TOURMALINE-GROUP MINERALS IN GRENVILLE ROCKS FROM NEW YORK: RELATIONSHIP BETWEEN COMPOSITION AND THE GEOLOGICAL SETTING

Tourmaline, a complex borosilicate, can record the chemistry of the host environment and the chemical changes in the composition of the generating fluids or associated minerals. A survey of the chemical composition of the tourmaline-group minerals from the Adirondack Mountains, Hudson Highlands and Franklin Marble in New York examines the range of naturally occurring tourmaline compositions in relationship to their local geological environment.

The dominant compositional groups of tourmalines, based on cation assignment to different crystallographic sites, are alkali (schorl, dravite, chromdravite and a member of the olenite-elbaite series) and calcic (uvite) tourmalines. Possible new species such as F-analogue of dravite (in brecciated quartz vein crosscutting a marble body), Mn-rich OH-analogue of uvite (tremolite – talc schists) and F-analogue of feruvite (Fe – B – F deposit) were identified.

Weak zoning is displayed by some crystals, but the difference in the chemical composition between zones is not significant; only one sample shows a core with uvite composition overgrown by a wide rim with schorl chemistry. Alteration is not common; only one specimen was found extensively altered to a glassy, yellow-green, sheet silicate along fractures and crystal faces.

The chemical composition shows extensive substitutions of Na and K for Ca in the X-site (from 0.92 apfu Ca and 0.09 apfu Na in marbles to 0 apfu Ca and 0.87 apfu Na in pegmatites). Ti4+ (up 0.27 apfu), Mn2+ (up to 0.61 apfu) both in the talc – tremolite schists, V3+ (up to 0.03 apfu), and Cr3+ (up to 0.1 apfu) are the most significant components in Y- and Z-site. Significant replacements of (OH)- and O2- by F- in W-site form the F-dominant species, prevailing in marbles.

The geological distribution of the tourmaline species shows their preference for a specific host, and indicates both the source of their components and the mineral-forming reactions. Schorl and some dravite specimens occur in pegmatites, uvite, dravite, OH-analogue of uvite and F-analogue of dravite are found in marbles, and Mn-rich OH-analogue of uvite and Mn-rich dravite are specifically associated with the talc – tremolite schists. Unusual occurrences of chromdravite in talc – tremolite schists and Al-rich (8 apfu Al) tourmaline in marbles are related to the local geologic setting.