HomeChemistry of Gem Materials

CHEMICAL 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       Atoms, Ions, Molecules, Compounds and Elements

Atoms at a basic level, consists 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

 

·       The Atomic Number =  Number of 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.

2       Ions and Chemical Bonding

2.1.1     Valency

2.1.2     Ionic

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

Cations are positively charged ions.

Anions (negatively charged ions) charged ions.

2.1.3     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 to the right.

2.1.4     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.

3       Isostructure

4       Ionic Substitution, Solid Solution and 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 colour, 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 colourless, 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.

 

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 colour. 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.

 

Picture above:Star ruby showing asterism.

 

 

5       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.

Some common examples are:

 

Substitution

Ionic Radii (C.N.) Å

Type

Examples

Fe+2 <=> Mg+2

Fe+2(6) 0.78

Mg+2(6) 0.72

Complete

High T favours Mg.

Olivines:  Mg2SiO4 - Fe2SiO4
Pyroxenes: MgSiO3 - FeSiO3
               CaMgSi2O6 - CaFeSi2O6
Biotite: KMg3AlSi3O10(OH)2
                     KFe3AlSi3O10(OH)2 
Tremolite Ca2Mg5Si8O22(OH)2 -
    Ferroactinolite Ca2Fe5Si8O22(OH)2 

Fe+2 <=> Mn+2

Fe+2(6) 0.78

Mn+2(6) 0.83

Complete, but
limited by amount of  Mn available.

Siderite  Fe(CO)3
        Rhodochrosite - Mn(CO)3

Mg+2 <=> Mn+2

Mg+2(6) 0.72

Mn+2(6) 0.83

Partial

 

Na+1 <=> K+1

Na+1 (8) 1.18

K+1(8) 1.51

Complete at  high T
Partial at low T

Alkali Feldspars:  NaAlSi3O8 - KAlSi3O8

Fe+3 <=> Al+3

Fe+3 (6) 0 .65

Al+3 (6) 0.54

Limited

Alkali Feldspar

Br-1 <=> Cl-1

Br-1(6) 1.96

Cl-1(6) 1.81

Complete

KCl - KBr

(OH)-1 <=> F-1 

(OH)-1 (4) 1.38

F-1(4) 1.31

Complete

Biotite: K(Mg,Fe)3AlSi3O10(OH,F)2 

 

 

 

 

 

 

 

 

 


  • Coupled Substitution

    Coupled substitution occurs if an ion of different charge is substituted.  This results in having to make another substitution in order to maintain charge balance.  Such coupled substitution is common in the silicate minerals where Al+3 substitutes for Si+4 in tetrahedral (C.N. = 4) sites.  Some examples of common coupled substitutions are given below:

 

Substitution

Extent

Example(s)

Na+1Si+4 <=> Ca+2Al+3

Complete 

Plagioclase: NaAlSi3O8 - CaAl2Si2O8

Ca+2Mg+2 <=> Na+1Al+3

Limited

Diopside: CaMgSi2O6 - Jadeite: NaAlSi2O6 

Mg+22Al+3 <=> 2Fe+2Ti+4

Extensive

Spinels

 

Another type of coupled solid solution involves filling a site that is normally vacant in order to achieve charge balance.  For example, in the amphibole mineral tremolite - Ca2Mg5Si8O22(OH)2, if Al+3 replaces one of the Si+4 ions then Na+1 can go into a site that is normally vacant to maintain charge balance.  The resulting formula would be NaCa2Mg5AlSi7O22(OH)2.  This would be called a sodic amphibole.

 

Interstitial Solid Solution

In some crystal structures there are sites that are not normally occupied by ions.  These are considered voids.  However, when an ion does occupy one of these voids it is called interstitial solid solution.  

 

Omission Solid Solution

Omission solid solution occurs when an ion of higher charge substitutes for an ion of lower charge.  In order to maintain charge balance, two of the lower charged ions will be replaced, but the higher charged ion will occupy only one site, thus the other site will become vacant, or omitted.  

An example of this type of solid solution is found in the blue variety of microcline, in which a Pb+2 ion replaces 2 K+1 ions. One of the K sites is replaced by the Pb+2 and the other site is left vacant.

 

Exsolution

As mentioned above, the extent of solid solution is sometimes dependent on temperature and pressure since the sizes of ions and the sizes of the crystallographic sites can change with temperature and pressure.  Thus, some minerals show complete solid solution under one set of temperature/pressure conditions, and only limited solid solution under different temperature/pressure conditions. When the conditions change to those where limited solid solution is favored, the mineral exsolves or unmixes.  But, because the process is taking place in the solid state, exsolution or unmixing cannot easily form two separate phases, because the ions must diffuse through the solid.  In fact, what happens is that two separate phases form in discrete domains within a single mineral grain.  These domains are crystallographically oriented, so they appear as lamellae or lines across the mineral grain.

Although we will explore this further in our discussion of phase diagrams, an example is given here.  The alkali feldspars (albite - NaAlSi3O8 - orthoclase - KAlSi3O8) form a complete solid solution at high temperature.  At lower temperature the solid solution becomes progressively more limited, and as a result lamellae of albite-rich feldspar begin to grow in the orthoclase-rich alkali feldspar.  This produces a texture called perthite, where the lighter coloured albite-rich feldspar is seen to occur as irregular lines or streaks (the lamellae) within the pink orthoclase-rich feldspar.

In the case of perthite, the exsolution lamellae are often large enough to see with the naked eye.  In other systems, the exsolution lamellae can only be observed with the petrographic microscope. 

Graphical Representation of Mineral Composition

For simple compositional variation it is often convenient to visualize the compositions in some kind of graphical form.  Most chemical analyses of oxide and silicate minerals are reported in weight % oxide components.  In weight percent because the classical technique of chemical analyses was once gravimetric, and in oxide components because it is difficult to obtain concentrations of Oxygen, so in oxides and silicates it is assumed that there is enough Oxygen to balance the cationic charges.

If the composition of a mineral can be expressed with 2 components, then a linear scale can be used as a graphical representation of composition.  For example, chemical analysis of the mineral kyanite shows that it is composed of about 36 weight % SiO2 and 64 weight % Al2O3.

 

We can plot this on a linear scale as shown here in the upper diagram.  Note that in this two component compositional diagram 0%  plots at the same point as 100 % .

 

We could divide each of these weight percentages by the molecular weight of each of the oxides and recalculate in the analysis in terms of molecular %.  But the chemical formula of Kyanite - Al2SiO5 - tells us that kyanite is made up of 1Al2O3  + 1SiO2 . So in molecular percent 50% of kyanite is  Al2O3 and 50% is SiO2, as shown in the lower diagram above.

Similarly, olivine can be thought of as a mixture of forsterite, Fo (Mg2SiO4) and fayalite, Fa (Fe2SiO4).  If these are the only two components involved, then note again that 100% Fo corresponds to 0% Fa, and vice versa.

An olivine solid solution that has 50% of the Mg+2 ions replaced by Fe+2 ions would be said to have a composition Fo50 or Fa50. Note that it could be expressed either way, because both ways indicate the same composition.  The chemical formula for such a composition would be written as MgFeSiO4.  Similarly, for an olivine composition where 20% of the Fe+2 ions are replaced by Mg+2 ions, the composition could be expressed as Fa20 or Fo80.  

 

If there are three components that need to be plotted, a triangular graph can be used.  Such a graph for the three components MgO, CaO, SiO2 is shown below.

 

 

Each of the corners of the triangular graph represent 100% of the component plotted at that corner, and 0% of the other two components.  Lines parallel to the sides of the triangle in this case are marked off in 10% increments, so that the horizontals lines represent the % of CaO starting from 0% at the bottom to 100% at the CaO corner.  Lines parallel to the SiO2 - CaO side of the triangle represent the %MgO starting from 0% at the SiO2 - CaO join to 100% MgO at the MgO corner.  Lines parallel to the MgO - CaO side of the diagram represent the % SiO2.  

Note that the composition 33%MgO, 33%CaO, 33%SiO2 plots at the exact center of the triangle.

Minerals that only contain 2 of the three components plot along the sides of the triangle, with the scale being similar to the 2-component graphs discussed above.  So, for example if we are using molecular percentages,  

  • CaSiO3 (wollastonite) which can also be written as 1CaO + 1SiO2, plots 50% of the way between CaO and SiO2.
  • MgSiO3 (enstatite) can also be written as 1MgO + 1SiO2, and plots 50% of the way between MgO and SiO2.
  • CaMgSi2O6 (diopside) can also be written as 1CaO + 1MgO +2SiO2.  There are a total of 4 molecules, with 1/4 as CaO, 1/4 as MgO, and 2/4 as SiO2.  So diopside plots at 25%CaO, 25%MgO, and 50%SiO2.

Triangular diagrams are often used to show the compositional ranges in minerals.  We here look at 2 examples.

 

The feldspars can be looked at in terms of the three components Albite (Ab) - NaAlSi3O8, Orthoclase (Or) - KALSi3O8, and Anorthite (An) - CaAl2Si2O8.  At high temperature, complete solid solution exists between Ab and Or, to form the alkali feldspar solid solution series.  But, as shown in the diagram, the alkali feldspar solid solutions can contain up to 5% of the An component.  Similarly a complete solid solution series exists between Ab and An, to form the plagioclase solid solution series.  Plagioclase can contain up to about 5% of the Or component.

 

Another example is shown by the pyroxene minerals.  These plot in the three component system Enstatite (En) - MgSiO3,  Wollastonite (Wo) - CaSiO3, Ferrosilite (Fs)  - FeSiO3.  Wo does not have a pyroxene structure  Complete solid solution exists between Diopside, Di,  (CaMgSi2O6) and Hedenbergite, Hd,  (CaFeSi2O6).  These pyroxenes are monoclinic and are thus called the clinopyroxenes.  Augite is also a clinopyroxene, but note that it is depleted in the Wo component relative to the Di - Hd series.  En - Fs also forms a complete solid solution series.  These minerals are orthorhombic, so the series is often referred to the orthopyroxenes..  Pigeonite is a monoclinic pyroxene that has slightly more of the Wo component than the orthopyroxenes.  

 

 

6       How are minerals grouped?

Minerals are grouped firstly based on what they contain.

These are the most important groups for this course:

  • silicates - silica (Si)-rich minerals
    • Examples include amethyst (quartz), tourmaline, beryl
  • oxides - anion is oxygen (O)
    • Examples include corundum (ruby and sapphire) and rutile
  • carbonates - contain carbon and oxygen (C)

Examples include calcite and aragontite (in pearls)

Other groups we will not talk about here (not required for this course):

  • phosphates - contain phosphorus (P)
  • borates - contain boron (B)
  • sulfides and sulfates - contain sulfur (S)
  • halides - contain chlorine (Cl) or other elements from group VIIa in the periodic table

Minerals are arranged into groups based on dominant anion (nonmetal) or anionic group. Minerals within the same chemical group have similar chemical properties, origins and occurrences, and physical properties.

Chemical Classifications

Chemical Class

Anion or Anionic Group

An Example

Silicates

Silicon and Oxygen

Tourmaline, (Mg,Fe)2 SiO4

Oxides

Oxygen

Corundum, Al2O3

Carbonates

Carbon and Oxygen

Rhodochrosite, MnCO3

Native Elements

One element, such as Carbon

Diamond, C

Sulfides

Sulfur

Sphalerite, ZnS

Halides

Halogen ions, such as Fluorine

Fluorite, CaF2

Phosphates

Phosphorus and Oxygen

Apatite, Ca5(PO4)3 (F,Cl,OH)

Sulfates

Sulfur and Oxygen

Gypsum, CaSO4 2H2O


7       Bibliography and Supplementary Reading

Webster, Practical Gemmology, Lessons 1,2

Read, Gemmology, Chapters 1,2,3

Hurlbut, C. S., & Kammerling, R. C. (1991), Gemology, Chapters 1,2, NY: John Wiley & Sons, Inc.

Gemmological Association of Great Britain, FGA Foundation course materials, Chapters 1,2

GIA Colored Stones course materials

CGA Preliminary course materials, Chapters 1,2

Schumann, Walter. (1997) Gemstones of the World, NY: Sterling Publishing Co, Inc.

Harvey, Anne (1981). Jewels. London: Bellew & Higton Publishers Limited.

 

Kunz, G. F. (1971). The curious lore of precious stones. NY: Dover.

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