TOURMALINE-GROUP MINERALS

Tourmaline is a chemically complex group of silicates. Essentially, it is an aluminum boron silicate with extensive substitutions at the different crystallographic sites that confer varied physical properties to the mineral. The general chemical formula of tourmaline can be written as

XY3Z6[T6O18][BO3]3V3W where

X = Ca, Na, K, vacancy
Y = Li, Mg, Fe2+, Mn2+, Al, Cr3+, V3+, Fe3+, Ti4+
Z = Mg, Al, Fe3+, V3+, Cr3+
T = Si, Al, B
B = B
V = OH, O
W = OH, F, O

The tourmaline-group minerals are a common presence in the rocks from the Grenville province of New York State. Although, many specimens are not of gem quality, the tourmaline minerals from New York are scientifically interesting by: (a) occurrence; (b) composition; (c) mode of formation; (d) crystal structure; and (e) potential discovery of new species. 

Eight tourmaline species were found in New York: uvite, schorl, dravite, feruvite, rossmanite, olenite, fluordravite, chromo-alumino-povondrite. Their geological distribution shows the 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 and dravite, are found in marbles, and Mn-rich uvite and Mn-rich dravite are specifically associated with the talc-tremolite-cummingtonite schists. Unusual occurrences of chromo-alumino-povondrite in talc-tremolite-cummingtonite schists and rossmanite and olenite (high-aluminum tourmaline) in marbles are related to the local geologic setting. 




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