Sea water contains about 0.12 ppm rubidium, most of which is incorporated in the potassium minerals of the evaporite sequence upon evaporation. The amount of rubidium entering a mineral such as sylvite will depend on the value of the partition coefficient, D = (Rb/K) _S /(Rb/K) _L , where (Rb/K) _S is the ratio of rubidium to potassium in the sylvite crystals, and (Rb/K) _L , the ratio of rubidium to potassium in sea water. The temperature dependence of the coefficient for the partition of rubidium between crystals and saturated aqueous solutions of potassium chloride was investigated experimentally. Results at six different temperatures in the range 0° to 100°C indicate that the value of D is related to temperature by the equation, In D = 0.2 77 − 0.8168 (10³/T°K) ± 0.001.

The effect of occluded mother liquor on the experimental value of D was also investigated. Under normal conditions of growth potassium chloride crystals have a cubic habit and tend to be cloudy, owing to the presence of liquid occlusions. If a small amount of bismuth is added to the potassium chloride solution, the resulting crystals develop an octahedral habit and are clear. A comparison of the values of D obtained with and without bismuth in solution at 40°C showed no essential difference within the limits of experimental error. The author concludes that the amount of liquid occlusion in sylvite crystals is insufficient to effect the value of D significantly.