CRYSTAL STRUCTURE OF A P21/M FERROMAGNESIAN CUMMINGTONITE AT 140 K

Abstract

We report a structure refinement of a P2/m ferromagnesian cummingtonite crystal with Fe/(Mg + Fe) = 0.37 at 140 K; this crystal was inverted from the C2/m structure, which is stable at room temperature. The phase-transition temperature is -240 K. The most significant structural changes associated with the C2/m-to-P2/m phase transition are as follows: (1) one crystallographically independent O-rotated silicate chain becomes two, with the A chain S rotated and the B chain 0 rotated; (2) the M4 site collapses, with a marked decrease (0.020 A) in the average M4-06 bond distance; and (3) the largest de-crease in the isotropic displacement factor is for the M4 cation. From 295 to 140 K, total changes of 14 and 11° occur in the 05-06-05 kinking angles of the A and B chains, respectively. The volume of the M1 octahedron decreases more than that of the other three M06 polyhedra, whereas the volume of the M3 octahedron is essentially unchanged. All four M06 polyhedra become more regular as a result of the C2/m-to-P2/m transfor-mation. These results provide new evidence that the effective size of the M4 cation controls the relative stability of the C2/m and P2/m structures. The results also provide a new explanation for the unresolved broadening of the cummingtonite Raman spectra measured at low temperatures by Wang et al. (1988).