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_0.36Na_0.06Mn_0.96Mg_0.57) Mg₅Si₈O₂₂ (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 groupP2₁/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 theP2₁/ctoC2/c transition between 700° and 800°C discovered recently in (Mg_0.30Fe_0.70) SiO₃ 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_0.04Fe_0.78Mg_0.18)SiO₃) at about 670°C. This is a significantly lower temperature than was found for the Ca-poor clinopyroxene.