Goldschmidt’s Classification
V. M. Goldschmidt and other geochemists in the early twentieth century believed that the Earth formed by processes involving interactions between various molten material, solids, and gases. They hypothesized that the Earth was originally completely homogeneous and molten, subsequently cooling and separating into the layered structure we know today. Although many of their ideas are now known to be incorrect, their basic observations were correct.
As a model for the internal processes of the Earth, Goldschmidt made observations of copper-smelting furnaces at Mansfeld, Germany, and found that molten materials often divided into several different sorts of liquid melts: one rich in Fe alloys, another rich in sulfide compounds, and one containing silicates. In most smelting processes, the latter two were considered waste, forming matte and slag, respectively. Goldschmidt also studied meteorites and igneous rocks, where he found minerals falling into the same three general chemical groups.
Goldschmidt and his colleagues devised a classification scheme for elements based on their behavior in a hypothetical iron alloy-silicate-sulfide-gas system (see Figure 1).
FIGURE 1 Typical ions in minerals. Elements with no indication of valence have no common ionic state in minerals. Small letters indicate classification of an element according to Goldschmidt : S = siderophile, C = chalcophile, L = lithophile, a = atmophile.
They divided elements into four groups: siderophile (elements that concentrate in an iron-rich liquid), chalcophile (elements that concentrate in a sulfur-rich liquid), lithophile (elements that concentrate in a silica-rich liquid), and atmophile (elements that form a gas). Although the distinction between siderophile, chalcophile and lithophile elements is sometimes ambiguous, this classification scheme is significant because it implies that properties and behavior of elements are not random. Since its origin, Earth has undergone differentiation, in large part due to magmatic processes. Lithophile elements have become enriched in the crust. Chalcophile and siderophile elements have become more concentrated in the mantle and core. Atmophile elements escaped Earth’s interior and formed the atmosphere we enjoy today.
Abundance of Elements
About a dozen elements account for 99.9% of Earth’s composition (Table 1).
TABLE 1 The Most Abundant Elements in the Earth’s Crust, Mantle, and the Entire Earth
Iron is the most abundant overall (32 wt %) but is mostly in the core. Iron is only about 5 to 6 wt % of the crust and mantle. Oxygen and silicon make up more than half the crust and mantle, so silicate minerals are common. The other major elements in the crust and mantle are similar but the proportions are different. The mantle contains substantially more magnesium and significantly less silicon, aluminum, sodium and potassium than the crust; consequently, the mantle is dominated by magnesium-rich mafic minerals (olivine, pyroxene, garnet, and spinel) while much of the crust contains quartz and/or feldspars.
Some elements are common in many different minerals. Oxygen (O) and silicon (Si) are perhaps the best examples. Many sedimentary rocks and nearly all igneous and metamorphic rocks are composed of multiple minerals containing O and Si. In contrast, because of their properties, some other elements tend to be found mainly in only a few distinct minerals. For example, titanium (Ti) may occur as a minor component in biotite, amphibole, or other minerals. In many rocks, however, Ti is concentrated in Ti-rich minerals such as rutile , titanite , and ilmeniteRocks rich in Ti always contain one of these latter three minerals. Similarly, rocks containing significant amounts of phosphorous usually contain apatite, (OH, F, Cl), or monazite.
ionic Complexes
While thinking of individual elements coming together to form minerals is convenient, in reality, atoms are seldom unbonded to others. Single atoms are very reactive. They tend, when possible, to bond to other atoms to form compounds. Sometimes, they bind to other atoms of the same element. For example, , composed of molecules containing two nitrogen atoms, dominates the Earth’s atmosphere. Small atoms with several valence electrons, such as silicon or carbon, are especially reactive. They seldom exist by themselves, readily combining with oxygen, and sometimes other elements, to form strongly bonded units called anionic complexes.Thus, silicon, carbon, phosphorous, nitrogen, and sulfur are usually found in silicate, carbonate, phosphate, nitrate, or sulfate minerals (Table 2).
TABLE 2 Common Anionic Complexes in Minerals
Anionic complexes such as (SiO4)4- and (CO3)2-are so strongly bonded that they behave just like individual anions in many minerals. Most mineralogical and geological texts (this one included) classify minerals based on their anions or anionic complexes because the properties of minerals with the same anions are generally very similar. Mineral formulas are usually written to emphasize any anionic groups that exist.
Mineralogists can acquire mineral analyses in many ways. In the past, most chemical analyses were determined by titration and other “wet chemical” techniques. Today we use sophisticated analytical machines, including atomic absorption spectrophoto meter sand electron microprobes.We normally report analytical results by listing oxide weight percents. These values must be normalized if we wish to have mineral formulas.