Mineral Chemistry of Chlorite Replacing Biotite Fromgranitic Rocks of the Canadian Appalachians

Chlorite flakes, as a product of alteration of biotite, the dominantferromagnesian mineral in the paleozoic granitic rocks of the canadianappalachians, have been analyzed by electron microprobe for major elements andby 57fe mossbauer spectroscopy for the coordination and oxidation state of fe.comparison of mossbauer fe^3+/fe ratios obtained from chlorite and its hostbiotite indicates that chloritization might have occurred under relatively oxidizingconditions. based on 54 analyzed samples, si cation totals of these sheet silicatesare less than 6.25 atoms per formula unit (apfu), and the sum of octahedral cationsis very close to 12 both an indication of trioctahedra1chlorite. the calculated molefraction of chlorite in interlayered phase, xc, ranges from. 0.72 to 0.98 confrrmingthat the chlorites are completely free of any smectite layers. compositionalvariations in chlorite are strongly controlled by host biotite and rock type.fe/(fe+mg) ratio ranges from 0.35 to 0.93 and si contents from 5.18 to 6.11 apfulead to the classification of chlorites mainly as ripidolite and brunsvigite. allmajor elements in the chlorite are strongly correlated with each other. fe/(fe+mg)ratio in biotite is well preserved by chlorite. chlorite thermometry based on thevariation in tetrahedral al content within the chlorite structure shows a largevariation in temperatures from 200 to 390 dc with an average of 340 dc. thechlorite from igneous rocks could also be used to detect reheating events andreveal the thermal history of the rocks.