Normal Materials

Standard atomic weights reflect the best knowledge of evaluated, published data applicable to normal materials. The IUPAC definition of "normal material" recognizes the fact that much of the atomic-weight variation of some elements is caused by isotopic fractionation processes that operate on many different time scales. This definition also excludes extraterrestrial materials from the determination of standard atomic weights.

The IUPAC definition of "normal material" anticipates that materials with radiogenic or nucleogenic isotopic variability may not be excluded, except in very exceptional situations, such as the anomalous occurrence of strontium-87 in rubidium-rich mica from Canada (giving rise to the assignment of footnote "g" to strontium). This observation by Mattauch (Naturwissenschaften 1937, 25, 189) predates by 35 years the discovery of natural nuclear reactor products, discovered in 1972 at the Oklo quarry in Gabon, Africa, and first discussed by the CIAAW in its 1973 Report.

The definition of "normal material" was last revised by the CIAAW in 2017 from its 1984 version. With this revised definition of normal materials, based on the evaluation of argon by Böhlke, the standard atomic weight of argon was changed in 2017 from 39.948 ± 0.001 to [39.792, 39.963] to include naturally occurring sources having nucleogenic and radiogenic isotopic variation.

Definition

Normal materials include all substances, except (1) those subjected to substantial deliberate, undisclosed, or inadvertent artificial isotopic modification, (2) extraterrestrial materials, and (3) isotopically anomalous specimens, such as natural nuclear reactor products from Oklo (Gabon) or other unique occurrences.

T.B. Coplen, N.E. Holden, M.E. Wieser and J.K. Böhlke. Clarification of the term "normal material" used for standard atomic weights Pure Appl. Chem. 90, 1221-1224 (2018)

CIAAW

Examples of exceptional natural variability in atomic weights for some elements occur at the Oklo mine site in the Gabon, Africa. Nuclear fission reactions occurred spontaneously two billion years ago in uranium ore, which at that time had a ²³⁵U abundance in excess of 3 %. Such exceptional geological occurrences are excluded by the CIAAW in recommending standard atomic weights.