Periodic Table--Lithium

Lithium has two naturally-occurring stable isotopes, ⁶Li (7.5 %) and ⁷Li (92.5 %). Lithium isotopes fractionate substantially during a wide variety of natural processes, including mineral formation (chemical precipitation), metabolism, ion exchange (Li substitutes for Mg and Fe in octahedral sites in clay minerals, where ⁶Li is preferential over ⁷Li), hyperfiltration, and rock alteration (Morozova and Alferovskiy, 1974; Chan and Edmond, 1988; Fritz and Whitworth, 1994). Therefore, isotopic compositions of Li can provide a basis for distinguishing various lithologic sources of cations in catchment waters and/or to trace water affected by the processes.

In the past, the ease with which Li fractionates has made laboratory isotopic analysis very difficult. Fortunately, recently developed techniques have been used to examine the Li isotopic compositions from thermal waters of Yellowstone National Park (Bullen and Kharaka, 1992). Because the isotopes will fractionate during hydrothermal processes, significant variations observed in the ⁷Li/⁶Li ratios distinguished water derived from marine sedimentary rocks and water derived from hydrothermally altered igneous rocks, thereby providing valuable information regarding regional ground-water flow paths.

In addition, lithium isotopes have recently become a source of quality control on manufactured lithium reagents (Qi et al., 1997). Several reagents were found to be artificially depleted in ⁶Li significantly compared to terrestrial materials, indicating that many lithium reagents used in chemical experiments are in fact ⁶Li-depleted and do not accurately reflect the atomic and/or molecular weights of these reagents. Large quantities of ⁶Li were removed from Li reagents for use in nuclear weapons. The remaining Li was then sold to chemical companies and found its way into reagents on chemists' shelves. Sometimes this Li makes its way into streams and it can be easily identified.

Source of text: This review was assembled by Eric Caldwell, primarily from Nimz (1998).