HomeChemical Composition of Gem Materials

An introduction to crystal chemistry helps in understanding what gems are made of and how this affects the gemstone environments and associations. Most gemstones are minerals, that is inorganic crystal treasures found in natural world.

 

1.1 Atoms, Ions, Molecules, Compounds and Elements

Recall:

 

atoms consist, at a basic level, of:

 

* a nucleus consisting of protons (positively charged) and neutrons (neutral)

* electrons (negatively charged), that surround the nucleus.

 

An electrically neutral atom contains equal numbers of protons and electrons.

 

An ion is formed by loss or gain of electrons. The valence of the ion tells us how many electrons have been gained or lost

 

. i.e., Carbon: 6 protons and 6 electrons, looses 4 electrons to become the cation C 4+

 

The ATOMIC NUMBER = NUMBER OR PROTONS

 

The ATOMIC WEIGHT = TOTAL WEIGHT OF PROTONS AND NEUTRONS

 

Isotopes

 

* The atomic number tells us what the element is (i.e., identifies its place on the periodic table).

 

* The atomic weights of an element tells us which isotope we have!

 

For example:

 

Here are three isotopes of carbon (C).

 

The atomic number for C is 6, so that all carbon atoms have 6 protons.

 

C 12: 6 protons plus 6 neutrons

 

C 13: 6 protons plus 7 neutrons

 

C 14: 6 protons plus 8 neutrons

 

The C 14 ISOTOPE IS UNSTABLE and thus, undergoes radioactive decay!

 

The result of radioactive decay is that C 14 -> N 14, an isotope of nitrogen.

1.2 Ions and Chemical Bonding

Ions form when atoms lose (cations) or gain (anions) electrons.

1.2.1 Valency

1.2.2 Ionic

Cations are positively charged ions

Anions (negatively charged ions) charged ions.

 

1.2.3 Covalent

Atoms are held together in crystals by atomic bonding.

The most important types of bonds are via electron exchange (ionic) or electron sharing (covalent), as shown simplistically below.

 

Chemical Bonding...

The forces that bind atoms, ions, or ionic groups together in crystalline solids are electrical, with their type and intensity responsible for the physical and chemical properties of minerals. The stronger the bond the harder the crystal and higher the melting point. The high hardness of diamond is because of the strong electrical bonding forces linking the carbon atoms. These electrical forces holding inorganic minerals together are chemical bonds, such as: ionic, covalent, van der Waals, metallic, hydrogen, or some combination.

1.3 Isostructure

1.4 Ionic Substitution and Solid Solution

Ionic Substitution, Solid Solution, Exsolution...

The solution or melt in which the mineral crystallizes can contain many elements not primary to the chemical composition. Such additional elements can be present in the crystal structure in minute amounts substituting for a major element within the mineral. This ionic substitution can cause color, such as chromium present in emerald (variety of beryl) creating green and iron present in aquamarine (also a variety of beryl) creating blue. When ionic substitution is extensive, it is termed solid solution. Substitution is common if the ionic radius differs by less than 15% assuming the overall neutral charge of the mineral is maintained. An example is with the olivine group of minerals, where forsterite is a magnesium silicate, and fayalite is an iron silicate. The iron and magnesium substitute for one another because they have like charges and similar ionic radii size. "With no iron, forsterite is colorless, but with increasing iron the mineral darkens, going from light-toned olive green to dark green to black in fayalite" (Hurlbut and Kammerling, 1991, p. 30). The gem variety of olivine, peridot, has 10% magnesium of forsterite replaced by iron.

 

 

1.5 Exsolution

Exsolution is responsible for adularescence and asterism in gemstones. When minerals crystallize at high temperatures, high internal thermal energy allows for less stringent space requirements and ionic substitution is extensive (Hurlbut and Kammerling, 1991, p. 30). When the mineral cools, the poorly fitting ions are forced to migrate through the crystal structure and a type of unmixing occurs. For example, a potassium-rich feldspar, called orthoclase, can tolerate sodium replacement of potassium at high temperatures, but forces these ions to migrate forming small localized areas of a sodium-rich feldspar, called albite. These pockets of albite intertwined with orthoclase result in an optical phenomenon called adularescence, which is an overall shimmery blue-white glow and localized flashes of color. This exsolution interaction gives the schiller or adularescence phenomenon to moonstone.

 

An example of asterism is found in corundum and referred to as star ruby and star sapphire. The aluminum and oxygen of corundum can accomodate titanium substituting for aluminum in the crystal structure. Upon slow cooling the titanium reacts with the oxygen producing needle-like crystals of the mineral rutile. The hexagonal crystal structure of corundum constrains the rutile crystals to orient 60 degrees to one another and if enough are present when the stone is cut en cabochon (a smooth convex top) perpendicular to the long c-axis direction, the star or asterism will result (Hurlbut and Kammerling, 1991, p. 30). Some corundum with titanium can be heat treated, slowly cooled and enhance the asterism, while some corundum is heated and cooled rapidly to reduce the star effect and improve the transparency of the gem.

 Star ruby showing asterism.

1.6 Relation between chemical Composition and Durability

Compositional Variation in Minerals

In our definition of a mineral we said that a mineral has a definite, but not necessarily fixed chemical composition.  Here we explore the "not necessarily fixed" portion of the definition.  Chemical compositional variation in minerals is referred to solid solution.  Although most of us think of solutions as a liquid containing dissolved ions, solids can form solutions as well, in which case we think of one solid as being dissolved in another solid.

Solid solution occurs as the result of ions substituting for one another in a crystal structure.  The factors that control the amount of solid solution that can take place in any given crystal structure are:

1. The size of the ions and the size of the crystallographic sites into which they substitute.  Generally ions of about the same size can substitute for one another, although the size of the crystallographic site can also play a role if one of the ions is of  nearly the same size, but is too large to fit into the site.

2. The charges on the ions that are substituting for one another.  If the charges are the same, then the crystal structure can remain electrically neutral.  If the charges are not the same then other substitutions must take place to maintain charge balance.

3. The temperature and pressure at which the substitution takes place.  In general there is a greater amount of substitution that takes place at higher temperature.  This is because the atoms vibrate at a higher rate and the size of crystallographic sites are larger.  Pressure can also have an effect because it can change the size of both the crystallographic sites and the ions, thus resulting in different substitutions than might take place at lower pressure.

 

Three different types of solid solution are recognized - substitutional, interstitial, and omission.

 

Substitutional Solid Solution

Simple substitution


When ions of equal charge and nearly equal size substitute for one another, the solid solution is said to be simple.  Generally if the sizes of the ions are nearly the same, the solid solution can occur over the complete range of possible compositions and the solid solution series is said to be complete.  If the sizes are similar, but still very different the substitution may only occur over a limited range of compositions and the solid solution series is said to be partial or limited.  Partial or limited solid solution can also occur because the substituting ion does not occur in high enough concentrations in the environment in which the mineral is formed.

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