Isotopes of lithium

Overview
Naturally occurring lithium (Li) (standard atomic mass: 6.941(2) u) is composed of two stable isotopes (6Lithium and 7Lithium, the latter being the more abundant (92.5% natural abundance). Seven radioisotopes have been characterized, the most stable being 8Lithium with a half-life of $838 ms$ and 9Lithium with a half-life of $178.3 ms$. All of the remaining radioactive isotopes have half-lives that are shorter than $8.6 ms$. The shortest-lived isotope of lithium is 4Lithium which decays through proton emission and has a half-life of $7.58 s$.

7Lithium is one of the primordial elements or, more properly, primordial isotopes, produced in Big Bang nucleosynthesis (a small amount of 6Lithium is also produced in stars). Lithium isotopes fractionate substantially during a wide variety of natural processes, including mineral formation (chemical precipitation), metabolism, and ion exchange. Lithium ion substitutes for magnesium and iron in octahedral sites in clay minerals, where 6Lithium is preferred to 7Lithium, resulting in enrichment of the light isotope in processes of hyperfiltration and rock alteration.

Lithium-4
Lithium-4 contains 3 protons and one neutron. It is the shortest lived isotope of lithium. It decays by proton emission and has a half-life of $9.1 s$. It can be formed as an intermediate in some nuclear fusion reactions.

Lithium-6
Lithium-6 is valued as a source material for tritium production and as a neutron absorber in nuclear fusion. Large amounts of lithium-6 have been isotopically fractionated for use in nuclear weapons.

Lithium-7
Some of the material remaining from the production of lithium-6, which is depleted in lithium-6 and enriched in lithium-7, is made commercially available, and some has been released into the environment. Relative lithium-7 abundances as high as 35.4% greater than the natural value have been measured in ground water from a carbonate aquifer underlying West Valley Creek, Pennsylvania (USA), down-gradient from a lithium processing plant. In depleted material, the relative 6Lithium abundance may be reduced by as much as 80% of its normal value, giving the atomic mass a range from $7.58$ to more than $6.94 u$. As a result, the isotopic composition of lithium is highly variable depending on its source. An accurate relative atomic mass cannot be given representatively for all samples.