Fluorine distribution in the hydrous silicate minerals of the Franklin Marble, Orange County, New York

Fifty specimens of the hydrous silicates norbergite, chondrodite, phlogopite, tremolite, edenite, and pargasite from 34 localities in the Franklin Marble of Orange County, New York, were analyzed quantitatively, and their F/(F + Cl + OH) ratios (X_F) determined. The average F/(F + Cl + OH) ratio (X_F) determined for each species follows the sequence: X_F Trem < X_F Phlog < X_F Amph < X_F Chond < X_F Norb where X_F Amph is the average X_F for amphibole species other than tremolite. The fluorine contents of the hydrous silicate minerals formed within the Franklin Marble are high by comparison with analyses of the same species from other localities. The Franklin Marble is a high-temperature (836 ± 40°C), high-pressure (4–7 kbar) metacarbonate. We suggest that the high fluorine content of the tremolite samples has increased the upper thermal stability limit for this species, thus preserving the assemblage tremolite + quartz + calcite under granulite facies conditions. There is an inverse relationship between the F/(F + Cl + OH) ratio and titanium content for chondrodite. This relationship has been noted by earlier investigators for other members of the humite group. The inverse relationship is not evident for Ti values below 0.02 atoms per formula unit.