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Geology and Mineralogy of Gemstones. David Turner R.Читать онлайн книгу.

Geology and Mineralogy of Gemstones - David Turner R.


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places. For example, silver has an atomic number of 47 and an atomic mass of 107.868. This is not to say that silver has 47 protons and 107.868 neutrons. Rather, 107.868 represents the average atomic mass of a sample of gold that includes gold atoms with different numbers of neutrons. Material comprised of high atomic mass elements will generally be of higher density, such as in the case of the native metals in Table 1.1.

      1.3.3 Atomic Structure, Electrical Charges, and Ions

Element Atomic number Average atomic mass Density (g/cm3) of native metal mineral
Copper, Cu 29 63.55 8.9
Silver, Ag 47 107.87 10.5
Gold, Au 79 196.97 19.3

      Protons have a positive electrical charge, neutrons have no electrical charge, and electrons have a negative electrical charge. The sum of their charges denotes overall ionic or atomic charge. In a basic atom of a given element with all of its allotted electrons, an atom is neutral. This means that all of the negative charges of the electrons are balanced by all of the positive charges of the protons.

      The electron cloud of an ion can be estimated to be in the shape of a sphere and its size is defined by the distance from the center of the nucleus to the limit of the cloud. This is called the ionic radius, which is measured in units called Angstroms or Å. An Angstrom unit is very short – it is equal to one tenth of a nanometer. Note that a nanometer is 0.000000001 meter or 10–9 meter!

      1.3.4 Elements

      There are 92 naturally occurring elements out of a total 118 identified, each with its own symbol that acts as a shorthand notation. Some familiar elements and symbols are Au for gold, C for carbon, Ag for silver, and Pt for platinum. Element abbreviations start with a capital letter and if a second letter is present it will always be lowercase. Many elements are already part of our everyday vocabulary, such as oxygen (O), carbon (C), nitrogen (N), and potassium (K) but others are much more obscure, such as beryllium (Be), scandium (Sc), and rhodium (Rh).

      1.3.5 Element Groups

      The elements of the second group are collectively called the alkaline earth metals. These elements usually lose two electrons, resulting in a characteristic +2 valence charge. Calcium (Ca) and magnesium (Mg), two of the important bone‐forming ingredients, are elements of this group.

      The middle block of elements (ranging from Sc down and across to element 112, Cn) are called the transition metals. These elements can have variable valence charges, usually up to +4 but sometimes as high as +6. Note how the precious metals Cu (copper), Ag (silver), and Au (gold) are all Group 1B transition metals and thus share similar physical properties. The metals Ni (nickel), Pd (palladium), and Pt (platinum) are similarly related as Group VII elements. The transition elements often endow gemstones with their striking colors.

      Elements classified as semi‐metals or other metals include aluminum (Al) and lead (Pb). The next group are the metalloids, including silicon (Si) and arsenic (As). Nonmetals include the biologically important elements carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S).

Schematic illustration of periodic table of the elements with atomic numbers and chemical abbreviations.

      1.3.6 Elemental Abundance in the Earth’s Crust


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