In 1889 Clarke wrote the first of his many publications on the geochemical distribution of the elements.
He assembled many chemical analyses of rocks from different continents, calculated average values, and showed that the overall chemical compositions of continental areas are remarkably similar.
The two major discontinuities that are universally recognized are the Wiechert–Gutenberg Discontinuity, which separates the mantle from the core.
It is not possible to give definite figures for the abundances of the elements in the Earth’s core.
It is certainly made up largely of metallic iron, however, probably with some nickel, a little cobalt, and appreciable amounts of such lighter elements as carbon and sulfur.
Siderophile elements are principally metals that alloy readily with iron; Goldschmidt explained their scarcity in the Earth’s crust by their concentration in the nickel–iron core (the siderosphere).
Chalcophile elements are those with strong affinity for sulfur; they occur mainly as sulfides.
The atomic number of iron is 26, so this implies that the core also contains elements of lower atomic number.
Sulfur, with atomic number 16, and carbon, 6, are relatively abundant in meteoritic matter, and the presence of minor amounts of these elements in the core would effectively reduce the mean atomic number.In estimating elemental abundances in the mantle, however, the same difficulty as with the core arises: direct sampling is not feasible.Much more geophysical data are available for the mantle, however, and some volcanic eruptions have brought rock fragments to the surface that have certainly been derived from this zone.Basic knowledge in this area was largely accumulated during the 19th century.As noted above, the concept of a limited number of chemical elements had been established by 1800, and the appearance of the periodic table, in 1869, provided a new insight into the limitations on the number of elements.It too is marked by a density increase from crust to mantle—a comparatively small one, from about 3 to 3.3.