Hawthorne, FC; Griep, JL; Curtis, L; ,
The three-amphibole assemblage gedrite-cummingtonito-hornblende occurs in an amphibolite sequence at the margin of the Tallan Lake sill, Peterborough County, Ontario. Mutual contacts, coarse, oriented intergrowths and mutual exsolution suggest a stable equilibrium association. Electron-microprobe analyses and calculation on the basis of 23 oxygens per formula unit using a Fe3+/(Fe2+ +Fe3+) ratio of 0.15 gave the following compositions: gedrite (K-0.009 Na-0.441) (Na-0.133 Ca-0.122 Mg-3.312 Mn-0.061 Fe2+ (2.119) Fe3+ (0.374) Ti-0.064 Al-0.815) (Si-6.366 Al-1.634) O-22 (OH,F)(2); cummingtonite, (K-0.002 Na-0.013) (Na-0.013 Ca-0.123 Mg-4.247 Mn-0.056 Fe-2.329(2+) Fe-0.166(3+) Ti-0.015 Al-0.051) (Si-7.751 Al-0.249) O-22 (OH, F)(2); hornblende, (K-0.022 Na-0.457)(Na-0.134 Ca-1.422 Mg-2.676 Mn-0.036 Fe2+ (1535) Fe3+ (0.271) Ti-0.086 Al-0.840) (Si-6.372 Al-1.628) O-22 (OH, F)(2). Cell dimensions, measured on a single-crystal diffractometer and by precession photography, are: for gedrite, a 18.696(5), b 17.913(4), c 5.278(2) angstrom; for cummingtonite, a 9.51, b 18.12, c 532 angstrom beta 101.9 degrees; for hornblende, a 9.832(3), b 18.037(5), c 5.302( 1) angstrom, beta 105.01( 2)degrees. Single-crystal photographs of gedrite show split reflections indicative of unmixing into an intimate intergrowth of gedrite and anthophyllite. Basal thin sections of gedrite show fine lamellae of anthophyllite parallel to ( 010) and ( 230). Three-dimensional counter-collected single-crystal X-ray data and a full-matrix least-squares method were used to refine the crystal structure and cation-ordering pattern of the calcic amphibole phase. The final R index for 1098 observed reflections is 4.8%. Comparison of mean bond lengths and constituent ionic radii, the A-site cation occupancy and R-index criteria suggest an Fe3+/(Fe2++ Fe3+) ratio of similar to 0.15 for the calcic amphibole phase, showine it to be a tschermakitic hornblende. This value was used to calculate the cell contents for tle gedrite and hornblende given above; the calculated cell contents are supported by the fact that the Mg/(Mg+Fe2+Mn) ratios in all three phases are very similar (similar to 0.63). Considerable Fe2+ occurs at the M(4) site in the tschermakitic hornblende, indicative of a considerable cummingtonite component in solid solution.