MINERAL SYSTEMATICS AND FLUORINE DISTRIBUTION IN THE AMPHIBOLES FROM TWO GRENVILLE MARBLE BODIES IN NEW YORK STATE

This study focuses on the mineral systematics and chemical compositions of the amphiboles hosted by two Grenville marble units from New York State: Franklin Marble (FM) in Orange County and Upper Marble Formation (UMF) in St. Lawrence County. These marble bodies share a common age (Late Precambrian) and common metamorphic conditions (700C-836C and 4-7 kbar) and have produced many fine mineral specimens, including amphiboles. According to the electron microprobe data, the amphiboles from FM belong to the calcic amphibole group with edenite, pargasite, ferropargasite, magnesiohornblende, hastingsite and magnesiohastingsite compositions. The amphiboles in the UMF are within the calcic amphibole group with tremolite, actinolite, edenite, pargasite compositions and the Mg-Fe-Mn-Li amphibole group with cummingtonite and manganocummingtonite compositions. Substitutions at OH, A or C crystallographic sites for the calcic amphibole group at both locations lead to florian/potassian/manganoan amphibole varieties. Manganoan (Mn3+) tremolite has a pink-dark pink color and the intervalence charge transfer (Fe2+-Fe3+ interaction) gives a bright green color to some tremolite specimens.

The F/(F+Cl+OH) ratio (Xf) calculated for each composition is generally higher for the FM (Xf=0.31-0.77) than that for the UMF (Xf=0.02-0.73) amphiboles. Three F-rich end members have been identified: fluoro-edenite (F1.54Cl0.01OH0.45) in FM and (F1.45Cl0.01OH0.54) in UMF, fluoropargasite (F1.01Cl0.03OH0.96) and fluoro-magnesiohastingsite (F1.2Cl0.02OH0.81) in FM. The lowest Xf values are found in cummingtonite (0.02-0.08), actinolite (0.05-0.06) and tremolite (0.06-0.12). The substitution of F for OH in the amphibole composition reduces the number of Mg-O bonds and increases the number of shorter and stronger Mg-F bonds, contributing to the higher thermal stability of the F-rich end members of the calcic amphibole group. Taking into consideration experimental data showing that edenite thermal stability is probably between 825C and 900C and that the presence of F extends the pressure stability field of pargasite, slightly higher metamorphic conditions for FM than for UMF could be assumed.