T H E TERNARY SYSTEM, KzSi03-Na~SiOs-Si0~ BY F. C . KRACEK
The equilibrium phase relations in the ternary system of silica with the two important alkali oxides, NazO and KzO, have not been previously subjected to a systematic study. The knowledge of the phase relations in this system has a bearing on petrology, since the two alkali oxides, NazO and KzO, are important constituents of the common silicate minerals occurring in rocks; the system is also of some interest in glass technology, the three components of the system entering in varying amounts into the composition of commercial glasses. From another point of view the phase relations in this system are of theoretical interest especially in that the results of the present work establish a unique type of solid solution sequence between the two disilicates, KzSiz06 and NazSiz05. The component-binary systems, KzSiOa-Si02 and NazSiOs-SiOz, have already been published from this Laboratory.lJ*a The information given in the papers quoted must be considered together with the additional data published in this paper in arriving a t the correct representation of the phase relations in the ternary system. The reader is referred to previous papers from this Laboratory for descriptions of the methods, apparatus, and technic employed in the somewhat specialized field of silicate research. No attempt is made in this paper t o describe the experimental technic in more detail than is needed to make each point a t issue clear. Preparation of Materials The mixtures studied were prepared in the form of glasses from NazCOa, KzC03 and quartz. The NazCOa used was obtained by dehydration of exceptionally pure N ~ z C O ~ . H Z O The . KzCOs was derived from KHC03, also of high purity. The quartz was taken from the special stock of this Laboratory, containing certainly less than 0.05 per cent impurities. The compositions were made up by heating mixtures of weighed amounts of the reagents, according to a definite procedure. Platinum crucibles were used throughout. Experience has shown that the best results are obtained when the mixed reagents are allowed to sinter slowly a t as low a temperature as possible for the reaction to proceed, usually not above 700°C. Most of the COzis driven off quietly when this is done, and mechanical losses due to bubbling or spattering are completely eliminated. When a large number of mixtures is being prepared, i t is best! to conduct the sintering in individual, controlled furnaces, allowing about 24 hours for the process. The crucible and 1
Kracek, Bowen, and Morey: J. Phys. Chem., 33, 1857 (1929).
2F. C. Kracek: J. Phys. Chem., 34, 1583 (1930). J
Morey and Bowen: J. Phys. Chem., 28, 1167 (1924).
2530
F. C . KRACEK
contents are then weighed to estimate the amount of COZ still undisplaced. About 99 per cent of the total COZshould be expelled before proceeding t o the next stage of the heating. The temperature is now raised slowly to drive off the rest of the COz. There should be no frothing of the melted glass, otherwise mechanical losses are unavoidable. In addition to the mechanical losses, there may be losses occasioned by the vaporization of the undissociated carbonates with the escaping COZ. After the COZis displaced, the temperature may be raised to 1zoo0C or higher for a brief period to facilitate diffusion in the molten mixture. It is essential to note that COz-free mixtures lose very little Na2O or K 2 0 by volatilization even a t moderately high temperatures. After the first melting the glass is crushed, thoroughly mixed, and replaced in the furnace for a second melting. The process is repeated until no inhomogeneity can be detected in the glass when examined under the microscope, using an immersion liquid matching it in refractive index. Control of the Composition of the Preparations During the preparation of the mixtures record is kept of the weights a t each stage of the process. It is assumed here that no Si02 is volatilized. Kracek showed2 that the volatility of Na2O from NazO-Si02 mixtures containing more than 60 per cent SiOz is negligible when an exact procedure is being followed. The volatilization losses in the preparation of the K20NaZO-SiOz mixtures in the present work were generally of the order of 0.1to 0.3 per cent, and the assumption is being made that the losses were due solely to the volatilization of K20. A number of the synthetic compositions were checked by direct analysis. Owing to the difficulties inherent in the separate determination of sodium and potassium when both are present in the same sample, the Na/K ratio can be determined with only moderate precision. It was found that the compositions as established by synthesis were correct to within the uncertainty of the analyses, which was of the order of 0.5 per cent on duplicate samples. The compositions given in the tables are for this reason the synthetic compositions in all cases. This procedure is assumed to be justified further by the general concordance of all the results leading to the derivation of the liquidus surfaces presented in Figs. I and 2. The assumption made, that all the volatilization losses are due to KzO, is definite, and the systematic errors thus introduced into the diagram are well within the experimental error of the analytical determination; they are of major importance only in the sense that a slight distortion of the fields in the region of equimolar Ka/K ratio may be produced. For example, let us assume that as much as 0.5 per cent volatilization loss is encountered with a hypothetical synthetic mixture represented by 2 5 per cent K20, 2 5 per cent NazO, and j o per cent SiOZ. If we assume that KzO and Nan0 have equal volatilities the composition of the mixture would be 2 5.25 per cent KzO, 2 4 . 7 5 per cent ru’azO,and j o per cent SiOz. The actual composition of the mixture would certainly lie between the two limits, and most probably nearer the z j : z 5 : 5 0 ratio, since KzO is distinctly more
THE TERNARY BYSTEM, K2SiOs-Na&3iOs-SiOa
25-31
volatile than NazO. Analysis can not decide between these possibilities; it is hence assumed, barring large accidental errors, that the compositions given in the tables are correct to within a maximum systematic error of 0.3 per cent.
Liquidus Determination The “quenching” method of liquidus determination, generally employed in silicate work in this Laboratory, has been used with all the preparations studied in this investigation. The individual preparations were first crystallized by appropriate heat treatments; the crystal aggregate was then powdered and well mixed to further insure good homogeneity of the preparation, and was then used in the quenching work to determine the liquidus. The quenching furnace control was thermostatic to within 0 . 5 O C . The temperatures were read by calibrated Pt VI. Pt-Rh (10% Rh) thermocouples in connection with the temperature scale of this LaboratoryP A Wolff-Feussner potentiometer with a highly sensitive galvanometer was used in the temperature measurements, all necessary precautions being observed. Compositions in the KaSi01 and NazSiO3 fields crystallize readily. In the disilicate fields, i e . , KzSi205 and NazSizOa, a variety of behavior is encountered. In general, those preparations whose compositions contain more K 2 0 or NazO than is required to yield a disilicate can be crystallized with ease in the dry state. Preparations with more Si02 than is required for a disilicate can usually be crystallized in the dry way only if the liquidus lies dbove 750”~or if they lie near the disilicate join. When a much larger excess of SiOais present, crystals will not grow even in seeded preparations in the dry state if the liquidus is much below 750°, and hydrothermal crystallization must be empl0yed.~~6~6Similar considerations apply to the fields of K2SiaOs and of quartz. Tridymite can usually be crystallized from the mixtures in which i t forms the stable phase, without any particular difficulty, in the dry way, if sufficient time is allowed; it is to be noted, however, that there is a preponderant tendency for cristobalite to be formed a t first, and sufficient time must be given in the initial crystallization to convert the (metastable) cristobalite to tridymite. Since diffusion in the more or less viscous vitreous melts is not very rapid, enough time must be given each preparation in the melting point experiments to come to equilibrium before quenching. One hour or less suffices for the attainment of equilibrium a t or near the liquidus temperature in the K2Si03, NazSiO3, and certain portions of the KzSiz06 and NazSi206 fields. In general when the Si02 content lies between 60 and 7 5 wt. per cent, very much greater length of time must be allowed for the attainment of equilibrium, the time required increasing inversely with the liquidus temperature. When the Intern. Crit. Tables, 1, 57; Day, Sosman, and Allen; Carnegie Inst. of Washington, Publ. 157. 6 Morey and Fenner: J. Am. Chem. Soc., 39, 1173 (1917). 6 Morey, Kracek, and Bowen: J. Soe. Glass Tech., 14, 149 (1930).
2532
F. C. KRACEK
liquidus temperature lies below joo°C a t least 6 to 1 2 hours must be allowed; in the quartz field a time of 48 hours or more often fails to dissolve the larger grains. Tridymite dissolves more promptly, and usually 2 to 5 hours are sufficient to reach equilibrium. I t is obvious that dynamic methods of liquidus determination are not applicable to materials of this type. The "quenching" method,' more accurately described as a static equilibrium method, when applicable, can give information not only about the liquidus or other transformation temperatures, but also with regard to the nature of the phases present at equilibrium. The principal requirement of the method is that nucleation of new crystals should be sufficiently slow so that the sample can be chilled to room temperature (by quenching in a non-reacting liquid when necessary) while retaining the structure reached during the thermostatic heating at the desired temperature. The sample is then examined under the microscope to determine whether crystals are present, and their character. By varying the temperature, the limits between which the sample exhibits one phase or another can be fixed as closely as desired. The great advantage of the method lies in the fact that the thermostatic heating a t each temperature can be prolonged until equilibrium is attained in the sample. The Experimental Results
It has been customary to present the results of phase rule studies on silicates from this Laboratory in the form of a more or less abbreviated record of the actual quenching experiments performed. In view of the large number of preparations studied in this work, and the relative simplicity of the system (there being no ternary compounds), it is deemed advisable here to present the results in a briefer form. The tables following present the synthetic compositions of the mixtures, the equilibrium transformation temperatures, and the temperature interval within which the transformation temperature is located. Thus, if the liquidus temperature is given as 764', interval f 3 - 2, it is meant that the preparation when quenched from 767" was all glass, and when quenched from 762" it contained crystals. Since the thermostatic control of the furnace is accurate to 0.jo,the limits 767" and 762" are intended to denote that these temperatures are known to the nearest 0.5" of the values given; the interval then denotes the limits within which the transformation was determined, and not the uncertainty of the temperature measurement. The data are given in Table I. In constructing the ternary diagrams, Figs. I and 2 , use was made of the already mentionedresults on the componentbinary systems, K2SiOs-SiOzand NazSi03-Si0~,'~2 together with the work of Kracek on the cristobalite liquidus?
' Originally described by Shepherd and Rankin: Am. J. Sci., 28, 308 (1909). SF.C.Kracek: J. Am. Chem. SOC., 52, 1436 (1930).
2533
THE TERNARY SYSTEM, KzSiOs-NazSiOa-SiO2
TABLE I Compositions and Liquidus Temperatures of Preparations studied Quenching Method Preparation number
Corn osition in weigKt per cent K 2 0 N a 2 0 Sios
Liquidus tepp.
C
Determined Time int:rval aIlowed C to reach Initial (see p. equilibrium condition* 2532) (hours)
A. Solid Phase: NazSi08
I1
10.7 17.7 24.6 32.0 39.1 42.3 38.6 34.9 37.1 26.9
I2
25.0
I3 I4 I5 16 I7
22.3 20.1 14.6 13.1 8.0
574 2
3 4
5 6 7 8 9 IO
-
50.82 41.5 36.2 31.3 24.5 18.7 15.8 16.7 17.5 13.9 19.4 17.7 23.2 20.9 29.4 26.9 31.3
49.18 47.8 46.1 44.1 43.5 42.2 41.9 44.7 47.6 49.0 53.7 57.3 54.5 59.0 56.0 59.9 60.7
1089 1009 963 912 858 842 759 780 754 682 729 672 788 689 847 765 796
+0.5-0.5
+4-4 +3-3
+3-3 +4-4 4-4 4-4
+ + +5-5
I
I I
3 2 2
3 3
+2-2
2
+4-4
3
+
2-2
+4-4 +3-3 +4-4 +4-4 +3-3 +3-3
2
3 2
4 3 2 2
CG C C G G G G G G C CG CG G C C C C
B. Solid Phase: NazSizOs
875 +0.5-0.5 2 C 34.05 65.95 3 . 8 31.1 65.1 827 2-2 2 C 7.2 28.6 64.2 1-3-3 2 C 23 789 9 . 5 26.8 63.7 758 1-2 2 C 24 1 2 . 1 2 4 . 8 63.1 25 739 +3-3 2 C 26 1 5 . 8 21.9 6 2 . 3 730 +2-2 4 C 1 8 . 8 19.7 61.6 27 723 1-1 12 C 28 2 2 . 8 16.5 6 0 . 7 +3-3 I2 C 711 23.9 1 7 . 1 59.0 695 2-2 12 C 29 30 12.5 25.9 61.6 729 1-1 12 C 7 . 8 29.9 62.3 776 31 2-2 3 C 7.0 27.2 65.8 792 2-2 6 C 32 6 . 6 25.8 6 7 . 6 33 787 1-1 6 C 34 780 6 . 2 24.5 69.3 1-1 8 C 808 3 . 7 26.4 69.9 +2-2 6 C 3s 5.8 2 2 . 7 71.5 755 1-1 IO C 36 11.2 23.0 6 5 . 8 37 7 53 1-1 8 C 1 0 . 5 21.4 6 8 . 1 38 7.51 1-1 I2 e * The s mbols G , C, B denote that the material used for the quenching experiments was G glassy, dcrystalliied dry, or B crystallized hydrothermally in bombs. CG denotes that 21
22
+ +
+ + + + + + + + + +
Iiquidus was determined starting with both glassy and pre-crystahzed materials.
c
2534
F. C. KRACEK
TABLEI (Continued) Compositions and Liquidus Temperatures of Preparations studied Quenching Method Preparation number
39 40 41 42 43 44 45 115 117
Composition in weight per cent K20 NalO SiO?
9.8 14.1
20.1
70.2
18.5 67.4
1 5 . 2 63.3 19.9 14.2 65.9 18.9 13.3 67.8 18.2 1 3 . 1 6 9 . 7 9.6 68.8 21.6 9.1 18.9 7 2 . 0 5.3 2 1 . 7 73.0 21.5
Liquidus
Determined interval
teT.
(seep.
735 726 707 679 649 63 8 597 723 m 741 m
"C
2532) +2-2
+
2-2
+2-2
+4-4 +2-2
+
2-2
$3-3 +3-3 $3-3
Time allowed to reach Initial equilibrium condition * (hours) IO
8 8 16 8 24 24
6 6
C. Solid Phase: &SiOa
M
(61.1)
51
50.6 47.3 41.7
52
53 251
61 62 63 64 65 66 67 68 69 70
43.8 42.3 38.4 36.8 34.6 32.9 30'.2 27.0
- (38.9) 8 . 7 40.7 12.1 40.6
976
-
902 833
$3-3 4-4
11.1
702
1.4 4.5
5.5 7.6 8.6 10.9 13.2 1.3 1.3
47.2
+
+ + + +2-3 + + + + + 2-2
0.5
I 2
C C
c B B B B C C
CG CG CG
D. Solid Phase: KzSizOs 56.2 56.3 57.2 57.7 57.8 58.5 58.9 59.8 57.8 59,8 61.9 63.0 64.2 65.0 66.1 67.0 60.2
1036 I 008 954 924 898 878 83 1 765
1-1
I
1-2
I
2-2
+ +
2 2
2-2
2
2-2
1-1
3 3
1-1
I2
CG CG CG CG CG CG C C CG CG CG CG CG CG C C CG C C C C
1000 1-1 I 40.9 978 1-2 I 38.9 973 2-2 2 36.9 1.2 951 +2-2 2 35,8 1.2 71 1.2 2 72 34.6 925 2-3 890 +2-2 2 33.9 1.1 73 32.8 1.1 74 861 +3-3 3 31.9 1.1 75 826 +3-3 3 928 2-2 76 35.6 4 . 2 3 8 4.0 6 2 . 4 77 33.6 883 2-3 2-2 I2 798 3.6 66.4 78 29.9 31.2 8 . 0 60.8 6 79 860 5-5 80 7 . 5 63.6 28.9 807 4-4 4 81 7 . 1 65.6 6 B 27.3 750 45-5 * The 8 mbols G, C,B denote that the material used for the quenching ex eriments was G glassy, 8crystallized dry, or B cfystallized hydrothermally in bombs. CC! denotes that
+
+
+
++ + +
liquidus was determined starting with both glassy and pre-crystallized materials.
2535
THE TERNARY SYSTEM, KZSiOa-Nad3iOa-SiOz
TABLE I (Continued) Compositions and Liquidus Temperatures of Preparations studied Quenohing Method Preparation number
Composition in weight per cent KO0 NazO Si02
82
25.7
83 84
24.7 44.8 47.3 48.4 49.8
85 86 87 88 89 90 91 92 93 94 95 96 97 284 IO1
50.7 51.6 46.9 40.9 45.0 35.6 39.2 40.5 34.7 31.9
67.7 63.6 1 . 5 53.7 1.5 51.2 1 . 6 50.0 1.6 48.6 1 . 7 47.6 1 . 7 46.7 3.7 49.4 4 . 9 54.2 5.3 49.7 9 . 3 55.1 6.6
11.7
Liquidus temp. "C
667 698 I002
975 93 5 865 816 789 860 948 864 843
Determined Time int$rval allowed to reach C Initial (see p. equilibrium condition* (hours) 2532)
+
2-2
+4-4 +z-2
+
2-2
+2-3 +5-5 +4-4 +4-4 +5-5 +3-3
+3-3 +4-4
50.8
785
10.8 48.8 13.1 52.2 15.5 52.5
+5-5
493 730
+3-3
10.0
28.1 26.9
-
24.5 23.4 20.9 16.0 13.3 9.1 8.2 5.3
2.9 6.0
3.2
715
+5-5 +5-5
24 6 I
I I I I I I I I I I I I I
B B CG CG CG CG CG CG CG CG CG CG CG CG CG CG
E. Solid Phase: KtSilOa 71.9 69.9
765
+0.5-0.5
700
+2-2
24 24
B B
F. Solid Phase: Quartz I10 I11
I12
113 1I 4 115 116 1'7
72.6 70.6 5.5 73.6 11.3 7 2 . 7 15.7 7 1 . o 18.9 7 2 .o 16 9 7 5 , o 2 1 . 7 73 .o
798 700
810 783 688 750 865 786
+4-4 +5-5 +4-4 +4-4 +5-5 +5-5
B B B B
+IO-IO
B B B B
+3-3
G. Solid Phase: Tridymite 2 . 6 75.0 902 +3-3
22.4 6 C 20.9 2 . 6 76.5 C 976 4-4 4 16.9 2.8 80.3 C 1168 +4-4 2 123 C 124 1 7 . 9 4 . 6 77.5 1035 +5-5 4 1 4 . 3 1 0 . 2 75.5 C 125 905 5-5 5 126 7.4 82.1 10.5 C 1235 5-5 3 C 127 7.2 15.4 77.4 980 +5-5 5 128 4 . 0 11.7 8 4 . 3 1310 Z C -5-5 _ * The symbols G , C, B denote that the material used for the quenching ex eriments was lassy, C crystallized dry, or B crystallized hydrothermally in bombs. C& denotes I21
I22
+
+ +
+
that !quidus
-
was determined starting with both glassy and pre-Crystallized materials.
F. C. KRACEK
2536
p
.e e
8
3
d c 13
uE
Y
THE TERNARY SYSTEM, KzSiOa-NazSiOa-Si02
P
1a
B
2531
FIG.I The ternary system, K2SiO3-Sa2SiO8-Si0z. This figure shows the compositions of the mixtures studied represented by open circles; the compositions of the binary potassium and sodium silicates by black circles, and the fields of the compounds.
FIG.2 The ternary system, KzSi03-KazSi03-Si02. This diagram shows the isotherms of the liquidus surfaces of the various compounds.
THE TERNARY SYSTEM, KzSiOs-Na&3iOs-Si02
2539
Discussion of the Ternary System The equilibrium diagrams of Figs. I and z present the stability fields of the crystalline compounds occurring in the system. It will be seen by reference to the figures that there are no ternary compounds and no incongruent melting relations between the adjacent binary compounds, hence the system is one of comparative simplicity. The only complications entering are due to the unique type of solid solution relations of the disilicates, and the polymorphic relations of the compounds, NazSizO6,KzSizO6, and the various modifications of silica, namely, high and low quartz (Quartz I and 11), tridymite, and cristobalite. I n addition to the component binary systems along the binary boundaries there are three binary systems within the ternary system, KzSiOrNaZSiOaSiOz, namely, KzSiOs-NazSiOs, K z S ~ Z O K - N ~ Z Sand ~ Z OK~Si206-NazSiOs. ~, Two other possible binary systems, K2SiOs-NazSi206 and & s i & N a ~ s i ~ O ~ , fail to appear. The first of these is excluded by the existence of the binary system, KzSiz06-Na&Os, the second fails to be realized a t the liquidus as a consequence of the excessive area covered by the liquidus surface of K~Si206. The melting relations for the ternary system accordingly are composed of the liquidus surfaces of KzSiOa, NazSiOa, KzSizO6,Na2Siz06, K2SiaOs,quartz, tridymite, and cristobalite. The ternary system may be thought of in terms of three subsidiary ternary systems: (I) K z S ~ O ~ - N ~ Z S ~ O ~ - K with I S ~aZ O ~ , ternary eutectic a t 40.1 wt. per cent KzO, 11.7 wt. per cent NazO and 48.2 wt. ~ O a~ternary - N ~ eutectic ~S~~O~, per cent SiOz, 645OC; (2) K ~ S ~ Z ~ ~ - N ~ ~ S with at 25.4 wt. per cent KzO, 17.2 wt. per cent NazO, and 57.4 wt. per cent SiOz, 665OC; (3) K z S ~ Z O K - N ~ ~ ~(Quartz ~ Z ~ ~ II), - S ~with O Z a ternary eutectic at 2 2 . 9 wt. per cent K20, 7.6 wt. per cent NazO and 69.5 wt. per cent SiOz, 54oOC. A list of the various invariant points encountered in the system is given in Table 11; Figs. 3 and 4 represent the binary systems, KzSiOs-NazSiOs and K z S ~ Z O ~ - N ~ Z SThe ~ Z Obinary ~. system, KzSizO6-Na~SiO3,was not separately investigated, and hence is not represented. The liquidus surfaces of KzSiOs and NazSiOsare simple curved sheets and need no further discussion. The liquidus surfaces of the different varieties of silica rise steeply in succession from the ternary eutectic for KzSiz06,NazSisOs, and quartz 11 a t 540°, with inversion boundaries 573' for quartz I1 and quartz I, 870' for quartz I and tridymite, 1470' for tridymite and cristobalite. The stability relations for the different varieties of silica are those determined by Fenner? The liquidus surface for cristobalite necessarily exhibits a fold in consequence of the difference in the liquidus curves for cristobalite in the component-binary systems, K ~ O - S ~ O Z and NazO-SiOl, the curve in the latter system exhibiting a reverse S curvature. The liquidus surfaces of tridymite, quartz I and quartz I1 are smooth sheets with no special features. The low ternary eutectic temperature for K2Si20K11, Na2Si20611, and quartz I1 is noteworthy. This was investigated by the method employed by Morey and Bowen in their study of the system, Na~SiO~-CaSiO~-Si0~.10 Crystallized C. N. Fenner: Am. J. Sci., 36, 331 (1913). lo
Morey and Bowen: J. Soo. Glass Tech., 9, 226 (1925).
2 540
F. C. KRACEK
1000
950
930
550
WO
750 10
30
FIG.3 Section through ternary diagram, showing the binary system, K2Si03-NazSi03.
FIG.4 Section through ternary diagram, showing the binary system, K2Si?Os-XazSi2Os.
THE TERNARY SYSTEM,
K2Si0~-Na$3iOa-Si02
2541
NazSiz05, KzSiz05,and extremely finely powderedquartz wereground together in the required proportions and samples of the mixtures were held at various constant temperatures to determine the point a t which sintering and glass formation take place. The most probable value thus determined is 540'c; the uncertainty is about f30'C. The reactions between silicates at such low temperatures are extremely slow, and even 5 to I O days heating at a given temperature above the eutectic does not serve to completely dissolve quartz crystals. The field of KzSi40goccupies only a small portion of the ternary diagram. This compound exists in two polymorphic modifications with an inversion a t 592' at the pressure of I atmosphere." The lowest liquidus temperature with K2Si409as the stable phase in the ternary system is 640' for the equilibrium between K2Si409I, K2Siz05I, quartz I and liquid, and hence the low temperature modification, KzSi40g I1 does not reach the liquidus. The disilicates, K2SizOaand Na2Siz05,present interesting features. Reference to Fig. 4 shows that the liquidus curves of both these compounds in their binary system exhibit breaks, a t 920' for KzSiz05, and a t 742' for NazSizO6. Reference to the work on the binary systems, KzSiOrSiOz and Naz0-SiOz1,2 reveals that in the potash system corresponding breaks are encountered a t 814' on the K 2 0side, and a t 993' on the Si02 side of the K~Siz05composition. I n the soda system lieat effects were located at 706' on the NazO side, and at 768' on the SiOz side of the NazSi205 composition, below the liquidus temperatures throughout. The interpretation was that bothK2&o6 and NazSizOs enter into solid solutions with excess SiO2, and excess K20 and NazO respectively, the different temperatures established being the unmixing temperatures of these solid solutions. Below the unmixing temperatures the crystals are essentially the pure disilicates, as evidenced by the constancy of inversion temperatures, 596'C for KzSiz05 I and KzSiz05 11, and 678' for NazSiz05I and NazSiz05 11. I n the ternary system a search was made for the existence of possible compounds which might explain the presence of the above-mentioned breaks at 920' and 742' in the subsidiary binary system, K Z S ~ Z O ~ - N ~ ZThis S~~O~. search revealed that no such compounds exist, but that the unmixing boundaries AB and CD (Fig. I ) are not constant temperature lines; they are screw curves, the temperature rising continuously from A, 814'C to B at 992'c, the intersection of the screw curve with the disilicate join being a t 920' at U, for the KzSiz05; in the case of the NazSiz06 the screw curve is entirely within the ternary system, rising from C at 740' through V at 742' to D at 765', the two ends of the screw curve being at 706' and 768' respectively, under the liquidus surface. The unmixing and the polymorphic inversions in these two compounds are prompt, hence no definite optical evidence for the existence of the solid solutions is adducible. I n the case of the NazSizO~a slight modification of the external form of the crystals was noticed, but this was not sufficient for a description. The relations may then be summarized in the statement that both NazSizOs and KzSiz05 take up limited excess of SiOz, l1 Goranson and Kracek: J. Phys. Chem.,36, 913 (1932).
F. C. KRACEK
2542
Na20, or K20, respectively, depending on the composition of the liquid from which they crystallize, in the component-binary syst,ems, and t,hat K2SizO takes in a limit'ed amount of Kaz0, while KazSizOstakes in a limited amount of KzO in the ternary system under description. In addition to the above-described solid solution phenomena, the liquidus surfaces of KzSiZOsand T\'azSizOjshow the inversion temperatures KeSizOsI to K2Si206I1 a t 596" and Na2Si205I to NazSizOjI1 at 678'. The temperatures of the eutectics El and Ez,namely K2Si03 K2Si2Oj I NazSi03 L and KzSiz05I NazSiOa PITazSizOj I1 L, were determined by quenching previously crystallized preparat,ions heated at the various appropriate constant temperatures and examining ait'h the microscope for the first appearance of glass. The compositions employed for this purpose were Nos. 53 and 9; for El and 1 2 for Ez. The compositions used do not, melt completely at the eut'ectic t'emperatures. These eutectic temperatures were located with an uncert'ainty of r ; t ~ o O , the uncertainty being due to the difficulty of recognizing the first traces of glass in the microscopic examination. From the physical-chemical point of view it is of interest to note that the liquidus surfaces of the silicate compounds of t'he syst'em are sheets with very flat' maxima at, the melting points, and generally with a large radius of curvature. This indicates considerable dissociation of these silicates in the liquid state.12 The extensive dissociation of the liquid is also indicated by the relatively low melting points, with the statistical implication that the silicates are not definite compounds in the melts, particularly a t higher temperatures. The situation is, in a sense, analogous to that encountered with other additive compounds; for example, while CaC12.6Hz0undoubtedly is present in CaC12H20 solutions in appreciable amounts a t the melting point' of CaCl2.6Hz0, it is probable from the point of view of statistical mechanics that the number of molecules of HzOassociated with one of CaClz (or Ca++)in solution depends upon average distribution governed by the intermolecular forces in the liquid, rather than upon t'he definite distribution governed by the directional forces in the lattice of a crystal. Summary The melting point relations in the ternary system, K2SiO3-Na~SiO3-SiO2, are simply-eutectoid with respect to the component-binary systems, K2Si03SiOz and NazSiOa-SiOz, there being no ternary compounds. A unique type of solid solution formation is encountered in the disilicate region, both K2Si205 and NazSitO; taking up a varying limited excess of the three constituents, K20, NazO, SiOz, dependent upon the composition of the liquid in equilibrium with the crystals. The equilibrium diagrams of Figs. I and 2 present the phase relations worked out by the method of quenching, the data being given in Tables I and 11.
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Geopliysical Laboratory, Carnegie Institution of Washington, J u l y , 1952. l2 W. Stortenbeker (upon suggestions of H. A. Lorents): 2. physik. Chem., 10, 183, 194 (1892), discussed by Lewis and Randall: "Thermodynamics," 217 (1923); see also A. Rmits:
Z. phy?$. Chem., 78, 708 (1912); hlorey and Bowen: Ref. 3; J. W. Gibbs: "Scientific Papers, 1, 135.
