Ramanspectroscopy, Virgo et al. (1983b) and Mysen et al. (1985a)found that the Fe3*/2Fe-dependent changes of the hy-perfine parameters were also associated with a rapidlydiminishing intensity of Raman bands from t4lFe3+-Obonds in alkali and alkaline-earth silicate melts. In thosespectra, the intensities ofthe t4lFe3+-O stretch bands near900 and 980 cm-' diminished rapidly with decreasingFe3*/)Fe in the Fe3*/ZFe range between -0.5 and -0.3.Neither the 900- nor the 980-cm ' band could be ob-served in the Raman spectra of glasses with Fe3*/2Fe

MYSEN AND VIRGO: Fe.BEARING ALUMINOSILICATE MELTS 65

0.3 (Virgo et al., 1983b; Mysen et al., 1985a). Those datafurther substantiate, therefore, the interpretation of theMdssbauer hyperfine parameters.

From the discussion of the isomer-shift variationsabove, it is concluded that Fe3* is in tetrahedral coordi-nation in silicate glasses with Fe3t/ZFe > 0.6 and in oc-tahedral coordination with Fe3*/)Fe < 0.3. An interme-diate Fe3*/)Fe range exists (0.6-0.3) where tetrahedralFe3* and octahedral Fe3* coexist in the melts. Similarconclusions, based on similar kinds of spectroscopic data(Mdssbauer and Raman spectroscopy) were reached formelts in the systems CaO-AlrOr-SiOr-Fe-O and MgO-AlrO3-SiOr-Fe-O (Mysen et al., 1985a). This type ofstructural interpretation has also been employed to ra-tionalize viscosity and volumetric data in the systemNarO-FeO-FerO,-SiO, (Dingwell and Virgo, I 987; Ding-well et al., 1988).

The coordination transformation of Fe3* is governedby the value ofthe Fe3*/)Fe and is, therefore, also a func-tion of any variable (intensive or extensive) that affectsthe Fe3*/)Fe. Interestingly, for certain oxygen fugacities,Fe3n may undergo a coordination transformation with in-creasing temperature. For example, aI.for: l0 6 bar, meltof NASIVF5 composition [All(Al + Si) : 0.334, NBO/T= 0.6, 5 wtolo iron oxide addedl has ISo.:r : 0.21 mm/saI 1250 "C, 0.35 mm/s at 1400 'C, and 0.59 mm/s at1550 "C (Table 2). Thus, Fe3* is in fourfold coordinationat 1250 "C and, most probably, in sixfold coordinationat 1550'C. At 1400'C, the intermediate value of IS..rmay result from a mixture of tetrahedral and octahedralFe3* in the melt.

Ferrous iron. From the results ofthe least-squares fit-ting of the absorption spectra (Table 2; Figs. 3-5), thestatistical parameters often indicate an improved fit whentwo Fe2* doublets are fitted to the spectra, observationsalso made in the M0ssbauer spectra of glasses in morecomplex systems (Spiering and Seifert, 1985; Dyar et al.,1987). The values of the hyperfine parameters for a singleFe'z* doublet represent the average values ofthe two dou-blets. It is not clear, however, whether this inclusion of asecond doublet has a structural interpretation or simplyreflects an improved fit to the distribution of the hyper-fine fields. Furthermore, it might be suggested that be-cause the difference in the hyperfine-parameter values isquite small (typically less than 0.1 mm/s), the differencemight be viewed as within the uncertainty.

Whether one or two doublets are fitted, the IS."zr valuesincrease in general by l0-20o/o and the QS."z- values de-crease somewhat with decreasing Fe3*/ZFe (Table 2). Therange in values is similar to that reported for Fe2* in otherglasses (e.g., Nolet et al., 1979 Mao et al., 19731, Calasand Petiau, 1983b; Seifert et al., 1979). Most of thesevalues (with an average near 0.95 mm/s) are, however,intermediate between those attributable to Fe2* in octa-hedral and tetrahedral coordination in crystals. Only inthe completely reduced samples are the hyperfine param-eters for Fe2* wholly consistent with those of octahedrally

coordinated Fe2* in crystalline silicates. Thus, the Mdss-bauer spectra of mixed-valence samples cannot be inter-preted uniquely in regard to the Fe2* coordination state.

An assignment of the Fe2t absorption doublet(s) inglasses predominantly to octahedral coordination mightbe justified, however, with the aid of results from otherspectroscopic techniques. For example, the infrared ab-sorption spectra of Fe-bearing NarO-SiO, glasses containtwo bands, at 10000 and 5000 cm-', which Fox et al.(1982) attributed to octahedrally coordinated Fe2*. Ra-man spectra of this and other glass compositions con-taining only Fe2* (Fox et al., 1982; Virgo et al., 1983a;Mysen et al., 1985c) showed no evidence of tetrahedrallycoordinated Fe'*. Finally, optical absorption spectra ofarange of simple synthetic and complex natural glass com-positions (e.g., Nolet et al., 1979; Nolet, 1980; Mao etal., 1973; Goldman and Berg, 1980) indicate that at thevery least, only a minor proportion of Fe2* iron could bein tetrahedral coordination. Thus, it is concluded that, ingeneral, Fe2* is in octahedral coordination in silicateglasses although there might be exceptions to this rule(see, for example, Raman data of nominally FerSiOocomposition glass; Cooney et al., 1987).

Redox equilibria

Melt compositional relationships. The Fe3*/)Fe ratioof silicate melts generally increases with oxygen ion ac-tivity (Larson and Chipman, 1953;, Douglas et al., 1965).The observed positive correlation between Fe3*/)Fe andNBO/T (at NBO/T > 0.4) of the melt (Fig. 7) is in accordwith those data. with NBO/T < 0.4, the Fe3+/)Fe de-creases, however, with decreasing NBO/T (Fig. 7). Sim-ilar reversals in the Fe3*/2Fe vs. NBO/T trends at lowNBO/T were observed in the systems NarO-SiOr-Fe-O,BaO-SiO,-Fe-O, and KrO-SiOr-Fe-O (Virgo et al., 1981).Under more reducing conditions, where Fe3* is predom-inantly in octahedral coordination, the limited data sug-gest that the Fe3*/ZFe may ilcrease with decreasing NBO/T. The positive correlation between NBO/T and Fe3*/>Fe (Fig. 7) has also been observed in a number of othersimple silicate systems (e.g., Larson and Chipman, 1953;Douglas et al., 1965; Holmquist, 1966; Virgo et al., 1981,1983a; Mysen et al., 1984, 1985a).

The principal relationship can be illustrated with theexpression (Holmquist, 1966)

4 F e O ; + 4 F e 2 * + O r + 6 o ' z . ( 1 )

This equation can be integrated with anionic equilibria(Virgo et al., 1980; Furukawa et al., l98l; Mysen et al.,1982a: Matson et al.. 1983) to illustrate the interactionbetween Fe and the melt structure. In its simplest form,such an equation is

4FeO; + l2SiO, + 4Fe2* + 6Si,O3- + Or. (2)

Thus, reduction of Fe3* to Fe2* is associated with depo-lymerization of the melt.

There is no evidence in the Mdssbauer data (Table 2)


o 2

A l , / ( A l * S i ) = 0 3 3 4

l o g f o r = - 6 . 6 6

) 4 0 6 0 8 t 0 r 2 l 4

NB O/T

Fig. 7. Relationships between Fe2*/Fer* and NBO/T as afunction of temperature at.fo.: l0-o6E bar.

or in the Mdssbauer and Raman data reported by Virgoet al. (1981) for binary metal oxide-silica melts to indi-cate that the minimum Fe3*/)Fe near NBO/T : 0.a (Fig.7) is associated with a change ofthe coordination state ofFe3* in the melts. One may speculate, however, that thetransition results from increasing difficulty of Fe2* to formt6lFe2+-O polyhedra as the NBO/T decreases in these highlypolymerized melts. The rationale for this speculation isas follows. Most likely the principal depolymerized(NBO/T > 0) anionic unit in the highly polymerizedNarO-AlrO.-SiO, melts has a SirOl- stoichiometry (My-sen, 1987; Mysen et al., 1982a; Furukawa et al., l98l).The stability of SirOl- units depends, however, on thetype network-modifying cations present (Fig. 8). In meltson binary metal oxide-silicajoins, the relative abundanceof SirO! units decreases rapidly with increasing Z/r2 ofthe metal cation so that for MgO-SiO, melts, for example,Si,O3 units can no longer be identified (Mysen, 1987;Mysen et al., 1982a). This reduction in abundance ofSirOl units caused by increasing Z/12 of the metal cationin binary metal oxide-silica melts is coupled with an in-crease in abundance of SiOl (NBO/Si : 2) and SiO,(NBO/Si : 0) units in order to maintain the overall bulkmelt polymerization (Fig. 8; see also Mysen, 1987). Thus,in highly polymerized MgO-SiO, melts, the principat de-polymerized structural unit has a SiOl- stoichiometry(NBO/Si : 2). Principally similar conclusions were madeby Liebau (1981) on the basis ofsize and charge distri-bution of the network-modifying cations in crystallinesilicates. In analogy with crystal chemistry, it is suggestedthat melts on the join FeO-SiO, structurally resemblethose on the join MgO-SiOr. As can be seen from Figure8, at the Z/r2 corresponding to that of Fer*, the abun-dance of Si,Ol- units is negligible. Thus, it is possiblethat the activity coefficient of Fe2* in melts may increasewith decreasing NBO/T. Fe3+, on the other hand, formsseparate tetrahedral complexes (Virgo et al., 1983a; Virgoand Mysen, 1985), the geometry and composition ofwhichappears insensitive to NBO/T and total Fe content. Onemight infer, therefore, that 7o":- (activity coefficient ofFe3*) is insensitive to NBO/T. Thus, there might be aminimum value of NBO/T of a melt below which the

si2o52-

Bulk mellNBO/SI=1 0

1 . 5

Zlr2

0 00 5 1 5 2 . 5 t - r 3 5

ZtP Fez* Mg

Fig. 8. Relationships between molar abundance of structuralunits in melts on metal oxide-silica joins and the Z/rz of themetal cation (data from Mysen et al., 1982a).

activity coemcient ratio, yr.t-/yr.tr, is so small that theconcentration ratio, Fe3*/Fe2+, decreases with a furtherdecrease in NBO/T. This model may rationalize the ob-served reversal in the relationship between NBO/T andFe3t/2Fe.

For melts with the same NBO/T (constant degree ofpolymerization), the Fe3*/)Fe is positively correlated withA1/(Al + Si) (Fig. 9) even when the Al/(Al + Si) varia-tions do not involve changes in the degree of polymeriza-tion of the melts. Such relationships have also been foundin the systems KrO-AlrO3-SiOr-Fe-O (Dickenson andHess, 1981), CaO-AlrOr-SiOr-Fe-O (Neumann et al.,1982), and MgO-AlrO.-SiOr-Fe-O (Seifert et al., 1982b)with K*/Al3* > 1.0 and M2+/Al3+ > 0.5 (Mr* : ca andMg). This dependence is more pronounced for reduced(Fe3*/)Fe < 0.6) than for oxidized (Fe3*/)Fe > 0.6) melts.The Fe3*,/)Fe becomes more strongly dependent on Al/(Al + Si) the higher the temperature.

In order to express the interaction between the anionicaluminosilicate network and Fe3* and Fe2*, the influenceof Al3* on the melt structure must be considered. Foraluminosilicate melts in the NBO/T range of the presentstudy, the Al3t will substitute preferentially for Sia* inthree-dimensional network units relative to other struc-tural units (Mysen et al., 198 l, 1985d; Domine and Pi-

1 0si02Bulk meltNBO/S|=1 0

t ^ -' J . CMg

I

N

oL

0 8

f i v o6;6 o 4E

0.2

0.0

1 0

0 8

i u od

o 0 4E

0 2

0 8

'E o.oo

i

E

0 2

1 5 z . s ltro2+

zC

0 0

1 0

2 5 t | 3 5Fe2* Mg

MYSEN AND VIRGO: Fe-BEARING ALUMINOSILICATE MELTS o /

(a) Neo/ r - 0 ,6I =1250 "C

0 - e

;---- lo-5l 0 - 0 . 6

a

l 0 * 3

0 0 , 1 o . 2 0 3 0 4

A l l (A l+s i )

riou, 1986). This structural behavior is related to the ob-servation that Al3* will preferentially substitute for Sia*in an interconnected network with the smallest availableassociated intertetrahedral angle (Brown et al., 1969).Among the structural units in silicate melts, three-dimen-sionally interconnected units have the smallest intertetra-hedral angle (Furukawa et al., 198 l). Consequently, alonga composition join with constant NBO/T, the proportionof three-dimensional network units increases with in-creasing Al/(Al + Si) (Fie. l0).

In order to maintain the same bulk melt NBO/T, thisincrease in abundance of three-dimensional network unitsis compensated by enhanced relative abundance of de-polymerized anionic units (their individual NBO/T val-ues are greater than the NBO/T value of the bulk melt)(Fig. l0). The equilibria involving Al may be expressedwith equations such as

SiO, + (NaAD,O3 = Sio3- + 2NaAlO,, (3)

and

Si,O3- + (NaAl),o3- + 2Sio3- * 2NaAlO,. (4)

Equations 3 and 4 may be combined with the trendsin redox equilibria to yield expressions that interrelatethe structural positions of Fe3* and Fe2* with the anionicstructure of aluminosilicate melts,

7SiOz + Si'O3 + 2(NaAl)'Ol + 4FeO;= 9SiO3- + 4NaAlO, * 4Fe2* * O,, (5)

with the equilibrium constant

o _ [SiOi ]"[NaAlOJ" [Fe,-]o ,. /A\" ' -@[Fe6 , 1oJo .

t ' l

In Equation 5, the network-modifying Fe2* most likely isassociated with nonbridging oxygen in the SirOl- com-plex. The remaining nonbridging oxygen (in bothSirOl- and SirOl structural units) is bonded to network-

(a) nno/r - 0 ,6l og f oz= -0 ' 68

1 5 5 0 0 ca

a

a

I 4 0 0 0 c

modifying Na*. With increasing Al/(Al + Si), both[NaAlO,]"/[(NaAl),O3-]'z and [SiO]-l'lsi,O3- will in-crease. The proportional increase ofSiO3- equals that ofSiO, to maintain constant bulk melt NBO/T. As a result,the Fe3*/2Fe will also increase.

The Fe3*/)Fe decreases with increasing temperature asis generally the case for silicate melts. For samples equil-ibrated with air, the slope of the log (Fe'z*/Fe3*) vs. l/T(K) curves is constant for a given composition (Fig. I l),but decreases slightly with increasing All(Al + Si) at sim-ilar NBO/T and with increasing NBO/T at the same AV(Al + Si). Atfo, < l0-3 bar, the data indicate that at orabove 1400 'C, the slope of the log (Fe2*,/Fer*) vs. l/Tcurves steepens. This apparent change in slope coincideswith the Fe3*/2Fe range where the hyperfine parametersfor Fe3* (Fig. 6) were interpreted to indicate the beginningof a coordination transformation of Fe3*.

Rnnox EQUILIBRIA AND MELT PROPERTIES

Melt polymerization and redox equilibria

Melt polymeizalion, relative proportions of anionicunits, and the distribution of Al3* between these unitsdepend on the Fe3*/)Fe of Fe-bearing silicate melts (seeEqs. 5 and 6). Furthermore, the transformation of Fe3*

Na2Si2O5-NaaAl2Os BulkmeltNBO/T=1.0

:-s

t v 3

0 1 0 . 2 0 . 3 0 4Al/(Al+Si)

Fig. 10. Relationships between molar abundance of struc-tural units denoted TOr, T2O5, and TO, (T : Al + Si) as afunction ofAl/(Al + Si) along the sodium disilicate-sodium di-aluminate join (data from Mysen et al., 1985d).

4+ 't to)u - 3

TN .

a zl!

I

0 . 5

o . 4

0 , 3

o . 2

0 , 1

0 . 00

00 I 0 2 0 . 3 0 4

A l l (A l+s i )Fig. 9. Relationship between redox ratio ofFe and AV(AI + Si) as a function oftemperature and oxygen fugacity.

1 0

tr

E.!

6 Q 4E

0 2

0 0

(a) nastv

68 MYSEN AND VIRGO: Fe.BEARING ALUMINOSILICATE MELTS

+ 0o

t\N

o .u - l

- 0 6

- 0 8

- 0 9

- 1 0

- l I

- 1 2

- 0 5

- u o

- o l

- 0 . 8

- u . :

- l o

(s ) Nas Ix

t 0 - 0 6 8

5 . 0 5 4 5 8 6 2 6 6

t / r x t o 4 ( K - 1 )

(c) ruasxrr r

I 6 - 0 . 6 8

- z

5

- 1 3

Fig. I l. Temperature dependence of redox ratio of Fe as a function of bulk melt composition (A: NASIVF5, B: NASIXF5, C:NASXIIIF5) at different oxygen fugacities (as indicated in figure).

from fourfold to sixfold coordination resulting from thereduction of some of the Fe3* to Fe2* will also affect themelt structure. Two examples to demonstrate the varia-tion of NBO/T with Fe3*/)Fe are shown (Fig. l2), wherethe dashed line indicates that calculated changes of NBO/Twith Fe3* in fourfold coordination in the entire Fe3*/DFerange from 1.0 to 0.0. The changes in NBO/T are calcu-lated relative to the NBO/T values with Fe3*/)Fe : 1.0(all Fe3* in tetrahedral coordination). The solid lines rep-resent the NBO/T trajectory of NASIV melt [All(Al + Si): 0.334, Fe-free NBO/T : 0.615, 5 wt0/o FerO, (tetra-hedral Fe3*) NBO/T : 0.5281 and NASIX [A1I(AI + Si):0.334, Fe-free NBO/T:0.166, 5 wto/o FerO, (retra-hedral Fe3t) NBO/T:0. 1l2l as a function of Fe3*/2Feand t6lFe3+/>Fe3* (calculated from ISo.r. vs. Fe3*/)Fe, Fig.6). Simple depolymerization by reduction of tetrahedrallycoordinated Fer* to octahedrally coordinated Fe2* is ev-ident. The relative changes in NBO/T are greater the more

0 5 4 5 8 6 . 2 6 6

t / Tx ro1 (K -1 )

0

5 . 0 5 4 5 8 6 . 2 6 6

t / T x t 9 + ( x - t )

polymerized the melt because the d(NBO/T)/d(Fe3*/2Fe)is approximately the same in both melt systems. Thechange in slope near Fe3"/ZFe : 0.6 (Fig. 1 2) results fromthe Fe3*/EFe--controlled transformation of Fe3* from four-fold to sixfold coordination (Fig. 6):

4SiO, + t4lFeot = I6lFe3t + 2SirO3 . Q)

Thus, in addition to depolymerization resulting from areaction such as illustrated by Equation 2, further depo-lymerization results from the coordination of transfor-mation (Eq. 7), and d(NBO/T)/d(Fe*/2Fe) will increaseat the onset of this transformation.

These two depolymerization efects are, however,countered by the fact that once Fe3* is in octahedral co-ordination, its reduction to Fe2' is associated with poly-merization of the melt:

4r6tFe3+ + 2SirO3- r+ 4t5tFe2+ + 2SiO, + Or. (8)

These three effects (Eqs. 2, 7, and 8) taken togetherresult in the maxima of the depolymerization curves nearFe3t/2Fe : 0.3-0.4 in Figure 12. The principal relationsbetween Fe3*/)Fe and NBO/T (Fig. 12) can be used tocalculate the degree of polymerization of silicate melts asa function of oxygen fugacity (Fig. 13) and temperatureGig. la). Decreasingf, is systematically related to a de-crease in Fe3*/)Fe (Fig. l5), and, therefore, degree ofpo-lymerization of the melt (Fig. l3). In Figure 13, the linemarked "M" represents the NBO/T trajectory of com-position NASIV (Table l) with 5 wto/o iron oxide addedas FerO, (NASM5). The dashed lines marked "IV" and"VI" represent hypothetical NBO/T trajectories under theassumption that Fe3r was fourfold and sixfold coordinat-ed, respectively, over the entire .fo, rarrge. The shadedregions are meant to highlight the NBO/T difference be-tween "M" and the NBO/T value of the hypothetical"IV" curve in the Fe3*/2Fe range where Fe3* undergoesits Fe3*/>Fe-induced coordination transformation. TheNBO/T values of the Fe-free NASIV and NASIVFS withFe3*/)Fe : 1.0 for fourfold and sixfold coordination arealso marked.

200

\ + NAS IVF5

F e r ' ( i i ) . . \ - n a s t x r s

r" i ; ' i rvl*- "

co)(Jt-A)6-

- t n n

o00z.FeFig. 12. Percent increase of NBO/T of melt compositions

NASIVF5 and NASIXF5 relative to value calculated with Fer*/ZFe equal to 1. Dotted lines represent hypothetical change inNBO/T of the two compositions with Fe3* in tetrahedral coor-dination [Fe]{IV)l throughout the Fer*/)Fe range. See text fordetailed description of symbols.

MYSEN AND VIRGO: FC-BEARING ALUMINOSILICATE MELTS

Fer./ r Fe

0.6 0 .4 0 .2 0 .1

69

FeJ'/ zFe0.8 0 .6 0 .4 0 .2 0 .1

ocoz.

FeJ./ : Fe

0 .4 0 .2 Q .1

- 2 - 4 - 6 - 8 - 1 0

l og Ioz

The topologies of the NBO/T vs./o, curves at the threedifferent temperature are qualitatively similar in thatNBO/T increases as the/o, is lowered. Furthermore, thereis an increase in d(NBO/T)/d(Fe3*/2Fe) as Fe3* begins toundergo coordination transformation and subsequentchange in sign when nearly all remaining Fe3+ has becomea network modifier. Note, however, that the f, corre-sponding to the onset and completion of Fe3* coordina-tion transformation decreases with increasing tempera-ture. This difference stems from the temperaturedependence ofFe3*/2Fe (Fig. I l).

The All(Al + Si) and NBO/T of NASIV compositioncorrespond to those of high-alumina basalt (see Mysen,1987, for compositional ranges and structure of naturalmagmatic liquids). The NASIV differs from naturalmagma in its lack of alkaline earths. Relationships quan-titatively similar to those illustrated in Figure 13 exist,however, for the analogous compositions CASIV (CaO-Al,O3-SiOr) and MASIV (MgO-Al,O.-SiO,) (Mysen et al.,1985a). Thus, it is suggested thatfo, variations ofabouttwo orders of magnitude may be sufficient to change theNBO/T of basaltic magma containing 5 wto/o iron oxideby as much as 2Uo/o.In terms of typical magmatic liquids,this change is comparable to the difference in NBO/Tbetween tholeiite and basaltic andesite liquids (see alsoMysen, 1987). With higher iron oxide contents, the effectoff, on the degree of polymerization of basaltic magmabecomes bigger.

o - 2 - 4 - 6 - 8 - 1 0

log toz

The oxygen fugacity is, therefore, not only importantin governing redox equilibria in natural basaltic magma,but is also an important variable in controlling the overalldegree of polymerization and, therefore, all melt prop-erties that are related to this variable (e.g., liquidus phaserelations, viscous behavior, crystal-liquid trace-elementpartitioning, and mixing properties).

In Figure 14, the NBO/T trajectories ofNASIVF5 melthave been calculated as a function oftemperature at threediferent oxygen fugacities. The symbols are the same asin Figure 13. The temperature range considered corre-sponds to that of most magmatic liquids at or above theirliquidii at I bar. If the melt is equilibrated with air, re-duction of I4lFe3+ ts [0lfgz+ is the only relevant processand a l0o/o increase in NBO/T is realized by reducing theFe from 1000/0 Fe3+ to l00o/o Fe2*. Atfor: l0-3 bar, how-ever, the temperature has a much greater effect because,at temperatures slightly above 1400 "C, the Fe3+/)Fe issufficiently low (see Figs. 6 and l1) that Fe3+ begins totransform from fourfold to sixfold coordination. As a re-sult, a nearly 400/o increase in NBO/T is realized in the1200-1700 "C temperature range. At.for: 10-6 bar, thetemperature effect is somewhat less pronounced simplybecause even at 1200'C, the Fe3*/)Fe is significantly low-er than at l0 3 bar (0.64 compared with 0.85). Thus, theamount of Fe3*/)Fe reduction and, therefore, the tem-perature increase necessary to convert all Fe3* from anetwork-former to a network-modifier are smaller at /o"

o - 2 - 4 - 6 - 8 - 1 0

l og f oz

Fig. 13. Calculated change in NBO/T (as in Fig. 12) of sodium aluminosilicate melt NASIVFS as a function of oxygen fugacity.See text for detailed description ofsymbols; (VI) : octahedral coordination; (IV) : tetrahedral coordination.

70 MYSEN AND VIRGO: Fe.BEARING ALUMINOSILICATE MELTS

FeJ*/ >Fe

0 .7 0 .6 0 .5 0 .4

Iron-f ree

fo2 : lO -o os ba r M=( lV )

. . Lqi:/.'.f.e: 1,-0.,.-f91:!lYl. .

I 200 1 300 1 400 r 500 1 600 1 700

Temperature, oC Temperature, oC

Fig. 14. Calculated change in NBO/T (as in Fig. 12) of sodium aluminosilicate melt NASIVF5 as a function of temperature.See text for detailed description ofsymbols; (VI) : octahedral coordination; (IV) : tetrahedral coordination.

FeJ' / r Fe

0 .9 0 .85 0 .8 0 .7s

: 10-6 bar than atfo,: l0 3 bar. A consequence oftheresults in Figure 14 is that as a magma cools (e.g., duringascent through the mantle and crust) along its liquidus,it becomes significantly more oxidized and, therefore,more polymerized. If, in addition, the effect of pressureon Fe3*/)Fe at constant,/o, is taken into consideration(see, for example, Mo et al., 1982, and Mysen and Virgo,I 978, I 985, for relationships between Fe3*/)Fe and pres-sure), the ascent itself (decompression) will also enhanceoxidation of Fe, thus further polymerizing the liquid.

Viscosity and redox equilibria

Most properties of silicate melts and melt-mineral sys-tems depend on the degree of polymerization of the melt.

F a J . / Y s a

0 .s 0 .4 0 .3 0 .2

An example is melt viscosity. Klein et al. (1983) andmore recently Dingwell and Virgo (1987) have demon-strated that the Fe3*/)Fe ratio ofFe-bearing silicate glass-es and melts affects the melt viscosity. In particular,Dingwell and Virgo (1987), in a study where results ofMdssbauer spectroscopy of quenched NarO-SiOr-Fe-Oglasses were combined with viscometry in the super-liquidus region, demonstrated that the viscous behaviorof those liquids was closely related to the NBO/T of themelt controlled by the Fe3*/)Fe. They showed, for ex-ample (their Figs. 2 and 3), that the melt viscosity de-creased systematically as a function of decreasing Fe3*/)Fe until Fe3*/2Fe reached -0.4-0.3. With a further de-crease of Fe3*/2Fe, no further decrease in melt viscosity

0 .6tJ. o

F

oz.

2In

MYSEN AND VIRGO: Fe-BEARING ALUMINOSILICATE MELTS ' t1

F e 3 * / t F e

0 8 0 6 0 4 0 2 0 1

F e I * / t F e

0 6 0 5 0 . 4 0 3

2 6

2 4

2 2

2 0

1 . 8

6

c)

o - 2 - 4 - 6 - 8 - 1 0

log fo2

was observed. This viscous behavior was correlated withmelt depolymerization as a function of Fe3*/)Fe as t4lFe3+/t6lFe2* decreased. The absence of such a relationship forFe3*/2Fe < 0.4 was ascribed to the Fe3*/)Fe

Rhyolite,345 analyses


o - -

6L

1 0

0

30

6 2 0oL

1 0

0

Dacite, 291 analyses

0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9

FeryxFe

Andesite, 1 987 analyses

0 0 0 1 0 2 0 3 0 4 o 5 0 6 0 7 0 8 0 9 1 0

Fe3vtFe

that defined by fourfold-coordinated Fe3* (but with Fe3*/ZFe decreasing) until the Fe3* coordination change begins(ruled area). In this Fe3*/2Fe range, the viscosity trajec-tory shifts from that defined by "IV" to that defined by"VI." Finally, the trajectory coincides with that definedby a melt where Fe3* is in octahedral coordination. Amaximum viscosity difference between "R" and "M" ofslightly more than 500/o exists at temperatures near 1500"C.

Redox equilibria in natural magmatic systems

The rock file mNrsys contains slightly above 16000analyses of Cenozoic volcanic rocks (Chayes, 1975a,1975b; Chayes, unpub. document, 1985). This rock fileprovides the opportunity to extract bulk compositionalinformation based on (l) rock names used in the originalsources, (2) geographic distributions, (3) age, or (4) chem-ical discriminants. For the following application, analysesfrom selected groups of rocks were extracted, and theirreported chemical compositions were treated as if these

0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0

Fe3*DFe

0 0 0 1 0 2 o 3 0 4 0 5 0 6 0 7 0 8 0 s 1 0

FesiDFe

represented the compositions of their respective liquids.It is recognized that this approach is a significant simpli-fication as subliquidus and subsolidus processes easilycould have altered the redox ratios of Fe. The data areused, however, to point out certain systematics that seemto exist between major groups of igneous rocks. No at-tempt has been made to refine or redefine the names usedin the original sources of the nrursvs. Furthermore, as itis considered unlikely that an unaltered extrusive igneousrock will contain more than 2 wto/o HrO, analyses withmore than this amount of water have not been used. Fi-nally, this collection of analyses is by no means exhaus-tive, but it is hoped to be representative.

The Fe3*/2Fe ratios of extrusive igneous rocks rangefrom near 0 to 1.0 (Fig. l7). From the data compilationin Figure 17, it is notable, however, that the distributionin Fe3*/)Fe appears grcater for the most felsic rock groupsand in particular for rhyolite and, in general, the averageFe3*/)Fe decreases for the more mafic melt compositions(Table 3).

0 0 0 1

620

L

1 0

Fig. I 7. Distribution of Fe3*/)Fe from extrusive igneous rocks as named (database Rr(NFsys from Chayes, I 975a, I 975b, unpub.document, 1985).

Teelr 3. Percentage of analyses (rock analyses from Chayes, 1975a, 1975b, unpub. document, 1 985) where Fe3*/>Fe falls withinthe brackets indicated

RhyoliteTholeiite and olivine

Andesite tholeiite Alkali basalt Nephelinite

Tholeiite & ol rholeiire287 analyses

No of analyses 367

Fe3* /2Fe . 063+0 .25

338 2068 1010 279 116

0.6..

13 .9't7.268.9

0.48 + 0.20

23.343.133.6

0.40 + 0.07

28.949.721.5

0.29 + 0.13 0 38 + 0.19 0.43 + 0.16

23.343133.6

58.3 36.634.7 40.17.0 23.3

- Average values for rock group... With Fe3*/)Fe < 0.3, all Fe3* is considered to be in octahedral coordination, with Fe3*/>Fe > 0.6, all Fe3* is considered to be in tetrahedral

coordination, and with Fe3*/>Fe : 0.3-0.6, tetrahedral and octdhedral Fe3* coexist in the melt.

MYSEN AND VIRGO: FC.BEARING ALUMINOSILICATE MELTS I J

0 0 0 1 0 2 0 3 0 . 4 0 5 0 6 0 7 0 8 0 - 9 1 0

Fig. 18. Relative abundance oftetrahedrally coordinated Fe3+[Fe'.(IV)] and octahedrally coordinated Fe3+ [Fe3+(VI)] in majorgroups of extrusive igneous rocks shown as a function of degreeof polymerization of the melt. Values are calculated based onthe average Fe3*/)Fe (from Table 3) for the rock groups indi-cated (database RKNFsys from Chayes, 1975a, 1975b, unpub.document. 1985).

Under the assumption that the analyzed Fe3*/2Fe rep-resents the Fe3*/)Fe of these materials prior to crystalli-zation, the proportion ofFe3* in octahedral and tetrahe-dral coordination can be estimated from the Fe3*/2Fedependence of Fe3* coordination in silicate melts. Fromthe chosen selection ofanalyses in rock file nxrvrsvs, thepercentage of analyses with Fe3'/2Fe > 0.6 (all Fe3* intetrahedral coordination) and Fe3*/ZFe < 0.3 (all Fe3* inoctahedral coordination) have been plotted against thebulk melt NBO/T (Fig. l8). It is evident from the resultsin Figure l8 that the more felsic the rock (and, therefore,the more polymerized the melt), the ore prevalent is tet-rahedrally coordinated Fe3*. For rhyolite melts, for ex-ample, Fe3* is commonly in tetrahedral coordination,whereas for tholeiitic and more mafic melts, octahedrallycoordinated Fe3* is common, and Fe3* in tetrahedral co-ordination is an uncommon situation. A consequence ofthis observation is that during fractional crystallizationof basaltic liquid toward andesite or rhyolite, the residualliquids become more polymerized not only because ofincreasing silica (and, perhaps, alumina) content. Theproc€ss also appears to be associated with increasing Fe3*/)Fe. This increase is so large that a significant fraction ofthe Fe3* undergoes a coordination change and becomes anetwork former, thus further polymerizing the liquids.

Sack et al. (1980) pioneered the use of laboratory-cal-ibrated Fer*/ZFe as a function of temperature and oxygenfugacity as a calibrant for oxygen fugacity during forma-tion of igneous rocks. These investigators employed anexpression ofthe form

ln(Xo,.or/ Xo,o) -- a (10)

where a, b, c, and d, are regression coefficients, ?n is ab-solute temperature, ln /o, is the natural logarithm of theoxygen fugacity, and X, is the concentration of the lthoxide component. By fitting 57 analyses from experimen-

30

b ' "I

A1 0

0

460 analyses

-" -'o -"o'"1,'l'l"ll'"1'""itl 30 40 50

Fig. 19. Distribution of deviation of calculated oxygen fu-gacities (from Eq. I l) relative to experimental values (databaseas in Table 4).

tally equilibrated liquids to this expression, Sack et al.(1980) found positive correlation of ln(Xr"ror/Xr.o) withX^^ro, Xrro, and X..o, whereas X*"o, Xorror, and X."o werenegatively correlated. In a subsequent refinement of thistreatment, Kilinc et al. (1983) concluded that the Fe3*/Fe2* ratio depended only on Xcuo, XN^zo, X*ro, Xo.o (allpositively correlated), and Xo,,o, (negatively correlated)'Kilinc et al. (1983) concluded that Xr"ror/Xrn was inde-pendent of Xrro content of the liquid. X.* was not iden-tified as a variable in the experimental results reportedby Thornber et al. (1980).

Mysen (l 987) suggested that the approach advanced bySack et al. (1980) could be reflned and linear regressioncould be applied to a rock composition after it has beenrecast to the relevant structural components. The proce-

dure used to calculate the relevant structural componentswas described by Mysen (1987) and will not be repeatedhere. The structural components are those found to gov-

ern Fe3*/)Fe in binary and ternary systems. An expres-sion of the form

I04hl n ( X e " : . / X 7 " t ) : c t + , + c l n f o .

/ v \ / w \+ dl - !i!-l * ' l

^"' )

\,ro, + X.,/ '\Xr", + X.,7

. >r/ryP), (u)j , \ r I '

TABLE 4. Regression coefficients for Equation 11

Coefiicient Standard error

C L No -ob s o(L

'10

0

h : _+7+c ln f .+

4Or .

a (constant)b ( 1 t Dc (ln fo.)d [AP./(AF. + Si)]e [Fes*/(Fe3- + Si)l

| (NBO/T)Mo

f I (NBo/rF"' I (NBO/T)N"|. (Neo/T)'".

15.435-2.848-0.3484

1.309-2.121

0.6662-0.5255-1. ' t25-3.215

0.7860.1380.1 200.4691.0550.09660.1 0840.1790.s38

Note.' Numbers of analyses in regression : 460. Experimental data fromKennedy (1948), Fudali (1965), Sack et al. (1980), Thornber et al. (1980),Kitinc et al {1983), Seifert et al. (1979), Virgo et al (1981), Mysen andvirgo (1983), virgo and Mysen (1985), Mysen et al (1980, 1984, 1985a,1 985b, 1 985c, 1 985d).


can be describe the relationship between Fe2+/Fe3+, tem-perature, oxygen fugacity, and melt structure. The/ and(NBO/T)j are the regression coefficients and NBO/T val-ues of the structural units associated with individual net-work-modifying cations (7), respectively. The coefficientsa, b, c, d, and e together with / are obtained with stepwiselinear regression. Equation I I takes into account each ofthe variables identified as affecting Fe3*/DFe.

The resulting coefficients are shown in Table 4. It isevident from this exercise that among the network-mod-ifying cations, the ln(X."2-lX..rr) is negatively correlatedwith the proportion of nonbridging oxygen associated withCa2*, Nat, and Fe2* (Table 4). The analysis shows thatFe2+/Fe3+ increases with increasing NBO/T associated withMg'zt. It is not clear why Mg2* is an exception among thenetwork-modifying cations in this respect. There is alsoa rapid decrease in ln(Xr.z-/Xr",-) with increasing X."r-l(X.a- + X'o-) and with increasitg Xor/(Xo, + Xr,).

The coefficients in Table 4 may be inserted in Equation1 I to be used as an oxygen-fugacity barometer. The cal-culatedf, values for the samples in the dataset are com-pared with the measured values in Figure 19. Between 85and 900/o of the calculated values are within + 1.0 log unitof the measured value, and 54o/o of the analyses are within+0.5 log unit of oxygen fugacity. About 950/o fall within+ 1.5 log unit. It is suggested that this model relating re-dox ratios of Fe to temperature and melt structure is anadequate description and that Equation I l, with the coef-ficients in Table 4, can be used with confidence to cal-culate oxygen-fugacity conditions of natural magmaticliquids at I bar.

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