Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 21

Although the periodic table appears to suggest that the elements are equally abundant and distributed proportionally on Earth, this is far from the case. The chemical composition of the Earth’s crust is in fact made up of eight dominant elements that comprise ~98.5%; all other elements combined make up the remaining ~1.5%. This distribution is shown in Table 1.2. Consequently, the bulk of the minerals commonly encountered have their base chemical formula closely associated with these eight elements.

The precious metal elements (e.g., Au, Pt, Ag, and Rh) occur very rarely in the Earth’s crust. Figure 1.4 is a graphic showing the relative abundance of the elements (vertical axis) against their atomic number (horizontal axis). Note the highlighting of the top eight rock‐forming elements, the rarest metals, and the Rare Earth Elements (also known as the Lanthanide Series). Because of the large variability in abundance of elements, the vertical scale in Figure 1.4 is logarithmic.

Table 1.2 Approximate abundance of dominant elements in the Earth’s crust. Data from Mason and Moore (1982).

Element, Symbol	Abundance in Earth’s crust (weight %)
Oxygen, O	46.60
Silicon, Si	27.72
Aluminum, Al	8.13
Iron, Fe	5.00
Calcium, Ca	3.63
Sodium, Na	2.83
Potassium, K	2.59
Magnesium, Mg	2.09
All others	1.41

Figure 1.4 Relative abundance of the elements in the Earth's crust as compared to silicon atoms. Haxel et al. (2002) / U.S. Geological Survey / Public domain.

Most elements are generally very rare; their concentrations are therefore commonly reported in parts per million, or ppm. The value of “1 ppm” indicates that there will be one gram for every million grams of the material (i.e., 1 gram in every tonne). A value of 10,000 ppm is equivalent to 1% (10,000 parts for every million). The level of concentration in the Earth's upper crust for gold is approximately 0.002 ppm. This is the same as 2 parts per billion (ppb), meaning for every billion atoms counted, only two will be gold!