The Chemistry of Gemstones

From The Gemology Project
Revision as of 16:05, 5 March 2006 by Anatase (talk | contribs) (Electronic Configuration of Atoms)
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An element is a substance that can not be seperated into a simpler form of matter by ordinary means. An element may be a solid, liquid or a gas.
Some common elements are carbon, gold, copper, iron, oxygen, silicon, etc.
An atom is the smallest part of an element. It is the smallest, indivisible particle of a chemical element. Each atom consists of negatively charged electrons and positively charged protons. There are a similar number of electrons and protons to produce an electrically neutral atom. An atom can not be subdivided by any chemical process. Atom.gif

Electronic Configuration of Atoms

Properties of an element, chemical compounds and the formation of crystals depends a great deal on the disposition of electrons and their interaction. The electrons circle the nucleus in all directions, like a cloud. The radius of the average of their movements, is defined as a shell or energy level. These shells will remain at a predictible level which is determined by quantum mechanics. Each shell contains only a given number of electrons. There is a maximum of 2 in the innermost shell (known as the k shell), and 8 each in the second and third shells (known as the l and the m shells respectively). There are 18 each in the next two shells (known as the N and the o shells) and 32 in the next full p shell.

Atomic Number

Each of the chemical elements is made up of like atoms, also known as the atomic number. They all have the same number of protonss. Hydrogen (atmoic No. 1), has one proton and one planetary electron. The heaviest naturally occurring atom, has 92 protons and 92 electrons.
Element 112 was discovered on 9th February 1996 in Darmstadt, Germany. The identified isotope currently is the heaviest atom ever produced by man and has an atomic mass of 277, that is, 277 times heavier than hydrogen.
Except for hydrogen, all the other nuclei of atoms contain neutrons, varying from 2 in the case of helium to 146 in the uranium atom. The atomic weight of an element is the weight of the protons and neutrons, which form the nucleus of the atom. If an atom gains or loses one electron, it ceases to be electrically neutral and becomes what is known as an ion. the gaining of electrons, produces a negatively charged ion, known as an anion. Loss of electrons results in a positively charged ion referrred to as a cation.

Molecules

Atoms of elements can combine together to form tiny fractions of new substances. These are known as compounds. A molecule is defined as the smallest fraction of a substance, consisting of two or more elements, chemically combined together, which define a substance's composition and chemical properties.

Valency

The valency of an element is determined by the number of its atoms which will combine with, or replace, one atom of hydrogen. It can be observed that some elements will have different valencies in different compounds. For example, the compound FeO (ferrous oxide) is divalent and Fe2O3 (ferric oxide) is trivalent.

Isomorphism & Isomorphic Replacement

When two ions are brought together, there is a distance where a force of repulsion abruptly sets in and resists any closer approach. It is convenieent to consider the ions as "spheres" in contact, and the distance betweeen their centers is taken as the sum of the radiis of the two ions. An anion, such as an oxygen ion, have large ionic radii, where cations have radii that are much smaller.
The common cations which enter the structure of crystals are often those with similar sized radii, and can replace each other within the structure without unduly straining the space lattice. This accounts for the phenomenon of isomorphism and isomorphic replacement.
In the garnet and spinel groups, magnesia (MgO) and ferrous oxide (FeO) replace one another extensively. Likewise, alumina (Al2O3) and ferric oxide (Fe2O3) can easily replace each other within a crystal's lattice. When such a replacement takes place there can be a a significant variation in chemical composition, without much effecting the internal structure. This accounts for the fact that in certain gemstones of the same species, the specific gravity and refractive indices are not constant specimen to specimen. In some species like garnet, spinel, tourmaline, and beryl the values for refractive index and specific gravity can vary within wide parameters. This is an important factor to keep in mind when testing gemstones.