Tourmaline crystals of the bizonal, lepidolite-bearing Brown Derby pegmatites no. 2 and no. 3 of Gunnison County, Colo., show a consistent variation in color, refractive index, and composition with position in the pegmatite. Starting with the outer wall of the pegmatite and going inward toward the core, the colors of the tourmaline crystals are black, dark green, blue, light green, and pink. The omega index decreases from 1.655 to 1.635 in the same direction.
The concentration of K, Rb, Cs, Pb, Be, and Li in tourmaline increases toward the pegmatite core. Fe, Mg, Ti, Na, Co, Ni, Cr, and V decrease in concentration toward the core; the last four are restricted to the crystals of the outermost part of the pegmatite. The main variation in tourmaline composition is a substitution of lithium for iron. Elements that remain more or less constant are Ga, Sr, Sc, Zr, Nb, Sn, Cu, Bi, and Zn. The concentrations of Mn, Y, and La show an interesting maximum at an intermediate position between the pegmatite wall and core.
Most of the spatial variations in composition of the tourmaline crystals may be explained on the basis of a gradual change in the composition of a single pegmatite fluid crystallizing in a closed system. Competition among different ions for sites in the crystal structure of tourmaline is invoked to explain the maxima in the variation trends. The variation trends of sodium and calcium, which are out of harmony with the variation trends in the composition of the pegmatite fluid, are apparently controlled by the charge requirements connected with isomorphous substitutions in the tourmaline structure.