Cummingtonite: A reversible, nonquenchable transition from P21/m to C2/m symmetry
Cummingtonite from the Gouverneur, New York, talc district (Sample No. U.S.N.M. 115046) with the composition, (Ca 0.36 Na 0.06 Mn 0.96 Mg 0.57 ) Mg 5 Si 8 O 22 (OH) 2 , unit cell parametersa = 9.550 ± 0.001, b = 18.007 ± 0.003, c = 5.298 ± 0.001Å, β = 102°39′ ± 1′, and space groupP2 1 /m, has been used in a high-temperature, single-crystal, X-ray diffraction study. When diffraction patterns are taken of the crystals heated in a sealed silica capillary to 45°C or higher, these reflections disappear, indicating a transition toC2/m symmetry. The transition appears to be analogous to theP2 1 /ctoC2/c transition between 700° and 800°C discovered recently in (Mg 0.30 Fe 0.70 ) SiO 3 clinopyroxene. In addition to the cummingtonite transition, we have also found a transition in pigeonite at higher temperatures. The transition occurs in this pigeonite (Gunflint Iron Formation, Minnesota, (Ca 0.04 Fe 0.78 Mg 0.18 )SiO 3 ) at about 670°C. This is a significantly lower temperature than was found for the Ca-poor clinopyroxene.