Cummingtonite from the Gouverneur, New York, talc district (Sample No. U.S.N.M. 115046) with the composition, (Ca0.36Na0.06Mn0.96Mg0.57) Mg5Si8O22 (OH)2, unit cell parameters a = 9.550 ?? 0.001, b = 18.007 ?? 0.003, c = 5.298 ?? 0.001 A ??, ?? = 102??39??? ?? 1???, and space group P21/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 to C2/m symmetry. The transition appears to be analogous to the P21/c to C2/c transition between 700?? and 800??C discovered recently in (Mg0.30Fe0.70) 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, (Ca0.04Fe0.78Mg0.18)SiO3) at about 670??C. This is a significantly lower temperature than was found for the Ca-poor clinopyroxene. ?? 1970.