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.