Cummingtonite: A reversible, nonquenchable transition from P21/m to C2/m symmetry
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Abstract
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 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 (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.
| Publication type | Article |
|---|---|
| Publication Subtype | Journal Article |
| Title | Cummingtonite: A reversible, nonquenchable transition from P21/m to C2/m symmetry |
| Series title | Earth and Planetary Science Letters |
| DOI | 10.1016/0012-821X(70)90150-0 |
| Volume | 8 |
| Issue | 6 |
| Year Published | 1970 |
| Language | English |
| Publisher | Elsevier |
| Description | 3 p. |
| First page | 448 |
| Last page | 450 |