T H E SYSTEhI POTASGIURI -\.IEThSILIChTE-SILIC.4 BY F. C. KRACEK, K. L. BOWEN, AND G. Fy. YOREY
Introduction The binary system K2SiOs-SiOzwas studied as a preliminary to the ternary systems K20-CaO-Si02 and K2SiO3-?;azSiO3-Si02, both of which are now completed. While this binary system had never been investigated it was thought that the essential features could be deduced from the study of the ternary system H20-K2SiO3-SiO2 by Norey and Fenner.' They determined the isothermal polybaric saturation curves from 28 j oto 6oo0C, and also the isobaric polythernial saturation curve at one atmosphere pressure of water vapor for mixtures containing Si02to K 2 0 in the ratios of approximately I :I to 4:1, that is, from the metasilicate composition to beyond the tetrasilicate. The latter curve was believed not t o be essentially different from the melting point curve of the binary system K2SiO3-SiO2,the expected difference being that of the melting point lowering caused by the solution of H 2 0 in the molten liquid. This melting point lowering was determined for the K20.Si02 and Kz0.2SiOr compositions, whose melting points were found to be 976' and 1041',~ while in presence of one atmosphere of H 2 0 vapor the values were 942' and 1034'C respectively. By extrapolation from the hydrous system the binary eutectic between Kr0.nSi02 and quartz was placed at 52jOC and 71.8 per cent SiO?, that is, at a SiOr/K20 ratio of 4.0:1. The present study of the anhydrous system, however, brought to light important additional features, making separate publication of the system desirable. Since the appearance of the work of Morey and Fenner3 there has been little other study of this system. Pukall' prepared glasses of SiO2/K20ratios of I : I , Z : I , 3:1, 4 : 1 , . 5 : 1and 6:1 by'heating mixtures of K H C 0 3 and SiO? without any attempts at systematic crystallization, and considered that the mixtures so prepared, whether crystalline or glassy, were compounds of the corresponding formulae, an altogether incompetent conclusion. Rice5 studied the "deformation" eutectic temperatures of cones of K20-Si02 mixtures prepared by melting I X 0 3 and Si02 to glasses, which were then powdered and sintered to make the cones. He gives 780' for the eutectic at 45 per cent SiOp, 5 5 per cent K 2 0 , with a I O per cent tolerance in composition, and 880' for a eutectic at 82. j per cent Sios, with a j per cent tolerance. The former temperature checks closely with our results.
' l l o r e y and Fenner:
J. Am. Chem. SOC., 39, 1173 (191;)
Compare p. 1871 l l o r e g and Fenner: op. c i ~ . ;Alorey: J. .Im. Chem. Soc., 36, allgem. Chem., 86, 305 (1914). Pukall: Silikat Z.,2, 6 j (1914). a Rice: J. Am. Ceram. $oc., 6 , jzj (1923).
2 1j
(1914); Z. anorg.
1858
F. C. KRACEK, S . L. BOWES, AND G. W. MOREY
Experimental I. Preparation of Materials. The mixtures studied were prepared in the form of glasses by decomposing K X O 3 with quartz in platinum crucibles. The quartz was partly that used in previous silicate syntheses' and gave a residue of 0.07 per cent on evaporation with HF and H2S04;partly another lot which gave a 0.05 per cent residue. The K2C03was a specially purified material] some of which was derived from especially pure KHC03. I n preparing the glasses it is advisable to allow the well mixed ingredients to sinter a t 700 to 800'C for several hours, in order to enable the reaction to proceed as far as possible without formation of appreciable amounts of liquid until most of the COSis driven off. The temperature then can be gradually raised until the mixtures become fluid enough to enable bubbles of the remaining COz to escape, without excessive frothing. For accurate synthesis of glasses of desired composition it is essential to guard against two main sources of loss, namely, mechanical loss caused by the bursting of bubbles of the escaping COS, and secondly, loss by volatilization of K 2 0 from the reacting mixture. The first loss can be kept to a minimum by accurate temperature control of the melting charge. The second loss is more difficult to control, but it also can be minimized if the melting after the preliminary sintering is done in electric furnaces where there are no violent gas currents to carry off the K 2 0 fumes, in preference to using gas furnaces. Without the preliminary sintering the violent escape of COz from the exceedingly viscous glasses formed causes such excessive frothing as to make the preparation of glasses of desired composition uncertain. After the first melting the glasses were crushed in a diamond mortar to pass a @-mesh sieve and examined under the microscope for inequalities in composition. When necessary, they were then remelted, and the operation repeated until homogeneous mixtures were obtained. The glasses were usually prepared of known composition by accurately weighing the amounts of ingredients used, followed by carefully checking the weight after each remelting. The slight loss by volatilization, assumed to be due to KzO alone, was corrected for by the addition of known amounts of KZCO3. Numerous analyses were made, usually by the method described by Morey and Fenner,l in which the powdered glass was treated twice with HF, the residue dried at 1 2jo and weighed as KnSiFs. This method is exceptionally convenient and accurate in this case. 4 s a special precaution most of the analyses were checked by conversion to K2S01in the usual manner. 2. Melting Point Determination Methods. Most of the thermal study was carried out by the quenching method3 using materials which had previously been crystallized by appropriate heat treatment, when necessary. In this method small quantities of the powdered material] amounting to a few
'Morey and Bowen: J. Phys. Chem., 28, 1167 (1924);J. SOC. Glass Techn., 9, (1925). * Morey and Fenner: op. cit. Shepherd and Rankin: Am. J. Sei., 28, 293 (1909)
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THE SYSTEM POTASSIUM METASILICATE-SILICA
1859
milligrams, are wrapped in thin Pt foil and held suspended in a furnace in close proximity to a thermocouple, at a desired constant temperature long enough for equilibrium to be attained. The charge is then quickly chilled to freeze the equilibrium, and examined under the microscope. The operation is repeated until a temperature is fixed such that just above it the charge remains in glassy condition, and just below it crystals are present. The crystals can be identified by the ordinary petrographic methods. I n addition to the quenching experiments, heating curves were made on several preparations by the differential thermocouple method. These will be described in detail later (see p. 1874). The temperatures were measured by means of P t vs. 90 Pt, I O Rh thermocouples, using a highly sensitive potentiometer system. The thermocouples were calibrated a t the fixed points NaCl 800.4’,’ gold 1062.6’, Li2Si032IZOI’, diopside 1391’,~ and anorthite 1550’~ and used with t h e aid of the tables of Johnston and Adams3 together with an appropriate deviation curve. The couples were checked frequently a t the NaCl and LizSiOa points to ensure that they had not suffered by contamination. The furnaces used were wound with Pt wire on alundum tubes and were maintained a t constant temperature by means of the thermoregulator developed by Roberts4 which has proved invaluable in the work of this Laboratory. The quenching charge was placed close to the hot thermocouple junction a t the point in the furnace where there is practically no temperature gradient. 3. Crystallization of the Preparations. The length of time required for the initial crystallization and for the attainment of equilibrium in the melt, and the speed with which it was necessary to chill the charge in order to freeze the equilibrium varied greatly with the composition. Mixtures whose composition lies between K20.Si02and K20.zSiOz, and those containing in excess of 90 per cent Si02 crystallize rapidly when the glass is held near the liquidus temperature; to freeze the equilibrium in such mixtures it was found necessary in many cases to quench the charges by dropping them into cold mercury. All other mixtures could be chilled quickly enough by simply lifting them rapidly out of the furnace. Preparations whose compositions are near that of K20.2SiOz devitrify readily enough for crystals to grow while the powdered glass is being brought up to the liquidus temperature. With increasing percentage of SiOz the crystallization velocity falls off very rapidly. Mixtures near K20.4SiOz are exceedingly difficult t o crystallize, and indeed, it was found that without seeding with the appropriate crystalline phase even several weeks’ time near the equilibrium temperature was insufficient to bring about the growth of an appreciable quantity of crystals. The pure crystalline compound K20.4Si02 Roberts: Phys. Rev., ( 2 ) 23, 386 (1924). Day, Sosman and Allen: “High Temperature Gas Thermometry,” Carnegie Institution of Kashington Publ. 157 (1911). Johnston and Adams: Am. J. Sci., 33, 534 (1912). * Roberts: J. Opt. SOC. America, etc., 11, 171 (1925).
1860
F. C. KRACEK, S . L. BOWES, AND G. IT.MOREY
melts readily, but to attain equilibrium between these crystals and a melt of a different composition several days may be required. Hydrothermal crystallization was found useful in inducing the initial formation of crystals in mixtures near the I
