V41B-3076
Effect of the Na:K Ratio on Melt Viscosity within the SiO2-NaAlSiO4-KAlSiO4 System

Thursday, 17 December 2015
Poster Hall (Moscone South)
Rebecca Anne Smith1, Genevieve Robert1, Paige Guevarra1 and Marlene Dreizler2, (1)Bates College, Geology, Lewiston, ME, United States, (2)University of Kiel, Experimental and Theoretical Petrology, Kiel, Germany
Abstract:
We synthesized eight compositions in the SiO2-NaAlSiO4-KAlSiO4 system, four along the feldspathoid join, and four along the pyroxene join, to test the effects of the Na:K ratio on the viscosity and heat capacity of these melts. Na:K ratios include [100:0], [75:25], [50:50], and [25:75].

Synthesis of each composition involved weighing and mixing powdered SiO2, Al2O3, Na2(CO3) and K2(CO3), slow decarbonation of samples in a Lindberg oven, and melting between 1100°C and 1745°C in a high-temperature box furnace. We measured the heat capacity of each sample, from room temperature to 1500°C, using a Netzsch 404 F1 Pegasus differential scanning calorimeter. We measured the viscosities of three of the feldspathoid compositions, NaAlSiO4, Na0.75K0.25AlSiO4, and Na0.5K0.5AlSiO4, and one of the pyroxene compositions, Na0.5K0.5AlSi2O6, in a Brookfield rotating viscometer at temperatures ranging from 1400°C to 1580°C.

NaAlSiO4 has a calorimetric glass transition temperature (Tg) of 833°C, and a viscosity of 101.58 Pa.s at 1557°C. Na0.75K0.25AlSiO4 has a Tg of 830°C, and a viscosity of 101.80 Pa.s at 1568°C. Na0.5K0.5AlSi04 has a Tg of 864°C, and a viscosity of 102.12 Pa.s at 1556°C. Na0.25K0.75AlSi04 has a Tg of 912°C. NaAlSi2O6 has a Tg of 834°C. Na0.75K0.25AlSi2O6 has a Tg of 834°C. Na0.5K0.5AlSi2O6 has a Tg of 845°C, and a viscosity of 103.11 Pa.s at 1553°C. Na0.25K0.75AlSi2O6 has a Tg of 899°C. Increase in potassium results in an increase in the calorimetric glass transition temperature, for both the feldspathoid and pyroxene compositions. K-rich pyroxene compositions have a slightly lower Tg than K-rich feldspathoid compositions. In the high-temperature range, however, the viscosity of feldspathoid is approximately one order of magnitude lower than that of pyroxene for the same Na:K ratio of 50:50. Additional experiments will be performed to determine the low-temperature viscosities of all samples using parallel-plate viscometry.

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