High-temperature study and thermal expansion of phlogopite

Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have bee n determined in the temperature interval of 27-1050 degreesC by X-ray powde r diffraction technique. Expansion rates vary discontinuously with temperat ure with a break at 412 degreesC. Below this temperature, the linear expans ions (alpha) for a, b and c axis lengths are 3.74 x 10(-5) K-1, 1.09 x 10(- 5) K-1, and 1.19 x 10(-5) K-1, respectively, and above that they are 0.86 x 10(-5) K-1, 0.80 x 10(-5) K-1, and 1.93 x 10(-5) K-1. The volume thermal e xpansion coefficients are 6.26 x 10(-5) K-1 and 3.71 x 10(-5) K-1 for low-t emperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with temperature could be due to the diff erent mode of structural changes. Thermogravimetric analysis of the sample indicates the oxidation of iron in the temperature range of 500-600 degrees C and dehydroxylation as well as decomposition of phlogopite in the tempera ture range of 900-1200 degreesC.