THE SYSTEM SODICM OXIDE-SILICA BY F. C. KRACEK I. During the course of a study of the three component system involving S a z O , XzO and Si02 it became advisable to amplify the work of Morey and Bowenl on the binary system sodium metasilicate-silica. As the binary system composed of NazO and SiOz is an end member of various polycomponent systems of considerable importance both in physico-chemical petrology and in industry, it was thought worth while to extend the study into regions hitherto untouched, namely, to compositions rich in KazO. It may be stated a t the outset that i t was not possible to study compositions richer in NazO than the orthosilicate with the methods and apparatus used, which are those currently employed in this Laboratory in melting point work. As will be evident from what follows, the removal of Con, present as the decomposition product of T\Ja&03 employed in preparing the various mixtures, is the principal obstacle. It is hoped that this difficulty will be overcome by suitable modifications of method so as to conduct the experiments under vacuum. An effort has been made in collecting the data presented in this paper to establish the invariant points (melting points, eutectics, etc.) accurately, and hence most of the compositions studied, with the exception of those on the silica melting point curves, are concentrated in the neighborhood of these points. These data, together with the data taken from Morey and Bowen's work, suffice to establish the course of the liquidus curves. The extension of the study to compositions in the orthosilicate region and an investigation of the phase relations in the disilicate region a t temperatures below the liquidus is new.
Experimental Materials. The preparations used were made up from selected pure 2. quartz as the source of Sios, and from especially pure XazCO3. The quartz gave a residue of 4.5 mg on evaporation of I O g with HF and H2S04. This residue was due principally to Fe. The N a z C 0 3contained 0.0024 per cent Si02, 0.0028 per cent combined Fe2O3and &OS, 0.01 per cent CaO, no detectable MgO, 0.002 per cent So3, and gave only a faint turbidity with AgNO3 (less than 0.001per cent Cl)? 3. Preparation of the Mzxtures. The materials were mixed in the required proportions by weight, placed in a platinum crucible, and gently heated to drive off the major portion of the COZpresent before appreciable quantities of liquid were formed. After this initial sintering the temperature of the charge was allowed to rise slowly until the mixture was wholly fluid. The initial sintering took place a t about 700'. The final melting was always Morey and Bowen: J. Phys. Chem., 28, 1167(1924).
* Analyak by E.G.Zies, on 2j g.
1584
F. C. KRACEK
done in a large platinum furnace, to avoid contamination. In order to make homogeneous preparations it is necessary to raise the temperature t o the point a t which rapid diffusion of the constituents takes place in the mixture. This temperature varies with composition, being generally higher the greater the silica content of t,he preparation. I t is advisable to heat no higher than is essential, if quantitative results are to be obtained, since at high temperature both the soda and silica are somewhat volatile. After heating the melted preparation the required length of time at a selected temperahre the glass is cooled, weighed, powdered, and examined under the microscope for homogeneity. It is then remelted, when necessary, until a homogeneous preparation is obtained. Finally, most of the preparations were crystallized before being used for melting work. This, however, is not essential except with mixtures from which quartz separates as the primary phase, or in which quartz forms one of the eutectic constituents. Quartz is a very reluctant crystallizer, and, when once obtained, disappears from the melt very slon4y. All the other crystalline phases occurring in the system grow with sufficient rapidity to insure equilibrium being obtained in relatively short time. 4. Control of Composition. Unlike other alkalis, Xa?O is only slightly volatile from mixtures containing in excess of 60 per cent SiO2. I n such mixtures the carbonate is decomposed easily at relatively lo~vtemperatures, and the CO, can be driven off completely at nioderntc temperatures. Mixtures in this region can be made up synthetically with accuracy which compares very well with analytical precision. The following experiments are selected at random : Per cent Si02 62.49 74.00 87.90
Weight calculated as oxides 5.0013
Weight after heating j ,0009
Loss g 0.0004
10.001~
Io.00oy
0.000.~
10 , 0 0 3 6
I 0 ,0033
0.0003
The losses given are those on first heating. If care is taken in the initial melting t o drive off the CO? compIeteIy, the losses on subyequent heating are quite negligible. When the S a p o content exceeds that needed for the metasilicate composition (I:I ratio of S a ? O to SiOe), the preparations tend to retain CO? with rapidly increasing tenacity as the SasO/SiOz ratio increases; when this ratio exceeds 2 . 0 the CO, can not bc driven off completely, I mol of SiO? replacing at most z mols of CO?. Heating in air at atmospheric pressure, considerable difficulty is experienced in driving off the CO? even from mixtures containing SiO? in moderate excess o w r that needrd for the orthosilicat,e composition. The best procedure appears to be that of holding the preparation between 1000' and 1100' for upward of r g hours. The orthosilicate crystallizes out at this temperature and settles to the bottom, leaving the lighter liquid at the top, where the CO,can slowly diffuse out. Heating to higher temperatures is actually a drawback because of oxidation, evidenced
THE SYSTEM SODIUM OXIDE-SILICA
1585
by rapidly increasing attack upon the P t crucible. It should be noted that in the more siliceous mixtures no attack on the Pt is noticeable even a t high temperatures. I n the Xa,O-rich mixtures the melts first turn a rose color which soon changes to a dark brown. Since free 0 2 does not attack Pt, whereas peroxides are known to do so, it may be assumed that the oxidation of the Pt takes place thru the medium of the melt. Alkali oxides are known to absorb free O2 with avidity, forming the corresponding peroxides. The peroxides may not be present in very great concentrations in the melts a t the higher temperatures, but since they give up oxygen in the activated state, even small concentrations suffice to facilitate the attack on Pt. Several melts were made using more than z mols of K a s C 0 3per niol of SiOz. In each of these trials the loss of weight corresponded to the evolution of at most z mols of CO,. This, together with the tet'ravalent nature of Si indicates that the orthosilicate, zKazO.SiOzis the most basic sodium silicate. This compound is not stable a t its melting point; the S a 2 0 set free in its dissociation tends to absorb CO,, so that, if there is any CO? present in the furnace it becomes uncertain how much of this gas is retained in the melt. The best preparation of the orthosilicate made still held z per cent COP. About 3 g of very much purer crystals were obtained from this mixture, by crystallizing at IO~O', and separating the crystals from the upper layer, which contained most of the COS retained by the melt. Analyses for the retained CO, were made by the usual absorption method, but, unfortunately, the analyses, although consistent among themselves, can not be taken too seriously, because in the melting point work the materials must be handled in air, and even a very brief exposure suffices to re-introduce C 0 2 into the prepwation. (See below, in connection with the description of heating curves.) The more important preparations were also analyzed by evaporation with HF, weighing the Xa2SiF6,followed by HzSOa to convert to XazS04. j. Methods of Studying the Equilibrium Relations. The various significant temperat'ures governing the phase equilibria were established by the method of quenching for the most part of the diagram. In the orthosilicate region, where it is difficult to freeze the equilibrium, and in studying the solid phase reactions in the disilicate region, use was also made of heating curves, employing the technique previously described.' The apparatus differed from that previously used only in that a Pt wound furnace was necessary to take care of the higher temperatures encountered in this work, and t,hat Pt vs. 90 Pt I O R h thermocouples were employed. The arrangement for reading the temperature of the charge and the differential temperature was the same as before. The method of quenching, introduced by Shepherd and Iiankin2 and repeatedly described in many papers from this Laboratory depends on the examination of "frozen" equilibrium in samples which had been held a suffi1 F. C. Kracek: J. Phys. Chem., 33, 1281 (1929); Kracek, Bowen and Morey: 33, 1857 (1929). 2 Shepherd and Rankin: Am. J. Sei., 28, 293 (1909).
1586
F. C. KRACEK
cient length of time a t a constant temperature and then quickly chilled, the examination being done with the aid of a petrographic microscope. The application of the met'hod is obviously limited only by the certainty with which the equilibrium in the charge can be "frozen," so that while ordinarily systems composed of rapidly crystallizing substances can not be studied with it, other systems, particularly certain portions of silicate systems, in which crystallization is usually too slow to give satisfactory heat effects on heating or cooling curves, depend upon it largely for their elucidation. I n the system with which this article deals the quenching method is applicable to the study of all the crystalline phases, including the orthosilicate. This last compound is the only one about which there is any question of being able t o "freeze" the equilibrium, and hence, in the region in which it is the primary phase, the heating curve method and the quenching method supplement each other. It was found that orthosilicate melts, free of all crystals, can be slowly undercooled at least 200' before crystallization suddenly sets in, which corresponds with the observation that small samples of such melts can be suddenly chilled t,o room temperature without deiitrification. On the other hand, if any orthosilicate crystals are present, or if the chilling is not rapid enough, the crystals will grow during cooling, and may give false indications. Sodium metasilicate can be quenched readily, and it crystallizes rapidly enough so t,hat crystals always appear on I j minutes heating. One hour heating at constant temperature appears to be ample for attainment of equilibrium between the crystal.. and melt. The disilicate is more sluggish in behavior, and two to five hours are needed for attainment of equilibrium, The crystals usually form in I j minutes, but do not disappear readily, because of the slow diffusion in the soinediat \-iscous melts in the disilicate region. The same is true of tridyinite or cristobalite, vihen these are formed metastably at low temperatures. ( tobalite usually crystallizes firpt, and is slowly converted to tridyiiiiti, on continued heating. For this reason it is preferable t o pre-crystallize the preparations from which tridyinite is expected t o form. At temperatures bclow 870' in the silica field quartz should crystallize as the stable phase, but because of its extreme reluctance to grow, it is possible to study the metastable tridj-mite liquidus down to the metastable tridymite-disilicate eutectic without an\- difficulty. JIorey and Bowen' were the first to obtain quartz in the dry ~ - a froin y sodium silicate mixtures, and they emphasize the slowness with which it dissolveu in the melts. In this xork quart,z was produced in several mixtures on 1 days heating a t 7 j o " dry, or hydrothermally on I day heating in bombs at .+jOo, in the manner described in connection with the crystallization of potassium tetrasilicate and quartz from potassium silicate niixtures.' Quartz crvstallized in the dry way was always found to be associated with inetastable tridyniite, and none of the mixtures were wholly crystallinc, e\-en though the temperature at which the hlorey and Bowen: J. Phys. Chem., 28. 1169-70 ( r g z 4 1 . Kracek, Bowen and More?: J. Phye. Chrm., 33, 18j; (19291
THE SYSTEM SODIUM OXIDE-SILICA
I587
preparations were held was below the eutectic with respect to sodium disilicate, which was always present. It is doubtful if the last traces of liquid could be removed in any reasonable length of time. 6 . Control and Measurement of Temperatures. The P t quenching furnace in which the charges were heated a t constant temperatures was controlled by a Wheatstone bridge type thermoregulator developed in this Laboratory.' The temperatures were measured with calibrated Pt vs. 90 P t I O Rh thermocouples, using a highly sensitive potentiometer system, shielded against external electrical disturbances. The thermocouple readings were converted to the gas thermometer scale2 with the help of standard tables3 7 . The Experimental Results. The data obtained from the quenching experiments are given in Table I. The heating curve results referred to in section 5 are described later, A graphical representation of all the available results of known reliabilit.y, combining the data of this investigation, with the data of Morey and Bowen, already quoted, and of Kracek on the cristobalite liquidus4 is given in Fig. I . ~ Discussion of the Melting Point Diagram The Field o j Silica Modifications. Silica crystallizes from sodium silicate preparations in three varieties, cristobalite, tridymite and quartz. The inversion temperatures are 14;o' and 8 j o o , according to Fenner.6 Cristobalite, the high temperature modification, melts' at I j 13'. The cristobalite liquidus descends from the melting point to the inversion point between cristobalite and tridymite, located at 88. j weight per cent SiOs (1470' =k IO'), in a characteristic reverse S type of curve, as shown recently by Kracek (op. cit.). The tridymite liquidus then descends smoothly from this point. At j j . j weight per cent SiOp (870' + IO') it meets the liquidus curve of quartz. The metastable prolongation of the tridymite liquidus below 870' is readily realizable, and has been studied down t o the disilicate-tridymite eutectic. The liquidus curve of quartz extends over a short range of temperature only, namely, from 8 70' to ; 9 3 O , ending at the disilicate-quartz eutectic. It should be remembered that only the high temperature modification of quartz, variously called a-quartz, quartz I, or high quartz, stable above 5;3', is the modification encountered at the liquidus. Because of the rapidity with which the 573' inversion takes place, the microscopic examination is always concerned only with the low temperature pseudomorphs of high quartz. 9. The Sodium Disilicate-Si2ica Eutectics. The location of the eutectics between the disilicate and quartz or tridymite was determined by noting the 8.
H. S. Roberts: J. Opt. Soc. America, 11, 171 ( ~ y z j ) . Day. Sosman and Allen: "High-temperature Gas Thermometry." Carnegie Institution of 'lvashington, Publication No. 157. L. H. Adams: "Int. Crit. Tables," 1, 57 (1926). F. C. Kracek: J. rlm. Chem. SOC.,52, 1436 (1930,. 5This figure also includes a number of points determined by Bowen, Schairer and Willems in their study of the system Na2Si03-Fe203-Si02. e C . N.Fenner: Am. J. Sci., 36, 331 (1913). J. 'lV. Greig: Am. J. Sci., 13, I (1927); Ferguson and Merwin: 46, 417 (1918).
1588
F. C. KRACEK
temperatures at which the disilicate disappeared from preparations previously crystallized a t temperatures below the eutectics. To assure the presence of quartz in sufficient quantities, hydrothermal crystallization was employed (see section 5 ) to save time. The data are given in Table I. After the tem-
40
50
50 FIG I
90 ut prcent
The Equilibrium Diagram for the Binaq Sjstem SazO-Si01 Black dots represent author's results
peratures were determined, it was a simple matter to locate the eutectic compositions. The disilicate-quartz eutectic is at 73.9 weight per cent SiOz and 703". The temperature is in agreement with the determination of hlorey and Bowen (op. cit.). The metastable eutectic between sodium disilicate and tridymite is at ;4.6 weight per cent Si02 and 782'.
THE SYSTEM SODIUM OXIDE-SILICA
1589
IO. The Field of S o d i u m Diszlzcate extends from the quartz-disilicate eutectic to the metasilicate-disilicate eutectic. The latter was determined in the same manner as the above-mentioned eutectics, and is located at 62.1 weight per cent SiOz and 846", a moderate correction of the values found by Pvlorey and Bowen. The compound, Na20.2Si02,melts at 874', in agreement with Morey and Rowen's value.
While studying the equilibrium relations at the liquidus in the ternary system Na2Si03-K2Si03-Si02 it became evident that the Xa20.zSi02liquidus surface is not of the simple type to be inferred from the liquidus curve of this compound in the binary system. Several binary compositions were therefore studied by the heating curve method, disclosing a situation analogous to that found to exist with KzO.zSiO2.' I n analogy with the latter compound, Na20.zSi02takes up into solid solution both excess X a 2 0 and Sios, with the difference that the unmixing temperatures lie entirely below the liquidus. Compositions containing excess K a 2 0 show unmixing at 706', those with excess SiO?, at 768'. The heat arrests are small, but perfectly well defined; the 706" arrest is obtained only on the N a 2 0 side of Na20.2Si02,the 760' arrest only on the Si02 side. These arrests correspond to no known transformations in either Sa20.SiOz or tridymite (the Si02 phase present under the conditions of experiment), and must therefore be assigned to be due to mixed crystal formation. The unmixing below these temperatures must be quite complete, if we can judge from the criterion that a n enantiotropic inversion, found a t 678", takes place a t the same temperature thruout the region. I I. T h e Field of Sodizim Metasilicate. Sodium metasilicate, ?ja20.Si02, melts, according to Jaeger2 a t 1088'. This value is based on a determination by the heating curve method, on a preparation which contained a slight excess of SiOa. The very sharply melting preparation made up in my work contained 49.18 per cent Si02 (theor. 49.207) and melted at 1089' i 0.5' (gold = 1063"). The liquidus curve of the metasilicate exhibits no unusual features. On the orthosilicate side it ends at the sodium orthosilicate-sodium metasilicate eutectic, 43.1 weight per cent Si02 and 1022'.
12. T h e Meltzng Relations of Sodium Orthoszlzcate. The melting curve of sodium orthosilicate, 2Sa20.Si02, rises steeply from the eutectic to I I 18' 5 , 40.7 weight per cent Si02. Above this temperature the orthosilicate ceases to be stable, the most obvious interpretation being that it decomposes into liquid and crystals of NazO. As was stated in section 4, it is extremely difficult t o remove CO2 from preparations in this region, the C 0 2 being readily re-absorbed at room temperature, and hence, the composition of the mixtures may vary somewhat, depending upon the amount of handling they are subjected to in contact with the C02-contaminated atmosphere. Further, since the orthosilicate crystallizes readily, good quenches were not always obtained, and the extremely hygroscopic nature of the quenched
*
'Kracek, Bowen and Morey: J. Phys. Chem., 33, 1857 (1929). 'F. M. Jaeger: J. F a s h . Acad. Sci., 1,49 (1911).
I590
F. C. KRACEK
samples made the microscopic examination so difficult that no reliable information could be obtained regarding the presence of K a 2 0 crystals, and their properties. To obtain more information, a number of heating curves (see section 5 ) on several preparations were investigated, with interesting results. All the heating curves gave the metasilicate-orthosilicate eutectic arrest, at 1 0 2 I' to 1 0 2 3 O , which agrees well with the temperature 1021' obtained by quenching. Preparations which contained appreciable quantities of COZ when introduced
into the furnace gave a quite sharp arrest at 96z0. On reapeated heatings this arrest gradually disappeared and, a t the same time, the upper arrest, corresponding to the dissociation of the orthosilicate, gradually rose, toward 1118' (see Table I1 and Fig. 2 ) . It is evident that as long as COz is present in the preparation, the composition is properly represented in the ternary system Na20-SiO2-C02,and the evanescent arrest at 962' then represents a n invariant point in this ternary system. Also, in such case, the lowering of the dissociation temperature of zXa2O.SiO2is to be expected, since we are dealing with a univariant curve in a ternary system, the amount of CO, being the composition variable, rather than with an invariant point in a binary system, I n all cases in which the 962' arrest was absent the dissociation temperature of zKan0.Si02 was found to be near 1118'. The best deter-
THE SYSTEM SODIUM OXIDE-SILICA
1591
minations were obtained by adding known amounts of the bottom portion of preparation j 7 j to known amounts of preparation 564. These preparations, as can be seen by reference to Table I, were practically Con-free. The niixtures of the two did not give the 962' arrest. By means of these two preparations it was established that the 1118' arrest remains constant for mixtures containing in excess of 59.3 weight' per cent T\;anO,i.e. less than 40.7 per cent S O n , and up to the orthosilicate composition. Beyond this composition it was not possible to make determinations, for reasons already stated (sect,ion 4). With preparation 564 a faint' arrest was also noted beyond I I IS', a t I 135', which would correspond to a point on the liquidus of the dissociation product of the sodium orthosilicate. I have not attempted to determine this liquidus for mixtures richer in S a n Owith the present arrangement. The Na?O-rich liquids are very mobile, moisten Pt readily, and creep up the sides of the container and up the thermocouple wires a t elevated temperatures so rapidly that the thermocouple circuit is soon short-circuited at a point outside the charge. There is little reason to doubt the correctness of interpretation of the results thus far obtained, but a confirmation under exactly controlled conditions is needed, particularly in the direction of a study of the dissociation pressures of Na2C03,with or Rlthout SiOs added. Such an investigation is planned. 13. General. Two of the binary compounds encountered in this system, the disilicate (Sa?0.0Si02)and the metasilicate (NazO.SiO?j are stable a t their melting points. The third, the orthosilicate (2Nan0.Sidzjdecomposes before its melting point is reached. The liquidus curves all indicate considerable dissociation of the liquid. This question has been discussed by Morey and Bowen for the disilicates. The question may be raised regarding what is the most st'able silicate in any given binary system, and whether any appreciable amount of the silicate compounds exists in the melts a t temperatures much above the melting points of these compounds. The data thus far obtained for the alkali silicate systems point to a large dissociation of al! the silicate compounds encountered a t their melting points, and to a progressive change of stability from the metasilicates to the disilicates as the atomic weight of the alkali increases. I n the case of Li and Ka,the metasilicate is unquestionably the most' stable compound, while in the case of K, it is the disilicat'e. The results for Rb silicates, now being studied, indicate also that the disilicate is the most stable compound. If we extend these considerat m s to the cases of the alkaline earth silicates, we find the results generally in accord with the above trend. The alkaline earth orthosilicates are probably all stable at their melting points, but, the metasilicates increase in stability as the atomic weight increases, hlg metasilicate being t,he only one with an incongruent melting point. -4s to dissociation in the liquid phase, t h e data are hardly precise enough to justify definite conclusions.
F. C. KRACEK
I592
T.4BLE
1
A. Preparations and Liquidus Determinations by Quenching Number Percent Si02 weight mol
t"C
Initial condition and time at constant temperature
Condition of quenched samples and other remarks.
Made up to be zNaZO.SiOZ. Melt was very brown, retained 3.97c Con. Erratic results on quenching. Used for heating curves. Made up to be zNazO.SiOZ. Preparation was heated a t 1050' and was about half crystalline at this temperature. The crystals which had settled to t,he bottom were separated (jTjb) from the top portion which contained most of the COZ, after cooling. 57jt 31.5
-
57jb 3 2 . 6
33.3
112j
cr. 30 in. cr. 3 0 n i . cr. 30 m.
zSayO.SiO?. 2 Sa20.Si02 and some glass. S o birefringent crystals, much attack by moisture from air. Contained less than 0.2% C O Z before using. Used also in heating curves.
111j
cr. 30"'.
0.3CC
1122
cr.
30111.
1114 cr.
30111.
1113 1120
564
40.0
40.8
Top portion of j i j . Contained 47; COa. Erratic results on quenching.
-
1125
112j
cr. 3om. cr. goin.
CO, before using. Orthosilicate, no glass. 1.0 orthosilicate; glass, some isotropic material of apparently higher indes than that of the glass. Much corrosion. Less than 0.2~: CO, before using. Aluch orthosilicate, glass doubtful. JJ-ell developed orthosilicate. S o orthosilicate, much corrosion.
T H E SYSTEM SODIUM OXIDE-SILICA
1593
TABLE I (Continued) A. Preparations and Liquidus Determinations by Quenching Percent Si02 weight mol
t"C
Initial condition and time a t constant temperature
Condition of quenched samples and other remarks.
42. I
42 . 9
1036 cr. Ih. Orthosilicate and some glass. 1061 cr. Ih. Orthosilicate and more glass 1069 cr. Ih. All glass. Orthosilicate Liquidus 106j o
43
44.35
gl. 30m. Metasilicate and glass. gl. 30m. Rare metasilicate and glass. 1038 gl. 30m. All glass. Metasilicate Liquidus 103jo
j j
1031 1034
1019 gl.
Ih.
1023 gl. Ih. Eutectic 1021'
45. I I
45 89
Ioj2 gl. Ih. Much metasilicate, and glass. 1063 gl. 30m. Rare metasilicate and glass. 1071 gl. 30m. Glass. IIetasilicate Liquidus 1065' IOIO
gl. Ih.
1022 gl. Ih. Eutectic 1021'
49 18 49 97
jj 91
58 68
59.98 60.74
ill1 crystalline, orthosilicate and metasilicate. Only metasilicate, and glass.
All crystalline, orthosilicate and metasilicate. Metasilicate and little glass.
1088 gl.
Ih. 1089 gl. Ih. 1090 gl. Ih.
All crystalline, metasilicate. Ifetasilicate, may be some glass. Practically all glass. This preparation is almost pure metasilicate. Metasilicate Liquidus 1089' 935 gl. Ih. 950 gl. Ih.
Much metasilicate, and glass. Evenly distributed metasilicate and much glass. 9;j gl. rh. Very rare crystals. AIetasilicate Liquidus 9 js'
901 gl. 3h. Metasilicate and glass. 907 gl. 3h. A11 glass. Metasilicate Liquidus 904'
F. C. KRACEK
I594
TABLE I (Continued) -1. Preparations and Liquidus Determinations by Quenching Sumber
Percent SiOI weight mol
t"C
844
Initial condition and time a t constant temperature
gl. 3h.
848 gl. 3h. Eutectic 846' jj7
61 ,go 62 . 2 4
gl. 862 gl.
Condition of quenrhed samplrf and other remarks.
XI1 crystalline, metasilicate and disilicat e. Metasilicate and glass.
Metasilicate and glass. Very rare crystals of metasilicate. 863 gl. 2h. All glass. Metasilicate Liquidus 862"
8jj
rh. xh.
846 gl. 4h. 849 gl. 4h.
Metasilicate, disilicate, may be some glass. Metasilicate, some glass.
559
62.49
63.23
Disilicate, metasilicat'e, may be some glass. 850 gl. gh. Rare disilicate, glass. 8 5 2 gl. gh. All glais. Disilicate Liquidus 85 I O
350
65.94
66.65
870
845 gl.
14h.
Disilicate, no glass. Disilicate, little glass. Glass, very rare disilicafe. This preparation is nearly pure disilicate. Ilisilicate Liquidus 874' 874
gl. gl. 875 gl.
2h. 18h. 6h.
561
71.49
72.13
827 gl. 828 gl. 830 gl. Disilicate
4h. Disilicate and glass. 4h. Rare disilicate, glass. 4h. All glass. Liquidus 829'
5j1
73.00
73.62
803 cr. cr. cr. Disilicate
3h. Disilicate, glass. 3h. Glass, rare disilicate. 3h. All glass. Liquidus 806'
805 808
Previously crystallized dry. Shows disilicate, tridymite, and some glass. 785 cr. 18h. Same as above. Shows only disilicate and glass. Eutectic (metastable) 782'
780
cr.
2h.
THE SYSTEM SODIUM OXIDE-SILICA
I595
TABLE I (Continued) A. Preparations and Liquidus Determinations by Quenching Number
jj6
Percent Si09 weight mol
74.00
74.61
t"C
Initial Condition and time a t constant temperature
788 cr. 7 9 j cr. 800 cr. Disilicate
Condition of quenched samples and other remarks.
3h. Disilicate and glass. 18h. Glass, very rare disilicate. 18h. All glass. Liquidus 795'
778 cr. jh. 783 cr. jh. 790 b. cr. 4h.
Disilicate, tridymite, glass. Disilicate, glass. Previously crystallized in bomb with little H20a t 540'. Shows disilicate, quartz, glass. 793 b. cr. 4h. Quartz, little disilicate, glass. 795 b. cr. 4h. Quartz, glass, no disilicate. Quartz Eutectic 793' Tridymite Eutectic below 783' True liquidus probably quartz as primary phase. 83 j b. cr. 13h. Glass, much eroded quartz. 835 d. cr. 18h. Glass, tiny grains of well formed quartz. 840 b. cr. zoh. Glass, no equilibrium quartz, only a few much eroded grains 840 d. cr. 24h. Glass only. Quartz Liquidus 838' 789 b. cr. 6h.
Well formed disilicate, quartz, some glass. 792 b. cr. 6h. Little disilicate, quartz, glass. 794 b. cr. 6h. Quartz, glass, no disilicate. Quartz Eutectic 793'
571 (contd.)
554
75.98
76.56
894 cr. 12h. Tridymite and glass. 904 cr. 8h. Very rare tridymite and glass 910 cr. 8h. All glass. Tridymite Liquidus 90 j'
b. cr. 6h. 793 b. cr. 6h. 792
Disilicate, quartz, glass. Quartz, glass, no disilicate.
993 cr. 6h. Tridymite, glass. 997 cr. 6h. All glass. Tridymite Liquidus 995'
I596
F. C . KRACEK
TABLE I (Continued) A. Preparations and Liquidus Determinations by Quenching Number
Percent Si02 weight mol
t"C
791 793 552
78 97
79 50
Initial condition and time at constant temperature
b. cr. 6h. b. cr. 6h.
Condition of quenched samples and other remarks.
Disilicate, quartz, glass. Only quartz and glass.
1064 cr. rzh. Tridymite, glass. 1075 cr. gh. Extremely rare tridymite. 1080 cr. gh. All glass. Tridymite Liquidus 1075" 7 7 7 d. cr. 6h. Disilicate, tridymite, some glass. 780 d. cr. 6h. Disilicate, tridymite, glass. 784 d. cr. 6h. Tridymite, glass. Tridymite Eutectic 782'
788 b. cr. 6h. 791 b. cr. 6h. 793 b. cr. 6h. 794 b. cr. 6h. Quartz Eutectic
Quartz, Quartz, Quartz, Quartz,
rare disilicate, glass. more disilicate, glass. rare disilicate, glass. no disilicate, glass.
793'
570
8j 44
85 83
gl. Ih. Kell formed tridymite, glass. 1364 gl. Ih. Rare tridymite, glass. 1368 gl. Ih. All glass. Tridymite Liquidus 1365'
560
87 97
88 30
gl. gl. 1 4 5 2 gl. Tridymite
1344
1415 1447
zom. hluch tridymite, glass. zom. Tridymite and glass. 20m. All glass. Liquidus 1450'
TABLE I B. Optical Properties of Compounds occurring in the System KapO - SiOz. z Sa,O.SiO,. (Determination by H. E. Xerwin.) y = 1.537, a: = 1 . 5 2 These are the highest and lowest values of y and a: observed. Lamellar
~
‘597
THE SYSTEM SODIUM OXIDE-SILICA
twinning, making determinations of interference figure impossible. y is perpendicular to twinning lines. Extinction angle is up t o 8’ with respect t o twinning plane.
3’azO.SiO~. (Norey and Bowen, op. cit.) Orthorhombic needles, prismatic cleavage in zone of y, optic axial angle 2Y is large, optical character is negative. Refractive indices: y = 1.528, fl = 1.520, cy = 1.513. NazO.zSz02. (Norey and Bowen, op. cit.) Orthorhombic plates and needles, pinacoidal cleavage / I $ and yo; optic axial angle 2J’ = j o - 5 j 0 , optical character is negative. Refractive indices (redetermination by K. L. Bowen):y = 1.508, cy = 1.497.
TABLE I1 Heating Curve Study of the Orthosilicate Region N-eight Percent Si02
Preparation Number j62
Cfl
Heat Arrests, t”C.
962,
1021,
1103
(962),
1022,
1115
0.1
First heating Fourth ” Sixth ”
-, 1022, 1115, 1130
-, -, 0 . 9 of 564
Remarks
962, 1023, 1082
32
40 . o
564
Curve’ Number
39.3
1021,
1116, 1132
1023, 1118, 1135
First *’ Second ” Third ’’
-, 1 0 2 1 , 1 1 1 5 ,
-
First
’’
-,
Second Third
”
First Second Third
”
of 575b 1118,
-
- 1022, I I I j ,
-
-,
-
1023,
,
”
0 . 6 of 564 0.4
Of
575b
37 . o
1022, 1 1 1 6 ,
-, 1 0 2 1 ,
-, 575b
1
72.6
Refers to curves in Fig.
1023, 1118,
-
1021,
1115,
-
-, 1023,
1117,
-
-,
1118,
-
(962),
2.
1117,
1021,
First Second Third
’’ ”
” ” ”
1598
I'. C. KRBCEK
TABLE I11 Invariant Points in the System S a 2 0 - S i 0 2 Type
9.
Reaction Eutectic Melting Eutectic Nelting Eutectic Eutectic Inversion Unmixing
IO.
Unmixing
I.
2.
3. 4.
5.
6. 7. 8.
Inversion Inversion 13. Melting I I.
I2 .
Phases
+
zNa2O.SiO2%Sa20 liquid zSa20.Si02,iYa20.Si02,liquid ?;a20.Si02, liquid S a 2 0 . S i 0 2 ,Na?0.nSi02,liquid Sa2O.2SiO2,liquid Ka20.zSi02,quartz, liquid Sa20.zSiO2, tridymite, liquid cuSa20.zSi0&$Sa2O.z Si02 Solid solutions of iYa20.nSi02 with excess S a 2 0 Solid solutions of Ka20.2Si02 with excess S O 2 Quartz, tridymite, liquid Tridymite, crietobalite, liquid Cristobalite, liquid
t"C. 1118 1022 I 089
Percent Si02 in liquid
40.7 43 1 49.21 '
62.I 6 j .96 73 .9 74.6
*13.5
88.j 100.0
(m) denotes metasitable.
Summary The system Sa?O-Si02 contains three binary compounds, the orthosilicate ( a S a 2 0 . S i 0 2 )the , metasilicate iSa?O.SiOz)and the disilicate (Sa&. zSi02). The orthosilicate decomposes at 1 1 2 0 ' rt j o(in mixtures containing less than 40.7 per cent SiO?) before its melting point is reached. A new value, 1089' =t o,;', is given for the melting point of the metasilicate, and Morey and Bowcn's value for the melting point of the disilicate, 874' =t IO, is confirmed. The ortho-metasilicate eutectic is at 43.1 per cent Si02 and 1 0 2 2 O , and the nieta-disilicate eutectic is at 6 2 . 1 per cent Si02and 846". The stable eutectic betlveen the di*ilicate and quartz is located a t i 3 . 9 per cent SiO. and ;93', and the metahtnbk one between the diailicate and tridymite at 74.6 per cent SiOz and j S z n . Sodium disilicate has a reversible inversion at 678'. -ittemperatures above 706" it takes up excess S a 2 0 ,above 768' exce into solid solution. The problenis connected with the investigation of the orthosilicate region are discussed in detail. Acknowledgment I am indebted t o niy colleagues G . JJ-. Morey, €1. E. l l e r w i n and E. G. Zies for help and discussion in connection with certain phases of this work. Geopli ysicul Luboruioui, Cnriirgie I i ( s ! i ! i i t i o i , oj l i - d i i t g ! i m ) February, 1980.
