Experimental study of phase equilibria in the PbO-MgO-SiO 2 system

Experimental Study of Phase Equilibria in the PbO-MgO-SiO2 System S. CHEN, B. ZHAO, E. JAK, and P. C. HAYES An experimental study of the PbO-MgO-SiO2 system has been carried out using high-temperature equilibration and quenching techniques, followed by electron probe X-ray microanalysis (EPMA). The phase equilibria were determined in the temperature range from 973 to 1673K. Nine primary phase fields have been investigated in this system, including three monoxides (PbO, MgO, and SiO2), five binary compounds (MgSiO3, Mg2SiO4, PbSiO3, Pb2SiO4, and Pb4SiO6), and one ternary compound Pb8Mg(Si2O7)3. Two other ternary compounds, PbMgSiO4 and PbMgSi3O8, were observed in some experiments; however, further experiments indicated that these two compounds are unstable in the temperature range investigated.

I. INTRODUCTION

INTEREST in the phase equilibria of the PbO-MgOSiO2 system stems from two principal sources. Magnesia refractories are used commercially to contain lead smelting slags during high-temperature processing. Magnesium is also a common impurity element found in the primary smelting of lead/zinc ores. Characterization of the phase equilibria in the PbO-MgO-SiO2 system is therefore useful in prediction and interpretation of phases formed in these reaction systems. The MgO-SiO2 system has been investigated by previous researchers.[1,2,3] The principal features of the MgO-SiO2 system include a region of liquid immiscibility on the silicarich side, a congruently melting compound forsterite, Mg2SiO4, and an incongruently melting compound protoenstatite, MgSiO3, which form binary eutectic liquid ⫹ MgO ⫹ Mg2SiO4 and peritectic liquid ⫹ Mg2SiO4 ⫹ MgSiO3, respectively. Wu et al.[4] have reported the thermodynamically calculated and optimized MgO-SiO2 phase diagram based on all available experimental results. The PbO-SiO2 system was systematically investigated by Smart and Glasser,[5] who reported compounds with PbO:SiO2 mole ratios of 4:1, 3:1, 2:1, 1:1, and 5:8. Subsequently, however, only the existence of the compounds Pb4SiO6, PbSiO3, and Pb2SiO4 have been confirmed by other researchers.[6–9] Jak et al.[10] have summarized the available experimental results and reported the thermodynamically optimized PbO-SiO2 system. No data were found in the literature for the MgO-PbO system. Argyle and Hummel[11] investigated the PbO-MgO-SiO2 system as the part of a study of the PbO-BaO-MgO-SiO2 system. Their investigations were concentrated on the subsolidus region, but some liquid was reported to form at temperatures below 1173 K. The ternary compound Pb2MgSi2O7 was reported to melt congruently at 1105 K, S. CHEN, Associate Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, the People’s Republic of China. B. ZHAO, Postdoctoral Research Fellow, E. JAK, Senior Research Fellow, and P.C. HAYES, Associate Professor, are with the Department of Mining, Minerals and Materials Engineering, The University of Queensland, St. Lucia, Queensland, 4072, Australia. Manuscript submitted July 25, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B

based on results of quenching and differential thermal analysis experiments. The ternary compound Pb2MgSi2O7 was reported to form extensive solid solutions with Pb2SiO4, PbSiO3, MgSiO3, Mg2SiO4, and SiO2. However, the method used to measure the solid solution and supporting details was not given in the article.[11] In a later study, Billhardt[12] pointed out from X-ray powder diffraction (XRD) study that this ternary compound Pb2MgSi2O7 belongs to the Pbbarysilite group, and the formula of this ternary compound should be Pb8Mg(Si2O7)3. Sugimoto and Kokusa[13] measured the activity of PbO in molten PbO-MgO-SiO2 slag at 1273 K. The objective of the present investigation is to characterize the liquidus and the phase relationships in the PbO-MgOSiO2 system. The research work has focused mainly on the low-MgO region of the system.

II. EXPERIMENTAL PROCEDURE The experimental procedures used in the present study are similar to those reported in a number of previous publications.[10,14–17] In brief, in order to reduce the vaporization of PbO during equilibration at high temperatures, lead silicate master slag was first prepared from pure oxide powders of 99.9 pct ⫹ purity. The master slag, with the mole ratio of PbO:SiO2 equal to 1:1, was heated at 1073 K and cooled rapidly in air. The final mixtures were prepared by mixing the crushed master slag with pure PbO, SiO2, and MgO powders in the desired proportions in an agate mortar. A pellet of the final mixture (0.5 g) was placed in a platinum crucible covered with a lid of platinum foil. The crucible was positioned into a muffle furnace, in air, close to a controlling thermocouple Pt/Pt-13 pct Rh. The temperature of the furnace was controlled within ⫾2 K. The controlling thermocouple was periodically tested with a calibrated standard thermocouple. The temperature accuracy was estimated to be within ⫾5 K. The weight of the sample was monitored before and after the experiment. Each experiment was usually carried out in two stages. The first stage was to premelt at a temperature higher than that of final equilibration for 10 to 20 minutes. In the second stage, the furnace temperature was lowered and the sample was equilibrated at the desired final equilibration temperature. The equilibrated VOLUME 32B, FEBRUARY 2001—11

sample was quenched into iced water, dried and weighed, and then mounted and polished for further analysis. Microstructural analysis of the samples was carried out using optical microscopy and scanning electron microscopy. The compositions of the phases were measured using a JEOL* 8800L electron probe X-ray microanalyzer with an *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

accelerating voltage of 15 kV and a probe current of 15 nA. The Duncumb–Philibert ZAF correction procedure supplied with the JEOL 8800L was applied. A lead-silicate glass standard* (Pb ⫽ 65.67 ⫾ 0.26, Si ⫽ *Office of Standard Reference Materials, National Institute of Standards and Technology, Gaithersburg, MD.

13.37 ⫾ 0.24, and O ⫽ 20.35 wt pct) was used for Pb, a wollastonite (CaSiO3) standard** (Ca ⫽ 34.50, Si ⫽ 24.18 **Charles M. Tayor Co., Stanford, CA.

Fig. 1—Backscattered electron image of the sample with 14.3MgO, 23.6PbO, and 62.2SiO2 mole pct equilibrated at 1173 K for 13 h illustrating the univariant equilibrium (liquid ⫹ MgSiO3 ⫹ SiO2) in the PbO-MgOSiO2 system (G ⫽ glass (liquid), MS ⫽ protoenstatite MgSiO3, and S ⫽ tridymite Si02).

and O ⫽ 41.32 wt pct) was used for Si, and a MgO standard (Mg ⫽ 60.31 and O ⫽ 39.69 wt pct) was used for Mg. The average accuracy of measurement is estimated to be within ⫾1 wt pct. During equilibration, lead oxide can vaporize at high temperatures, which changes the bulk composition of the mixture. However, since the phase compositions were measured after rather than before the experiments, any changes to the bulk mixture composition during equilibration did not affect the results. X-ray powder diffraction analysis was used in some cases to confirm phase identification. The XRD measurement was carried out with a PHILIPS* PW1130 X-ray diffractometer *PHILIPS is a trademark of Philips Electronic Instruments, Mahwah, NJ.

with a graphite monochromator using Cu K␣ radiation. III. RESULTS AND DISCUSSION Experiments were carried out in most of the primary phase fields in the PbO-MgO-SiO2 system from 973 to 1673 K. Some typical microstructures observed in the equilibrated and quenched samples are presented in Figures 1 through 4. Figures 1 through 3 show the following univariant equilibria between liquid and two solid compounds: liquid ⫹ SiO2 ⫹ MgSiO3, liquid ⫹ Mg2SiO4 ⫹ MgSiO3, and liquid ⫹ Mg2SiO4 ⫹ Pb8Mg(Si2O7)3, respectively. Figure 4 shows the microstructure of the subsolidus univariant equilibrium between Pb4SiO6, Pb2SiO4, and Pb8Mg(Si2O7)3. The experimental results obtained for the equilibria in the PbO-MgO-SiO2 system in air are given in Table I. Using the information on the chemical compositions of the phases, the liquidus surface has been constructed and is shown in Figure 5. Figure 6 shows an enlargement of the lead-rich region of the phase diagram. In some cases, experiments were carried out with different starting compositions and different equilibration time, but with the same final equilibration temperatures. These experiments give the same final phase compositions and provide additional evidence and confidence in the final results reported. Nine primary phase fields have been established in the PbO-MgO-SiO2 system: three mono-oxides (PbO, MgO, and 12—VOLUME 32B, FEBRUARY 2001

Fig. 2—Backscattered electron image of the sample with 24.4MgO, 26.5PbO, and 49.1SiO2 mole pct equilibrated at 1373 K for 2 days illustrating the univariant equilibrium (liquid ⫹ Mg2SiO4 ⫹ MgSiO3) in the PbOMgO-SiO2 system (G ⫽ glass (liquid), M2S ⫽ forsterite Mg2SiO4, and MS ⫽ protoenstatite MgSiO3).

Fig. 3—Backscattered electron image of the sample with MgO 4.9, PbO 65.1, and SiO2 30 mole pct equilibrated at 1073 K for 21 h illustrating the univariant equilibrium (liquid ⫹ Pb8Mg(Si2O7)3 ⫹ Mg2SiO4) in the PbOMgO-SiO2 system (G ⫽ glass (liquid), P8MS6 ⫽ Mg-Pb barysilite Pb8Mg(Si2O7)3, and M2S ⫽ forsterite Mg2SiO4). METALLURGICAL AND MATERIALS TRANSACTIONS B

Table I. Liquidus Compositions Determined from Experiments on the PbO-MgO-SiO2 System in Air

Temperature (K)

Fig. 4—Backscattered electron image of the sample with 4.9MgO, 65.1 PbO, and 30SiO2 mole pct equilibrated at 973 K for 12 days illustrating the univariant equilibrium (Pb8Mg(Si2O7)3 ⫹ Pb4SiO6 ⫹ Pb2SiO4) in the PbO-MgO-SiO2 system (P4S ⫽ tetralead Pb4SiO6, P2S ⫽ lead orthosilicate Pb2SiO4, and P8MS6 ⫽ Mg-Pb barysilite Pb8Mg(Si2O7)3).

SiO2), five binary compounds (Mg2SiO4, MgSiO3, PbSiO3, Pb2SiO4, and Pb4SiO6), and one incongruently melting ternary compound Pb8Mg(Si2O7)3. Data on the binary systems PbO-SiO2 and MgO-SiO2 have been taken from Jak et al.[10] and Wu et al.,[4] respectively. The MgO-SiO2 system has a congruently melting binary compound Mg2SiO4 and an incongruently melting compound MgSiO3, which form the binary eutectic liquid ⫹ Mg2SiO4 ⫹ MgO and the peritectic liquid ⫹ MgSiO3 ⫹ Mg2SiO4, respectively. These phase relations extend into the ternary region and form the primary phase fields of MgO, Mg2SiO4, MgSiO3, and SiO2, which dominate this ternary phase diagram. The Mg2SiO4 primary phase field extends almost to the PbO-SiO2 binary and the liquidus rises rapidly with increasing MgO. Electron probe X-ray microanalysis (EPMA) measurements show that there is no solubility of PbO in the Mg2SiO4 or MgSiO3 phases. Likewise there is no solubility of MgO in Pb2SiO4 and PbSiO3. Neither the olivine nor pyroxene phase fields therefore extend over the entire width of the diagram. In the low-MgO region, the primary phase field of Pb8Mg(Si2O7)3 is adjacent by the primary phase fields of Mg2SiO4 and MgSiO3, as well as PbSiO3, Pb2SiO4, and Pb4SiO6. From the experimental results in the low-MgO region (Table I), it has been deduced that Pb8Mg(Si2O7)3 ⫹ SiO2 ⫹ MgSiO3, Pb8Mg(Si2O7)3 ⫹ Mg2SiO4 ⫹ MgSiO3, Pb8Mg(Si2O7)3 ⫹ Pb4SiO6 ⫹ Mg2SiO4, and Mg2SiO4 ⫹ PbO ⫹ Pb4SiO6 form the ternary peritectic points A, B, C, and D, respectively, while Pb8Mg(Si2O7)3 ⫹ Pb2SiO4 ⫹ Pb4SiO6, Pb8Mg(Si2O7)3 ⫹ PbSiO3 ⫹ Pb2SiO4, and Pb8Mg(Si2O7)3 ⫹ PbSiO3 ⫹ SiO2 form ternary eutectic points E, F, and G, respectively (shown in Figure 6 and Table III). It has not been possible in the present study to determine the exact temperatures and compositions of these invariant points. These points are in the temperature ranges presented in Table III. The ternary compound Pb2MgSi2O7 was reported by Argyle and Hummel.[11] However, based on XRD and infrared spectra studies, Billhardt[12] has pointed out that this compound should have the chemical formula Pb8Mg(Si2O7)3 METALLURGICAL AND MATERIALS TRANSACTIONS B

Phases in equilibrium

Liquidus Compositions (Mole Pct) MgO

Bivariant Equilibria Tridymite primary phase field 1573 L⫹S 5.7 1473 L⫹S 6.5 1473 L⫹S 9.8 1273 L⫹S 6.7 1173 L⫹S 5.4 Protoenstatite primary phase field 1573 L ⫹ MS 18.5 1473 L ⫹ MS 13.1 1373 L ⫹ MS 9.2 1273 L ⫹ MS 8.0 1173 L ⫹ MS 5.4 1073 L ⫹ MS 3.8 Forsterite primary phase field 24.1 1673 L ⫹ M2S 19.7 1673 L ⫹ M2S 15.1 1673 L ⫹ M2S 1573 L ⫹ M2S 19.8 1573 L ⫹ M2S 17.2 1573 L ⫹ M2S 16.9 1573 L ⫹ M2S 11.7 1573 L ⫹ M2S 3.6 1523 L ⫹ M2S 10.3 1523 L ⫹ M2S 11.0 1473 L ⫹ M2S 14.9 14.8 1473 L ⫹ M2S 1473 L ⫹ M2S 12.2 1473 L ⫹ M2S 4.8 1473 L ⫹ M2S 3.2 1373 L ⫹ M2S 10.9 1373 L ⫹ M2S 11.2 1373 L ⫹ M2S 8.8 1373 L ⫹ M2S 8.9 1373 L ⫹ M2S 6.4 1373 L ⫹ M2S 10.1 4.4 1373 L ⫹ M2S 1373 L ⫹ M2S 3.9 1273 L ⫹ M2S 8.1 1273 L ⫹ M2S 8.1 1273 L ⫹ M2S 6.3 1273 L ⫹ M2S 3.4 1273 L ⫹ M2S 2.1 1273 L ⫹ M2S 2.0 1273 L ⫹ M2S 1.8 1173 L ⫹ M2S 5.0 5.2 1173 L ⫹ M2S 1173 L ⫹ M2S 4.8 1173 L ⫹ M2S 4.6 1173 L ⫹ M2S 4.3 1173 L ⫹ M2S 1.4 1123 L ⫹ M2S 4.5 1123 L ⫹ M2S 4.5 Mg-Pb barysilite phase field 1023 L ⫹ P8MS6 3.9 3.9 1023 L ⫹ P8MS6 Univariant Equilibria 1373 L ⫹ MS ⫹ S 9.4 1273 L ⫹ MS ⫹ S 7.2 1273 L ⫹ MS ⫹ S 7.0 1173 L ⫹ MS ⫹ S 5.0

PbO

SiO2

27.4 29.4 45.4 30.3 32.7

66.9 64.1 44.8 63.0 61.9

20.6 23.6 27.3 31.9 34.3 35.5

60.9 63.3 63.5 60.1 60.3 60.7

26.0 36.1 48.1 26.8 31.7 37.2 49.8 75.2 44.1 44.7 29.5 31.3 40.1 62.1 76.2 30.4 30.4 45.3 45.5 52.5 53.2 61.8 70.8 33.1 38.4 50.6 68.6 76.3 78.1 79.9 46.4 48.7 49.4 51.4 52.0 77.3 48.7 49.0

49.9 44.2 38.8 53.4 51.1 45.9 38.5 21.2 45.6 44.3 55.6 53.9 47.7 33.1 20.8 58.7 58.4 45.9 45.6 41.1 36.7 33.8 25.3 58.8 53.5 43.1 28.0 21.6 19.9 18.3 48.6 46.1 45.8 44.0 43.7 21.3 46.8 46.5

38.5 38.8

57.6 57.3

26.9 30.2 30.6 33.1

63.7 62.6 62.4 61.9

VOLUME 32B, FEBRUARY 2001—13

Table I. Continued

Temperature (K) 1073 1023 1373 1373 1373 1073 1023 973 1073 1073 1073 1023 1003 973 973 973 973 973 973 973

Phases in equilibrium

Liquidus Compositions (Mole Pct) MgO

Univariant Equilibria L ⫹ MS ⫹ S 4.0 L ⫹ MS ⫹ S 3.5 L ⫹ M2S ⫹ MS 10.5 L ⫹ M2S ⫹ MS 10.2 L ⫹ M2S ⫹ MS 10.6 L ⫹ M2S ⫹ MS 4.8 3.7 L ⫹ MS ⫹ P8MS6 L ⫹ S ⫹ P8MS6 2.1 L ⫹ M2S ⫹ P8MS6 3.9 L ⫹ M2S ⫹ P8MS6 4.3 L ⫹ M2S ⫹ P8MS6 1.8 L ⫹ M2S ⫹ P8MS6 0.9 L ⫹ M2S ⫹ P8MS6 0.7 Subsolidus Phase Equilibria P4S ⫹ M2S ⫹ P8MS6 — — P4S ⫹ M2S ⫹ P8MS6 PS ⫹ M2S ⫹ P8MS6 — P4S ⫹ P2S ⫹ P8MS6 — P4S ⫹ M2S ⫹ P8MS6 — MS ⫹ S ⫹ P8MS6 — MS ⫹ S ⫹ P8MS6 —

PbO

SiO2

34.8 36.6 32.3 32.9 34.4 37.7 38.8 37.5 44.2 44.5 63.3 74.1 75.3

61.2 59.9 57.2 56.9 54.9 57.5 57.5 60.4 51.9 51.2 34.9 25.0 24.0

— — — — — — —

— — — — — — —

L ⫽ liquid; P ⫽ PbO, S ⫽ SiO2, PS ⫽ alamosite PbSiO3, P2S ⫽ lead orthosilicate Pb2SiO4, P4S ⫽ tetralead silicate Pb4SiO6, MS ⫽ protoenstatite MgSiO3, M2S ⫽ forsterite Mg2SiO4, and P8MS6 ⫽ Mg-Pb barysilite Pb8Mg(Si2O7)3.

rather than Pb2MgSi2O7. In the present study, it was confirmed from EPMA and XRD measurements that Pb8Mg(Si2O7)3 is the stable ternary compound in this system. From more than 100 EPMA measurements in 20 samples, the composition of this ternary compound is determined to be 6.7 ⫾ 0.2 MgO, 53.3 ⫾ 0.5 PbO, and 40 ⫾ 0.5 SiO2 in mole pct (Table II). No solid solution of Pb8Mg(Si2O7)3 was found in the temperature range from 973 to 1073 K. It can be seen from Figure 5 that Pb8Mg (Si2O7)3 has an incongruently melting point between 1073 and 1123 K. The conclusions of the present study validate the view of Billhardt,[12] that “Pb2MgSi2O7” belongs in fact to the Pb-barysilite group having the formula Pb8Mg(Si2O7)3. Two other ternary compounds PbMgSi3O8 and PbMgSiO4 have been observed in some samples quenched from 973 K (shown in Figures 7 and 8). However, it has been found that these compounds are unstable at this temperature because they all disappeared if the samples were kept at the same temperature for a longer time. For example, a mixture having bulk composition of 35PbO, 10MgO, and 55SiO2 in mole pct was first premelted at 1473 K for 10 minutes and then finally equilibrated at 973 K for 17 days. PbMgSi3O8 was observed in the quenched sample (Figure 8). This compound, however, disappeared after the sample was kept in the furnace at 973 K for an additional 24 days. There is similarity between the systems PbO-CaO-SiO2[14] and PbO-MgO-SiO2, since the melting points of CaO and MgO are 3172 and 3099 K, respectively,[18] and they are located in the alkaline earth group of the chemical periodic table. In these two systems, the primary phase fields of the binary compounds MgSiO3 and Mg2SiO4, and CaSiO3 and

Fig. 5—The phase diagram on the PbO-MgO-SiO2 system in air based on the results of the present investigation. 14—VOLUME 32B, FEBRUARY 2001

METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 6—Lead oxide-rich region of the PbO-MgO-SiO2 system based on the results of the present investigation.

Table II. Compositions of Solid Phases Measured by EPMA Compositions (Mole Pct) Phase Name

Ideal Formula

Tridymite Magnesia Lead oxide Alamosite Lead orthosilicate Tetralead silicate Protoenstatite Forsterite Mg-Pb barysilite

SiO2 MgO PbO PbSiO3 Pb2SiO4 Pb4SiO6 MgSiO3 Mg2SiO4 Pb8Mg(Si2O7)3 PbMgSiO4 PbMgSi3O8

MgO 0 0 0 0 0 0 50.0 ⫾ 66.6 ⫾ 6.7 ⫾ 33.4 ⫾ 20.0 ⫾

PbO ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ 0 0 53.3 ⫾ 33.3 ⫾ 20.0 ⫾ 0.1 0.1 99.9 50.0 66.7 80.0

0.1 0.1 0.2 0.2 0.1

SiO2

0.1 0.1 0.2 0.1 0.1 0.4 0.5 0.4 0.2

99.9 99.9 0.1 50.0 33.3 20.0 50.0 33.4 40.0 33.3 60.0

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 0.3 0.1 0.1 0.1 0.2 0.1 0.1 0.5 0.2 0.4

Table III. The Temperature of Ternary Invariant Points in the PbO-MgO-SiO2 System Phases in Equilibrium Liquid-Pb8Mg(Si2O7)3-SiO2-MgSiO3 Liquid-Pb8Mg(Si2O7)3-Mg2SiO4-MgSiO3 Liquid-Pb8Mg(Si2O7)3-Pb4SiO6-Mg2SiO4 Liquid-Mg2SiO4-PbO-Pb4SiO6 Liquid-Pb8Mg(Si2O7)3-Pb2SiO4-Pb4SiO6 Liquid-Pb8Mg(Si2O7)3-PbSiO3-Pb2SiO4 Liquid-Pb8Mg(Si2O7)3-PbSiO3-SiO2

Temperature Invariant (K) Points 973 1023 973 973 973 923 923

to to to to to to to

1023 1073 1003 997 990 997 1002

A B C D E F G

Ca2SiO4, dominate the ternary phase diagrams, respectively. However, CaSiO3 is a congruent melting compound and MgSiO3 is an incongruent melting compound; therefore, the relative size of the primary phase field of CaSiO3 is larger than that of MgSiO3. Ternary compounds Pb8Ca(Si2O7)3 and Pb8Mg(Si2O7)3 belong to the barysilite group. However, a METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 7—Backscattered electron image of the sample illustrating the nonequilibrium phase assemblage (Pb8Mg(Si2O7)3 ⫹ SiO2 ⫹ PbMgSiO4) in the PbO-MgO-SiO2 system at 973 K (S ⫽ SiO2, P8MS6 ⫽ Mg-Pb barysilite Pb8Mg(Si2O7)3, and PMS ⫽ PbMgSiO4). VOLUME 32B, FEBRUARY 2001—15

no solid solution of PbO in the binary compounds of MgO and SiO2, nor MgO in the binary compounds of PbO and SiO2. ACKNOWLEDGMENTS The authors thank the Australian Research Council through the ARC-SPIRT scheme for providing the financial support to enable this research to be carried out. REFERENCES

Fig. 8—Backscattered electron image of the sample illustrating the nonequilibrium phase assemblage (liquid ⫹ PbMgSi3O8) in the PbO-MgOSiO2 system at 973 K (G ⫽ glass (liquid), PMS3 ⫽ PbMgSi3O8).

difference between the PbO-CaO-SiO2 and PbO-MgO-SiO2 systems is that Ca2PbSi3O9 and Ca2Pb3Si3O11 are stable ternary compounds in the CaO-PbO-SiO2 system, while in the MgO-PbO-Si02 system, the corresponding magnesiumcontaining compounds are not stable in the temperature range investigated. In the PbO-CaO-SiO2 system, a significant amount of Pb2SiO4 dissolves into Ca2SiO4. In contrast, in the PbO-MgO-SiO2 system, no Pb2SiO4 dissolves into Mg2SiO4. IV. CONCLUSIONS Phase equilibria in the PbO-MgO-SiO2 system were experimentally determined in the temperature range from 973 to 1673 K. The results enabled the primary phase fields and the liquidus surface in this system to be defined. Three ternary phases have been observed in this system: Pb8Mg(Si2O7)3, PbMgSiO4, and PbMgSi3O8, but the ranges of stability of the latter two were not determined. There is

16—VOLUME 32B, FEBRUARY 2001

1. J.W. Greig: J. Am. Sci., 1927, 5th ser. vol. 13, pp. 1-154. 2. N.L. Bowen and O. Andersen: Am. J. Sci., 1914, vol. 37, pp. 487-500. 3. C.M. Schlaudt and D.M. Roy: J. Am. Ceram. Soc., 1965, vol. 48, pp. 248-51. 4. P. Wu, G. Eriksson, A.D. Pelton, and M. Blander: Iron Steel Inst. Jpn. J., 1993, vol. 33, pp. 25-34. 5. R.M. Smart and F.P. Glasser: J. Am. Ceram. Soc., 1974, vol. 57, pp. 378-82. 6. R.F. Geller, A.S. Creamer, and E.N. Bunting: Research Paper No. RP705, National Bureau of Standards, Gaithersburg, MD, 1934, vol. 13, pp. 237-44. 7. U. Kuxmann and P. Fischer: Erzmetallurgy, 1974, vol. 27, pp. 533-37. 8. P.D. Calvert and R.R. Shaw: J. Am. Ceram. Soc., 1970, vol. 53, pp. 350-52. 9. K.A. Krakau, E.J. Mukhin, and M.S. Geinrich: Fiziko-Khimicheskie Svoistva Troinoi sisteyemy: Okis Natriya-Okis Svintsa-kremnezem, Grebenshchikov, Moscow, 1949, pp. 15-38. 10. E. Jak, S. Degterov, P. Wu, P.C. Hayes, and A.D. Pelton: Metall. Mater. Trans. B, 1997, vol. 28B, pp. 1011-18. 11. J.F. Argyle and F.A. Hummel: Glass Industry, 1965, Dec., pp. 710-18. 12. H.W. Billhardt: Am. Mineral., 1969, vol 54, pp. 510-21. 13. E. Sugimoto and Z. Kosuka: Trans. Jpn. Inst. Met., 1978, vol. 19, pp. 275-80. 14. E. Jak, N. Liu, and P.C. Hayes: Metall. Mater. Trans. B, 1998, vol. 29B, pp. 541-53. 15. E. Jak, S. Degterov, P.C. Hayes, and A.D. Pelton: Can. Metall. Q., 1998, vol. 37, pp. 41-47. 16. E. Jak, H.G. Lee, and P.C. Hayes: Kor. IMMJ, 1995, vol. 1, pp. 1-8. 17. E. Jak, S. Degterov, B. Zhao, A.D. Pelton, and P.C. Hayes: Proc. Zinc and Lead Processing Symp., Calgary, CIM, Montreal, 1998, pp. 313-33. 18. D.R. Lide: CRC Handbook of Chemistry and Physics, 81st ed., CRC Press, Boca Raton, FL, 2000, pp. 4-50 and 70.

METALLURGICAL AND MATERIALS TRANSACTIONS B