NOTES
3360
Acknowledgments. We gratefully acknowledge the assistance of Larry Feldman, Earl Hansen, and Jay Rynbrandt in solution preparation and data treatment.
I 1000
I
I
900
The System Potassium 8 00
Carbonate-Magnesium Carbonate
-Y 7
by S. E. Ragone, R. K. Datta, Della M. Roy, and 0. F. Tuttle
W
700
a 3
I-
Department of Geochemistry and Mineralogli and Materials Research Labbratory, The Pennsylvania State U n h m a y , UniversBy Park, Pennsylvania (Received June 88,1966)
As a part of an investigation of glass-forming carbonate systems,1 phase equilibria have been determined for the K2COs-MgC0a join of the MgO-K20-C02 system. Starting materials were Fisher certified reagent grade chemicals (see Table I). The usual hydrothermal equipment2 with cold-seal pressure vessels was used, enabling an over-all temperature control of h lo", with a pressure control of +500 psi. Initially, samples were K2C03-iClgC03 mixtures held in sealed Au capsules, but better results were obtained with KzCOr MgO mixtures in unsealed Au capsules open to COZ pressure so that complete carbonation could take place.
;600 W
a I
E
500
n + . +
+
+
+
460' t
* IO
t
400
K2C0,
t MgC03
t I:I
I
300
K,C03 20 K2C03
I : I + MgCO,
40
M 0L
60
E
80
/'e
Figure 1. Temperature-composition projection of the pseudobinary system K&03-MgC03 in equilibrium with 15,000 psi of COZ. Because of experimental difficulty, the melting point of K&03 at this pressure has not been checked. Lower part of diagram represents equilibria a t about 500 psi, where the 1:1 compound,EKzC03.hfgCOs is stable.
Table I : Analysis of Reagents KzCOs
MIZO
Ba, Not, Ca, K, Na, Sr Iron (Fe) Chloride (Cl) Sulfate/sulfite (aS
sod
Heavy metals m (Pb)
P.T. 0.002% 0.005% 0.002%
0.002%
Chloride/chlorate C1) Sodium (Na) Heavy metals &s (Pb 1 Sulfur compounds
(as sod
P.T. P.T. 0.0001% 0.0003%
The quenching method was used, in which a charge is held at the desired temperature and pressure for a period of time, quenched rapidly in water to room temperature under the pressure of the run,and examined under a petrographic microscope and by X-ray diffraction. Glass-forming compositions are particularly suited to the quenching method, as they yield a homogeneous glass when quenched from above the liquidus. In borderline cases, it is frequently possible to distinguish between primary phase and quench phase crystal^,^ the latter usually appearing brownish and The Journal of Physical Chemistry
slightly birefringent under the polarizing microscope, or having dendritic and spherulitic growths. Failure to distinguish between primary and secondary (quench) magnesite (MgC03) crystals in preliminary work caused the dissociation temperature to be placed about 100" too high. Distinction can be made on the basis of microscopic form and birefringence. Quench magnesite appears as rhombohedral cleavage fragments formed in preparing slides, the quench mass breaking along cleavage planes when ground. Primary magnesite displays no such cleavage, forming subhedral single crystals instead. Proper high order white birefringence is observed in all primary crystals while secondary crystals are usually high first to second order. Wavy, irregular isochromes provide another indication of secondary quench growth. All compositions near the 1 :1 ratio KZC03-MgC03 (1) R. K. Datta, D. M. Roy, S. P. Faile, and 0. F. Tuttle, J . A m . Ceram. SOC.,47 (3), 153 (1964). (2) R. Roy and 0. F. Tuttle, "Physics and Chemistry of the Earth," Vol. I, Pergamon Press, London, 1956, Chapter VI, pp 138-180. (3) P. J. Wyllie and 0. F. Tuttle, J. Petrol., 1 , 1 (1960).
NOTES
3361
and give rise to an invariant point involving the five phases: AIgCO3,MgO, K&Oa, liquid, and vapor. At temperatures above 755", the system is definitely ternary and the probable phase relations are shown in Figure 2e. At 755", there is an isobaric invariant point involving 1fgC03,MgO, vapor, and liquid (Figure 2d). This represents one point on a univariant pressuretemperature curve for the dissociation of RIgC03 by the reaction MgC03 --t MgO vapor, and as this temperature falls near the value for the dissociation curve determined by Harker and T ~ t t l e ,it~ is asassumed that there is very little, if any, solid solution of I
