Niobium hardening element

Conservative elements (C,) include zirconium (Harden, 1987), titanium (Johnsson et al., 1993), rare earth elements, and niobium (Brimhall and Dietrich, 1987). Considerable disagreement occurs in the literature as to the relative mobility of these elements under differing weathering conditions (Hodson, 2002). Also, these elements are concentrated in the heavy mineral fractions and are often not suitable for describing weathering ratios in depositional environments where selective concentration and winnowing occurs. For such conditions, relatively inert minerals such as quartz can be considered (Sverdrup, 1990 White, 1995). 

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