STRUCTURAL VANADIUM IN SYNTHETIC GEOTHITE

A significant correlation between Fe and V in many weathered profiles suggests a close geochemical association of the two elements. Because V3+ is the dominant form of V in primary rocks, and because an isostru ctural VOOH (montroseite) exists, we suspect that goethite, FeOOH, mig ht incorporate V3+ in its structure. We synthesized Fe1-xV(x)OOH with x up to 0.06. Because the ionic radii of V3+ and high spin Fe3+ are ve ry similar (64.5 vs. 64.0 pm), even a careful measurement of the unit cell parameters did not reveal a statistically significant difference. Structural incorporation was, therefore, demonstrated by congruent di ssolution in 6 M HCl. Equivalent amounts of Fe2+ and V found in the ac id digest were presumably formed by the reaction V3+ + Fe3+ + H2O = VO 2+ + Fe2+ + 2H+. An EPR spectrum did not indicate V4+. For these and s teric reasons, it is postulated that V is incorporated into goethite a s a trivalent cation rather than as a combination of V4+ and Fe2+.