GemChem Chemistry for Gemmology

 Introduction ….

Chemistry is divided into various sections as follows

Inorganic Chemistry, dealing with the elements, their properties and reactions.

Physical Chemistry, dealing with interactions between matter and energy.

Organic Chemistry, dealing with the compounds of carbon, although a few of these, mainly the oxides of carbon, carbon in carbonates and carbon combined with metals are generally dealt with under Inorganic Chemistry.

Nuclear Chemistry, dealing with the radioactive elements.

Quantitative Analysis, dealing with the determination of the amount of each element in substances.

 

Qualitative Analysis, dealing with the determination of what elements are present in substances.

In this work, which is subject to addition and change, the author has tried to extract the elements of chemistry that have a bearing on the science of Gemmology and present them as a text for the non-chemist wishing to have a better understanding of the gemstones with which he/she is dealing.

The author is considering, for presentation sometime in the future, an addendum to what is presented here to include the qualitative and quantitative analysis of gem material.

 Contents

 Part 1                  The Fundamentals

Part 2                  Organising the Elements – the Periodic Table

Part 3                  Bonding – joining atoms together

Part 4                  Covalent Vs Ionic

                             Nomenclature

Part 5                  Isomorphism

Part One   The Fundamentals

Definition  Chemistry is the study of the Earth’s material.

Definition Matter, the material of the Earth, is anything that has mass and occupies space (has volume).

Definition  Mass is the measure of the inertia of a body (how difficult it is to move).

World  (made up of)

Elements                                 Compounds                             Mixtures

~120                                        ~3.5 million                              Infinity

Liquids 2

Gases 11

Solids ~107

(Smallest piece of element or compound)

Molecule

(is composed of)

Atoms

(composed of)

Protons   (have unitary positive charge)

Neutrons (have no external charge)

Electrons (have unitary negative charge)

The matter of the world is divided into three types, elements, compounds and mixtures.

 Elements

 There are about 90 naturally occurring elements with the rest being man-made.  The number of these latter continues to rise but since they are measured in micrograms, in most cases can safely be ignored for gemmological purposes.

Definition  An element is a substance that cannot be divided into simpler substances by chemical means.

 

Two of the elements are liquids, Bromine and Mercury, neither of which is found in gemstones.

Eleven of the elements are gases.  These are the six on the right hand side of the periodic table (more about this later) and are called the Inert or Noble Gases.  Their names are Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) and Radon (Rn).  The other five are Hydrogen (H) Nitrogen (N) Oxygen (O)  Fluorine (F) and Chlorine (Cl).

Only one gemstone is composed of a single element, this is the Diamond, a crystalline form of Carbon, and even this stone generally has impurities of other elements, eg Nitrogen and Boron, or uncrystallised carbon.

Compounds

Definition  Compounds are substances that can be broken down chemically to simpler substances.

About 90% of the world’s compounds are solids and it among these that all the gemstones, excepting Diamond, are to be found.

Mixture

Definition  A mixture is two or more elements or compounds together without being chemically combined.

The first task of a chemist is the separation of mixtures into their pure forms and generally mixtures may be ignored in Chemistry.  In Gemmology most gemstones are solid mixtures with one substance dominating but the lesser influencing colour, spectra etc.

Molecule

 Definition  The smallest piece of an element or a compound that still has the properties of that element or compound is called a molecule.

 Atom

Definition An atom is the smallest piece of matter that takes part in a chemical reaction.

In chemistry the atom remains indivisible with the molecule being made up of a discrete number of atoms, eg. The Carbon dioxide molecule is composed of one carbon atom combined with two oxygen atoms.  A half or portion of an atom is never to be found in a molecule.

The atom is comprised of three particle forms, the proton, the neutron and the electron and these are not generally to be found free of the atom.

Proton

Definition The Proton is an elementary particle of an atom with a unit positive charge and a mass of ~ 1 amu. (atomic mass unit)

Electron

Definition The Electron is an elementary particle of an atom with a unit negative charge and a mass some 1800 times smaller than the proton.

Neutron

Definition The Neutron is an elementary particle of an atom with no external charge and a mass very slightly greater than the proton.

The number of protons in the atom determines what atom it is.  The Hydrogen atom has only one proton, that of Oxygen has 8 while the Uranium atom has 92.

Atomic Number

Definition  The atomic number of an atom is the number of protons it contains and is always an integer.

A neutral atom will always have the same number of protons and electrons but the number of neutrons may vary leading to isotopes.   If the number of protons and electrons in an atom do not match, the atom is called an ion.  All gemstones, except Diamond, are composed of ions.

Ion

Definition  An ion is an atom, or group of atoms, which has lost or gained one or more electrons.

Definition Isotopes are atoms with the same number of protons but different number of neutrons.

Eg Oxygen 16 has 8 protons, 8 neutrons and 8 electrons.  The isotope oxygen 15 has 8 protons, 7 neutrons and 8 electrons, while the isotope oxygen 17 will have 8 protons, 9 neutrons and 8 electrons.

The Table below gives the actual masses of the three elementary particles.  A gram is included for convenience.

Table  1

Kg                                  Atomic Mass Units

Proton         1.672621637 * 10-27                               1.00727646677

Neutron       1.67492729 * 10-27                    1.0086649156

Electron       9.10938215 * 10-31                    5.4857990943 * 10-4

1 gram                   1 * 10 -3                          6.02214179 * 1023

 The actual mass of an atom is less that than the sum of its individual elementary particles.  This is due to energy being lost as the particles are forced into combination.

Atomic Weight (Relative Atomic Mass)

Definition  The atomic weight is the mass of the atom compared to 1/12th that of an isolated carbon atom, at rest and in its ground state, containing 6 protons, 6 neutrons and 6 electrons.

As a rough measure assigning unitary mass to the protons and neutrons in an atom the sum of these two particles can be taken as the atomic weight in amu (atomic mass units).  This number is known as the Mass number.

Mass Number

Definition  The Mass Number is the sum of the protons and neutrons in the atom.

Definition Nucleon a particle found in the nucleus of an atom, a proton or a neutron.

Number of each fundamental particle in the atom.

 The Mass Number of an element is got by first taking the integer of the Atomic Weight (Relative Atomic Mass).  Then the Atomic Number is subtracted from this and one has the number of neutrons in the atom.

e.g  Lead has an atomic number of 82 and an atomic weight of 207.21. From the atomic number one knows that the atom of lead has 82 protons in it and as it is neutral it must also have 82 electrons in it as well.  The integer of the atomic weight is 207  and this is the mass number.  Subtracting the atomic number (82) from this gives 125 and this is the number of neutrons in the atom.  Thus an atom of lead has 82 protons, 125 neutrons and 82 electrons in it.

 Note  IUPAC, the International Union of Physical and applied Chemistry on being asked to formally recognize Relative Atomic Mass as the proper nomenclature for the widely used Atomic Weight, refused to do so even though it is mass that is measured not weight (weight = mass * gravity) saying that the term atomic weight had been in use from the dawn of chemistry and that everyone knew that it was a mass anyway and so why bother to change the term.  Thus either term can be used.

Part 2

Lay out of the Elements   The Periodic Table

An atom is composed of three forms of elementary particles, Protons, Neutrons and Electrons.  The protons and neutrons are found in the centre of the atom, the nucleus, and the electrons are found orbiting this nucleus in an arrangement of Shells and Sub-Shells at varying distances from this nucleus.  The nucleus is tiny, in terms of volume, compared to that of the atom itself.  To get an idea of the vast “emptiness” of an atom if the nucleus of a hydrogen atom is represented by a cent then the electron, represented by a grain of rice, would be found in a spherical orbit around it of about radius 330 metres.

The various shells occupied by the electrons are numbered 1 to 8 from the nucleus and up to four different sub-shells may be found in each shell.

The Maximum number of electrons to be found in a shell is given by the formula   2 N 2  where N is the number of the shell.

The first shell is full when it has 2*12 , ie 2 electrons

The second shell is full when it has 2 * 22 , ie 8 electrons

The third shell is full when it has 2 * 32  , ie 18 electrons and

The fourth shell is full when it has 2 * 42 , ie 32 electrons in it.

The remainder of the shells could have 50, 72, 98 and 128 electrons respectively but in fact are largely unfilled at this time.  Remember there are only about 120 known elements to date.

 These sub-shells are called S, P, D and F derived from spectroscopic lines called Sharp, Principal, Diffuse and Fundamental.

The number of electrons that may be found in the sub-shell is

S       2        P       6        D      10       F       14

The S sub-shell is the least complicated and may be visualised by a sphere.

The P sub-shell is more complicated and may be visualised by a figure eight dumbbell.

The D sub-shell may be visualised by two figure eight dumbbells crossing at their centres.

The F sub-shell may be visualised by three figure eight dumbbells all intersecting at their centres.

The Periodic Table, devised by Dimitri Ivanovich Mendeleev in 1867, puts an order on the known elements and is one of the most powerful tools in use in Chemistry.

The Elements are inserted into the table in increasing Atomic Number, the number of protons in the Nucleus (the heart of the atom), and columised by the number of electrons in the outermost shell.  For certain elements e.g. numbers 21 to 30 (transition elements) where the number of electrons in the outside shell is the same in all, the members in the columns are placed according to their immediate inner subshell.

Short form of Periodic Table

group

1

2

3

4

5

6

7

0

period

1

H

He

2

Li

Be

B

C

N

O

F

Ne

3

Na

Mg

Al

Si

P

S

Cl

Ar

4

K

Ca

*********

Ga

Ge

As

Se

Br

Kr

5

Rb

Sr

*********

In

Sn

Sb

Te

I

Xe

6

Cs

Ba

*********

Th

Pb

Bi

Po

At

Rn

7

Fr

Rd

Transition Elements in Period 4
Sc Ti V Cr Mn Fe Co Cu Ni Zn

Each group (Column) member has the same number of electrons in its furthest out shell, this is the shell that is involved in bonding.  Thus all the elements in group 5 for example have five electrons in their outermost shell although having a different number inside.

Element       Name           Outside electrons            Inside electrons

N                 Nitrogen                5                                     2

P                 Phosphorus           5                                     10

As               Arsenic                 5                                     28

Sb               Antimony              5                                     46

Bi                Bismuth                 5                                     78

The periodic table shown above, taken from the Mathematical tables issued by the Government Publications Sales Office for use in State examinations shows each element, identified by its chemical symbol, in its place with the atomic number (Z) above it and its atomic weight (rel.at. Mass), based on the oxygen 16 atom, below.  Note the missing elements between Z57, Lanthanum and Z72 Hafnium and following Z89, Actinium.  These are the Inner Transitional Elements and are rarely met with in Chemistry although some do appear occasionally in gemstone spectra.

Note  1  Hydrogen, which has been placed in group 1, only has 1 electron in its atom and should really be placed alone separate from all the groups since, at times, it can behave as if it were in group 7 as well.

Note  2   The metal elements are to be found on the left of the table with the non-metal elements being found on the right.

It is easier for atoms of elements on the left of the periodic table to lose one, or more, electrons becoming positively charged ions than it is for those on the right.  Generally the atom will not lose electrons from the inner shells..  The periodic table below shows the energy required to remove the most loosely bound electron from neutral, gaseous atoms of an element.  It is measured in kilojoules per mole of electrons removed.

Note 3  The atoms of the elements in group 1 and 2 have their furthest out electrons in S sub-shells.

Note 4  The atoms of the elements in groups 3 to 0 have their last electrons filling the P sub-shells.

Note 5 The atoms of the elements Z21 to Z30 and those underneath them have their last electrons filling the D sub-shells and these are known as Transition elements.

Note 6 The atoms lying between Z57 and Z72 and the 14 following Z89 are known as the inner Transition elements and have their last electrons filling the F sub-shells.

Definition A Mole is the number of atomic mass units in a gram.  6.02 * 1023

Definition A mole of atoms of an element will have a mass equal to the atomic weight (Rel.At.Mass) of that element.

 Symbols of the elements

12 of the elements have single letters used as their chemical symbol, these are Hydrogen (H), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Phosphorus (P), Sulphur (S), Vanadium (V), Yttrium (Y), Iodine (I) and Tungsten (W).

The other elements up to 103 use two letters as their chemical symbol, usually the first and second or first and last.

11 of the elements have symbols which at first glance have no bearing on their names. These have been shown in bold in the list of element names and symbols further on.

The symbol used in 10 of the cases is derived from the elements latin name showing that it was known to the ancients.  These 10 are

Sodium (Na)  Natrium,              Potassium (K) Kalium,              Iron (Fe) Ferrum,

Copper (Cu) Cuprum,               Silver (Ag) Argentum,               Tin (Sn) Stannum,

Antimony (Sb) Stibium,            Gold (Au) Aurum,

Mercury (Hg) Hydroargentum,  Lead (Pb) Plumbum.

The final element, Tungsten (W) is derived from the German Wolfram, a mineral in which it is found.

First Ionisation Energies of the Elements

 Since it is easier for an atom of an element on the left to lose an electron than for an atom on the right it should be no surprise to find that two atoms joining together, one from the left and one from the right that the left hand atom should lose one or more electrons to the atom on the right leaving a pair of oppositely charged ions which remain together by electrostatic attraction.

If one chooses the elements sodium and chlorine, ionization energies of 494 and 1260 kj mol-1 respectively, both poisonous to humans and combine them one gets the Na+Clgroup, sodium chloride which not only is non poisonous but, indeed, is essential for the human.

Other periodic tables giving the size of the ionic radius are available for the different ionic charges of those atoms exhibiting variable valancy.

Definition Valency is the number of electrons lost, gained, or given to share by an atom in the course of bonding (joining together).

 List … Names of the elements with their chemical symbols

 1        Hydrogen    H

2        Helium         He

3        Lithium        Li

4        Beryllium     Be

5        Boron                   B

6        Carbon        C

7        Nitrogen      N

8        Oxygen        O

9        Fluorine       F

10      Neon           Ne

11      Sodium       Na

12      Magnesium  Mg

13      Aluminium   Al

14      Silicon         Si

15      Phosphorus P

16      Sulphur        S

17      Chlorine       Cl

18      Argon                   Ar

19      Potassium  K

20      Calcium       Ca

21      Scandium    Sc

22      Titanium      Ti

23      Vanadium    V

24      Chromium   Cr

25      Manganese   Mn

26      Iron            Fe

27      Cobalt         Co

28      Nickel          Ni

29      Copper       Cu

30      Zinc             Zn

31      Gallium        Ga

32      Germanium  Ge

33      Arsenic        As

34      Selenium      Se

35      Bromine       Br

36      Krypton       Kr

37      Rubidium     Rb

38      Strontium     Sr

39      Yttrium        Y

40      Zirconium    Zr

41      Niobium      Nb

42      Molybdenum Mo

43      Technetium  Tc

44      Ruthenium   Ru

45      Rhodium     Rh

46      Palladium     Pd

47      Silver                   Ag

48      Cadmium     Cd

49      Indium         In

50      Tin              Sn

51      Antimony    Sb

52      Tellurium     Te

53      Iodine                   I

54      Xenon         Xe

55      Caesium      Cs

56      Barium         Ba

57      Lanthanum   La

58      Cerium        Ce

59      Praesodymium  Pr

60      Neodymium Nd

61      Promethium Pr

62      Samarium    Sm

63      Europium     Eu

64      Gadolinium  Gd

65      Terbium       Tb

66      Dysprosium Dy

67      Holmium      Ho

68      Erbium        Er

69      Thulium       Tm

70      Ytterbium    Yb

71      Lutetium      Lu

72      Hafnium       Hf

73      Tantalum     Ta

74      Tungsten    W

75      Rhenium      Re

76      Osmium       Os

77      Iridium         Ir

78      Platinum      Pt

79      Gold           Au

80      Mercury     Hg

81      Thallium      Tl

82      Lead           Pb

83      Bismuth       Bi

84      Polonium     Po

85      Astatine       At

86      Radon         Rn

87      Francium     Fr

88      Radon         Ra

89      Actinium      Ac

90      Thorium      Th

91      Protactinium  Pa

92      Uranium       U

93      Neptunium   Np

94      Plutonium    Pu

95      Americium   Am

96      Curium        Cm

97      Berkelium    Bk

98      Californium  Cf

99      Einsteinium  Es

100    Fermium      Fm

101    Mendelevium Md

102    Nobelium     No

103    Lawrencium Lr

Part 3

 Bonding

 Definition Bonding is the joining of two atoms together.

Atoms bond, join together, by means of electrons in their outermost shells.  These electrons form the “skin” of the atom and just as when humans shake hands it is the skin that makes contact, not the inner material so too in bonding, it is only the outermost electrons that come in contact with each other.  A bond involves two electrons, one from each atom and the number of bonds an atom can make depends on the number of electrons in its outside shell.

As a general rule an atom will lose, gain or share electrons so as to have the nearest inert gas (noble gas) configuration, which is generally 8 electrons in the outside shell.

If one examines the second period of the periodic table one sees that the number of electrons held by each of the atoms in their outside shell is as follows …

Li   1,   Be   2,   B   3,   C   4,   N   5,   O   6,   F   7 and   Ne   8.

Neon (Ne) already has 8 electrons in its outermost shell and so will, except in very unusual circumstances, not bond with any element, even itself.  This lack of reactivity is the basis of the group name, Inert or Noble gases (since all are gases at normal temperatures).

Fluorine with its 7 electrons only requires to gain one electron to have 8 (as Neon) electrons in its outermost shell but would need to lose 7 to become like the previous Inert gas, Helium.

It is known that no bonding in which more than three electrons are gained or lost is known to occur.  Any atom requiring more than that will share electrons so as to come to the “magic” eight number (2 in the case of Helium).  In fact it is very difficult to remove/add even three electrons.

Lithium (Li) with one electron in its outermost shell would have to gain 7 to become like Neon but losing just one would give it the Helium electronic configuration.

If, then, Lithium and Fluorine come together the Lithium will lose an electron to the Fluorine and both atoms (now ions) will have attained inert gas configuration and be “happy”.

Beryllium will lose two electrons, Boron three, C will give four and get four sharing eight, Nitrogen will gain three, and Oxygen two thus forming 2, 3, 4, 3 and 2 bonds respectively.

All the other members of these groups will make the same bonds as the above.

Definition Valency is the number of bonds made, or, in other words the number of electrons the atom gains, loses or gives in to share during bonding.

In the case of the Transition elements which all have two electrons in their outermost shell and are filling an inner sub-shell, they all display a valency (number of bonds made) of two.

If the above held universally chemical life would be easy but unfortunately variations occur.  Two types of variation occur.

In the first variation the number of electrons after bonding exceeds the Inert Gas rule in elements of group 5, 6 and 7 lying in periods of greater number than 2.  As seen earlier Shell two can contain a maximum of 8 electrons but shell three can have 18 and the later periods even more.  Thus taking the period three elements of groups 1 to 0 one has, as before

1,  2,  3,  4,  3,  2,  1,  0 bonds being made by the elements Na, Mg, Al, Si, P, S, Cl and Ar  respectively where the integer is the number of electrons of the atom in use in bonding.   However the P, S and Cl atoms may also make use of all the 5, 6 and 7 electrons in their outermost shell for bonding.

The later members of groups five, six and seven may also display this variation.

Examples of this variable valency are

1        Phosphorus and Chlorine which lead to two different compounds PCl3 and PCl5 (phosphorus tri-chloride and phosphorus penta-chloride) where the Phosphorus is using three electrons for bonding in the PCl3 and five in the PCl5.

2        Sulphur, Hydrogen and Oxygen which lead to two compounds H2S and H2SO4 (hydrogen sulphide and sulphuric acid) where the sulphur is using two electrons for bonding in the H2S but six in the H2SO4.

3        Chlorine, Hydrogen and Oxygen which lead to HCl and HClO3 (hydrogen chloride and hypochlorous acid) where the chlorine atom is using one electron for bonding in the HCl but seven in the HClO3.

The second variation involves the Transition elements.  As stated previously all show a valency of two from the fact that they have two electrons in their outermost shell.  However, if excited, by heat, light or other energy, one, or more of the electrons filling the inner sub-shell can be “promoted” into the outermost shell making them available for bonding.  Thus for all the group from Scandium to Zinc, apart from poor Zinc, these elements exhibit variable valency and it is this variation which allows these elements to force gemstones in which they exist, either as part of the composition of the host itself or as impurities, to exhibit different colourations.  Zinc never shows any valency other than two and most books refuse to accept it as a Transition Element.  The Transition Elements are noted for their colour change with valency and the atoms of these elements have a great influence on the colour of gemstones.

Three principal forms of chemical bond exist.  These are

Covalent, where each atom gives one or more electrons to be shared and share the given electrons equally.

Polar Covalent, where each atom gives one or more electrons to be shared but have an unequal share of the given electrons, one atom becoming slightly positive the other slightly negative.

Ionic, where one atom loses one, or more, electrons to another, positive and negative ions forming held together by electrostatic attraction.

Note:  Two sub-sections are found, Hydrogen bonding and Dative or Co-ionic bonding, but there is no need to look at these under the heading of this work.

The question of deciding which of the bonds above exist in a chemical is difficult at first sight but fortunately Linus Pauling devised a easy reckoner for the problem in 1932.  His scale is given below.

Definition Electronegativity is the “pulling” power of an atom for shared pairs of electrons.

The degree of ionization in a bond varies from 0% to 100% but very few bonds exist at the extremes.  Most bonds are partly ionized.

Pauling discovered that when the electronegativity difference between two atoms was 1.7 they displayed 50% ionization while a difference of 0.5 showed only a 4% ionization.

If one uses these two figures (0.5 and 1.7) as the “marks” one can say

Atomic radius decreases → Ionization energy increases → Electronegativity increases →

Group (vertical)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Period (horizontal)

1

H
2.20

He

2

Li
0.98

Be
1.57

B
2.04

C
2.55

N
3.04

O
3.44

F
3.98

Ne

3

Na
0.93

Mg
1.31

Al
1.61

Si
1.90

P
2.19

S
2.58

Cl
3.16

Ar

4

K
0.82

Ca
1.00

Sc
1.36

Ti
1.54

V
1.63

Cr
1.66

Mn
1.55

Fe
1.83

Co
1.88

Ni
1.91

Cu
1.90

Zn
1.65

Ga
1.81

Ge
2.01

As
2.18

Se
2.55

Br
2.96

Kr
3.00

5

Rb
0.82

Sr
0.95

Y
1.22

Zr
1.33

Nb
1.6

Mo
2.16

Tc
1.9

Ru
2.2

Rh
2.28

Pd
2.20

Ag
1.93

Cd
1.69

In
1.78

Sn
1.96

Sb
2.05

Te
2.1

I
2.66

Xe
2.60

6

Cs
0.79

Ba
0.89

*

Hf
1.3

Ta
1.5

W
2.36

Re
1.9

Os
2.2

Ir
2.20

Pt
2.28

Au
2.54

Hg
2.00

Tl
1.62

Pb
2.33

Bi
2.02

Po
2.0

At
2.2

Rn
2.2

7

Fr
0.7

Ra
0.9

**

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Cn

Uut

Uuq

Uup

Uuh

Uus

Uuo

Lanthanoids

*

La
1.1

Ce
1.12

Pr
1.13

Nd
1.14

Pm
1.13

Sm
1.17

Eu
1.2

Gd
1.2

Tb
1.1

Dy
1.22

Ho
1.23

Er
1.24

Tm
1.25

Yb
1.1

Lu
1.27

Actinoids

**

Ac
1.1

Th
1.3

Pa
1.5

U
1.38

Np
1.36

Pu
1.28

Am
1.13

Cm
1.28

Bk
1.3

Cf
1.3

Es
1.3

Fm
1.3

Md
1.3

No
1.3

Lr
1.3

Periodic table of electronegativity using the Pauling scale

If the electronegativity difference between the two atoms in a bond is less than or equal to 0.5 then the bond is pure covalent.

If the electronegativity difference between the two atoms in a bond is 1.7 or greater then the bond is Ionic and

If the electronegativity difference between the two atoms in a bond lies between 0.5 and 1.7 then the bond is Polar Covalent.

In determining which bonding should be ascribed to a chemical compound of more than two atoms the rule is to take the atom with the highest electronegativity value and compare it with the atom of least electronegativity value.  Eg

Spinel, Magnesium Aluminium Oxide, MgAl2O4,  the electronegativities are

Mg  1.31,  Al  1.61,   O  3.44.

Taking the largest value, that of oxygen 3.44 and the lowest, that of magnesium 1.31 and subtracting the lower from the higher one gets an Electronegativity Difference (E.D.) of 2.13.  This figure is a good deal higher than the “mark”  of 1.7 and so the compound is an ionic one.

Even taking the aluminium value with oxygen still gives an ionic result.

The magnesium is not directly linked to the aluminium.

The result of the formation of spinel is that the magnesium atom, from group 2, loses 2 electrons to oxygen while each of the two aluminium atoms lose three electrons to the oxygens.  Each oxygen ends up with two extra electrons and the full formula can be written

Mg2+Al23+O42-

In Gemmology the only gemstone in the Covalent category is the diamond which forms a giant covalent crystal, each of its carbon atoms’ four outermost electrons shared with four other carbon atoms to form this spectacular gemstone.

The only other gemstone not fully in the Ionic group is Quartz which is highly polar covalent and this could be taken as a giant polar covalent crystal but it is easier to regard it as an Ionic one.

Below is a representative list of gemstones and their chemical formulae

 

Stone

EN

Max

Min

ED

Bond
Diamond

C

2.55

2.55

0

Covalent
C

2.55

Fluorite CaF2

Ca

F

3.98

1

2.98

Ionic
Calcium Fluoride

1

3.98

CorundumAl2O3,

Al

O

3.44

1.61

1.83

Ionic
Aluminium Oxide

1.61

3.44

Quartz , SiO2,

Si

O

3.44

1.9

1.54

Pol.Cov.
Silicon Dioxide

1.9

3.44

Spinel, MgAl2O4,

Mg

Al

O

3.44

1.31

2.13

Ionic
Magnesium Aluminium Oxide

1.31

1.61

3.44

Chrysoberyl , BeAl2O4,

Be

Al

O

3.44

1.57

1.87

Ionic
Beryllium Aluminium Oxide

1.57

1.61

3.44

Taaffeite  Mg3BeAl8O16

Mg

Be

Al

O

3.44

1.31

2.13

Ionic
Magnesium BerylliumAluminium Oxide

1.31

1.57

1.61

3.44

Zircon,      ZrSiO4,

Zr

Si

O

3.44

1.33

2.11

Ionic
Zirconium Silicate

1.33

1.9

3.44

Peridot ,  (Mg,Fe)2SiO4,

Mg

Fe

Si

O

3.44

1.31

2.13

Ionic
Magnesium iron Silicate

1.31

1.83

1.9

3.44

Sinhalite, MgAlBO4,

Mg

Al

B

O

3.44

1.31

2.13

Ionic
Magnesium Aluminium Borate

1.31

1.61

2.04

3.44

Topaz Al2SiO4(F,OH)2

Al

Si

F

H

O

3.98

1.61

2.37

Ionic
Aluminium Silicate (Fluoride, Hydroxide)

1.61

1.9

3.98

2.2

3.44

Garnet,     A3B2(SiO4)3

Mg

Al

Si

O

3.44

1.31

2.13

Ionic
Magnesium Aluminium Silicate

1.31

1.61

1.9

3.44

Tanzanite , Ca2Al3(SiO4)3(OH),

Ca

Al

Si

O

H

3.44

1

2.44

Ionic
Calcium Aluminium Silicate Hydroxide

1

1.61

1.9

3.44

Benitoite,   BaTiSi3O9,

Ba

Ti

Si

O

3.44

0.89

2.55

Ionic
Barium Titanium Silicate

0.89

1.54

1.9

3.44

Emerald, Al2Be3Si6O18

Al

Be

Si

O

3.44

1.57

1.87

Ionic
Aluminium Beryllium Silicate

1.61

1.57

1.9

3.44

Feldspars
 Kspar endmember KAlSi3O

K

Al

Si

O

3.44

0.82

2.62

Ionic
Potassium Aluminium Silicate

0.82

1.61

1.9

3.44

Moonstone
Albite endmember, NaAlSi3O8

Na

Al

Si

O

3.44

0.93

2.51

Ionic
Sodium Aluminium Silicate

0.93

1.61

1.9

3.44

Jadeite
Anorthite endmember, CaAl2Si2O8

Ca

Al

Si

O

3.44

1

2.44

Ionic
Calcium Aluminium Silicate

1

1.61

1.9

3.44

Sunstone
 
Cubic zirconia ZrO2

Zr

O

3.44

1.33

2.11

Ionic
Zirconium dioxide

1.33

3.44

Part 4

Covalent Vs Ionic and Nomenclature

Properties of Covalent and Ionic compounds.  

A table of general properties of the extremes in bonding is given.  The Polar Covalent bonded compounds will lie somewhere in between.

Please remember that the following is general, exceptions do occur.

Covalent                                           Ionic

Liquids, gases or soft solids               Hard, brittle solids

Individual molecules                           Crystalline

Low melting points                             High melting points

Low boiling Points                             High boiling points

Slow partial reactions                          Fast complete reactions

Non conductors of electricity              Non conductors of electricity

in all states                                         when solid, conduct electricity

when molten or in solution

Soluble in covalent solvents                Soluble in polar solvents

Non-soluble in polar solvents              Non-soluble in covalent solvents.

Note.. covalent solvents are those based on carbon, i.e. the organic solvents, e.g. acetone, ether, ethanol etc. (CH3OCH3 ,  C2H5OC2H5 , C2H5OH )

Note.. polar solvents are those which have partial ionisation such as water, H2O, which has an electronegativity difference of 1.22 (3.44 – 2.20), between its hydrogen and oxygen atoms…This solubility is the basis of the hydrothermal process for the preparation of synthetic gemstones

Naming Compounds

In inorganic chemistry compounds are named by using the names of the constituent atoms.  The general rule is that the lower electronegativity atom is placed at the beginning of the chemical formula and then the other atoms in the compound in increasing electrochemical sequence.

All compounds will have the same number of positive and negative charges since they are electrically neutral overall.

1.       Compounds of two atoms taken one from each of two different elements

If both atoms are from the same element the compound is known by its elemental name, e.g. O2  oxygen,  H2 Hydrogen etc.

Where only one atom from each of two different elements combine, e.g. Iron and Sulphur, the lower electronegative atom is named first followed by the higher with its ending truncated and the suffix IDE attached.  Thus in the example the compound is Iron Sulphide FeS.  Here the first syllable of the atom/element of higher electronegativity is taken and –ide added.  This is the general form and give

Ox – ide  (Oxygen) e.g. magnesium oxide  ( MgO )

Nitr –ide  (Nitrogen) e.g. aluminium nitride ( AlN )

Chlor – ide (Chlorine) e.g. sodium chloride ( NaCl )

Sulph – ide (Sulphur) e.g. iron Sulphide ( FeS )

Fluor – ide (Fluorine) e.g. potassium fluoride ( KF )

Brom – ide (Bromine) e.g. lithium bromide ( LiBr )

2.       Compounds with more than just two atoms taken from two different elements.

These compounds are named as in the previous section but when the two elements form more than one compound a prefix is attached to differentiate between them.

Calcium Chloride  CaCl2  One atom of calcium to two atoms of chlorine. (Calcium is in group 2 and can lose two electrons, the chlorines are in group 7 and wish to gain one electron each.)

Sodium Oxide Na2O Two atoms of Sodium (group 1) combine with one atom of Oxygen (group 6)

Other examples, taken from gemstones, are

Fluorite … Calcium and Fluorine giving Calcium Fluoride   Ca F2.

Quartz (all varieties)…. Silicon and Oxygen giving Silicon Dioxide SiO2

Corundum (Ruby, Sapphire) … Aluminium and Oxygen giving Aluminium Oxide Al2O3

Cubic Zirconia …. Zirconium and Oxygen giving Zirconium dioxide ZrO2

Note The prefix “Di” is added to the oxide in the silicon example to avoid confusion, Silicon commonly linked to 4 oxygens is also to be found.  Other prefixes are “Mon” as in carbon monoxide CO  (one oxygen) and “Tri” as in sulphur trioxide SO3 (three oxygens)since CO2 (carbon dioxide) and SO2 (sulphur dioxide) also exist.

3.       Compounds composed of atoms from three elements, one of which is Oxygen 

Type 1  The two non-oxygen elements are from opposite sides of the Periodic table.

These compounds can be taken to be formed from one of the previous types the  -ides with oxygen added.  The compound is now named by changing the –ide to –ate.  The suffix –ate always means that oxygen has to factored into the formula.

Examples  ..

Calcium sulphate  CaSO4  (Based on calcium sulphide CaS)

Hydrogen sulphate H2SO4  (Based on Hydrogen Sulphide H2S, the stink of rotten eggs.)

Type 2   The two (or more) non-oxygen elements are from the left-hand side of the Periodic Table

In this type the compound is named by giving the names of the non-oxygen elements, in order of rising electronegativity followed by Oxide.  Examples from gemstones are as follows …

Spinel :  Magnesium Aluminium Oxide , MgAl2O4  (E.N. 1.31, 1.61, 3.44)

Chrysoberyl : Beryllium Aluminium Oxide , BeAl2O4 (E.N. 1.57, 1.61, 3.44)

Taaffeite : Magnesium Beryllium Aluminium Oxide , Mg3BeAl8O16 (E.N. 1.31, 1.57, 1.61, 3.44)

4.       Compounds involving an inorganic radical.

Definition: An inorganic Radical is a group of atoms which remain together during chemical reactions.  E.g. in the chemical reaction

Mg + H2SO4 à MgSO4 + H2

(Magnesium and sulphuric acid (hydrogen sulphate) react forming Magnesium Sulphate and Hydrogen)

The SO4 (sulphate radical) remains unaltered.

The inorganic radicals are all ionised having from 1 to 5 negative charges.

One negative charge        -OH             Hydroxide

-ClO3           Chlorate

-NO3           Nitrate

Two negative charges      SO4            Sulphate

CO3            carbonate

Three negative charges    PO4             Phosphate

Four negative charges      SiO4                     Silicate

Five negative charges       BO4             Borate

Compounds of these are named from the other elements in increasing electronegativity (if more than one) followed by the radical.  (Occasional exceptions do occur usually because it is easier to pronounce the formula in that way.)

Sodium Hydroxide (caustic soda) NaOH

Sodium Chlorate (weed killer) NaClO3

Potassium Nitrate (nitre) KNO3

Potassium Sulphate  K2SO4

Calcium Carbonate (lime-stone)  CaCO3

Lithium Phosphate  Li3PO4

Zirconium Silicate  (Zircon)  ZrSiO4

Magnesium Iron Silicate (Peridot)  (Mg, Fe)SiO4

Magnesium Aluminium Silicate (Pyrope)  Mg3Al2(SiO4)3

Calcium Aluminium Silicate Hydroxide (Tanzanite) Ca2Al3((SiO4)3(OH)

Magnesium Aluminium Borate (Sinhalite) MgAlBO4

5.       Radicals with one less attached oxygen

The mono-negative NO2   (from nitrate NO3 ) as in NaNO2 is named Sodium Nitrite.

The di-negative SO3  (from sulphate SO4 ) as in Na2SO3 is named Sodium Sulphite.

6.       Other silicates

Various multiple combinations of silicon with oxygen exist, but, in all each silicon is quadrivalent, making four bonds.  Silicon, with its electronegativity of 1.9 compared to oxygen’s 3.44, is highly positive and must be counted with the metals to balance out the negativity of the oxygen ions in the compounds.

Some gemstones, without the one to four silicon to oxygen ratio are given below.  If one adds all the “positives” they will exactly match the “negatives”

Benitoite:  Barium Titanium Silicate,  BaTiSi3O9   (+2 +4 +3*4 –9*2  =0)

Emerald:   Aluminium Beryllium Silicate Al2Be3Si6O18 (+2*3 + 3*2 +6*4 -18*2 = 0)   (The beryllium E.N. 1.57 should come before the aluminium E.N. 1.61 but it is easier to pronounce as written.)

Moonstone: Potassium Aluminium Silicate,

KAlSi3O8 ( +1 + 3 + 3*4 – 8*2 =0)

Jadeite:  Sodium Aluminium Silicate,

NaAlSi3O8  ( +1 + 3 + 3*4 – 8*2 =0)

Sunstone: Calcium Aluminium Silicate,

CaAl2Si2O8 (+2 +2*3 +2*4 – 8*2  = 0)

 

Part 5

Solubility, Isomorphism

Solubility in Water

In Part 4 the inorganic radical was introduced and eight negative radicals were given, viz. Hydroxyl (OH), Chlorate (ClO3), Nitrate (NO3), Sulphate (SO4), Carbonate (CO3), Phosphate (PO4), Silicate (SiO4) and Borate (BO4).

One inorganic radical was not given.  This is the ammonium radical NH4 and it has a single positive charge.  Since the ammonium radical is positive and all the rest negative one should not be surprised to find that it is found in combination with each of the negatives giving  NH4OH, NH4ClO3, NH4NO3, (NH4)2SO4, (NH4)2CO3, (NH4)3PO4, (NH4)2SiO4 and (NH5)BO4 respectively.

Since all gemstones, excepting diamond, are ionic in nature one would expect to find representative specimens of gemstone of each of the nine radicals above.  However on examination this is not so.  The reason is simple.  A gemstone needs to be durable and not affected by the surroundings, among other factors.  In the case of many of the inorganic radicals, one finds their compounds to be soluble in water.  Thus the very perspiration of the skin would be sufficient to dissolve them.

For this very same reason a number of very common metals are not to be found in gemstones since the presence of these metals in a compound renders it very soluble in general.

All group 1 metals, Lithium, Sodium, Potassium, Rubidium, Caesium and Francium and the positive radical, ammonium (NH4) are soluble in water at ordinary temperatures and pressures.  They are  more so at higher temperatures and pressures.   So no surprise not to find any gemstone containing the ammonium radical.  A number of gemstones belonging to the feldspar group do exist but these fall into the low solubility section and ought to be kept separate from water.

The group 1 metal and ammonium radical hydroxides are water soluble as are the combination with Barium (Ba2+), Strontium (Sr2+) and Tellurium (Tl+) all the others are insoluble, Tanzanite and the Tourmalines are two of the groups found with the hydroxyl (OH) group in them.

(The Oxides act in the same manner as the hydroxides above.  Gemstone examples are the Corundums, the Spinels, the Chrysoberyls and Taaffeite.)

The inorganic compounds of Chlorine, Bromine and Iodine are all soluble while those with Fluorine are deemed insoluble.  Thus Fluorite, Calcium Fluoride (CaF2) is used as a gemstone the others of the Halogen (group 7) family are not.

All the chlorates are soluble in water.  No gemstone contains this radical.

All the nitrates are soluble in water.  No gemstone contains this radical.

The large majority of compounds containing the sulphate (SO4) radical is soluble in water.  Only those sulphates combined with Silver(Ag+), Lead (Pb2+), Barium (Ba2+), Strontium (Sr2+) and Calcium (Ca2+) are found to have low solubility.

The Carbonate compounds are all insoluble except for those joined to the group 1 metals, hydrogen or the ammonium radicals.

The Phosphate compounds of group 1 metals, hydrogen and ammonium radical are all soluble.  All other members have low solubility.

The silicates are generally insoluble in water but become increasingly soluble as the temperature and pressure rises.  Even quartz, which is the least soluble dissolves to about 6 ppm (parts per million) in pure water at 25 degrees Celcius.  An interesting feature of the Silicates is that while their solubility increases with temperature and pressure generally, between 340 and 550 degrees Celcius and 0 to 800 bar of pressure their solubility decreases.

The Borate compounds are generally very to moderately soluble in water.  The solubility is affected by pH, particularly between 6 and 8.  Since this is about the pHs found on the human such gemstones ought to be worn over clothing.  Sinhalite (MgAlBO4) is a gemstone of this group.

Isomorphism

The notion of isomorphism was first put forward by the German Eilhard Mitscherlich (1794 to 1863) who stated that compounds crystallizing together probably have similar structures and composition.  His contemporary, Jons Berzelius, made use of the idea in attempting to assign atomic weights to the elements.

Isomorphism is the property shown by some minerals in crystallising together in variable chemical proportions.  For example in the Garnet family, Pyrope (Mg3Al2(SiO4)3) and Almandine (Fe3Al2(SiO4)3) both of whom show Cubic crystallisation may exist as an in-between form, written (Mg,Fe)3Al2(SiO4)3, which has both Magnesium and Iron, in variable amounts in it leading to in-between properties, such as specific gravity, refractive index, hardness, colour etc..  The in between form is called Rhodalite.

Definition: Endmember is a pure, or unsubstituted, compound forming one end of a family of isomorphic compounds.  E.G. the Pyrope or the Almandine of the the Garnet group mentioned above.

Definition: Midmember is the partially substituted compound formed by the partial substitution of an element of one endmember for that of another. E.G. The Rhodalite compound formed by the substitution of some of the magnesium of the Pyrope by the Iron of the Almandine.

To belong to an isomorphic family the gemstones must

  1. Have the same general formula
  2. Have ions of the same valency
  3. Has ions of approximately the same ionic radius
  4. Have the same crystal structure.

Taking the Garnet group as the example is is seen that all the family members gave the same general formula, i.e. A3B2(SiO4)3 where A is a di-valent positive ion e.g. coming from Group 2 of the Periodic Table, and B is a tri-valent positive ion, coming from Group 3 of the Periodic Table or from the first Transition Series.  They all display the Cubic form of crystallization.

While all the ions just mentioned theoretically could be involved in Garnet gemstones, it is found in Nature that only a few actually become involved.  These are the dipositive Iron, Magnesium, Manganese and Calcium and the tripositive Iron, Chromium and Aluminium.

Table 1    Ionic Radii of the Garnet metals in picometers

(0.000,000,000,001 m = 1 pm)

Fe+2             77                         Fe+3                       63

Mg+2            86                         Cr+3             75.5

Mn+2            89                         Al+3             53.5

Ca+2            114

Table 1 above shows that while the ionic radii of Iron, Magnesium and Manganese are roughly of the same size and should be able to substitute for each other the Calcium is so much larger and would be unlikely to be substituted as it would distort the crystal. Thus one finds the Pyralspite series of Garnet isomorphs, based on the endmembers Pyrope (Mg3Al2(SiO4)3), Almandine (Fe3Al2(SiO4)3) and Spessartine (Mn3Al2(SiO4)3).

Like wise the tripositive Iron, Aluminium and Chromium can lead to the Ugrandite series of Garnet isomorphs, based on the endmembers Uvarovite (Ca3Cr2(SiO4)3),  Grossularite (Ca3Al2(SiO4)3) and Andradite (Ca3Fe2(SiO4)3).  However substitution by the Chromium tripositive ion causes, due to its large size, distortion and is rarely found in any size.

Table 2  values for Pyrope, Rhodolite and Almandite

Pyrope        Rhodolite     Almandite

Specific Gravity              3.62 (low)    3.84   (ave)  4.30 (high)

Average                 3.78                                4.05

Refractive Index              1.720 (low)  1.760 (ave.) 1.820 (high)

Average                 1.740                              1.790

Table 3  values for Pyrope, Malaia (Malaya) and Spessartine

Pyrope        Malaia          Spessartine

Specific Gravity              3.62 (low)    3.80 (ave)    4.26 (high)

Average                 3.78                                4.16

Refractive Index              1.720 (low)  1.758 (ave.)  1.820 (high)

Average                 1.740                              1.810

Table 4  values for Spessartine, Maralambo and Almandite

Spessartine  Maralambo  Almandite

Specific Gravity              4.26 (low)    4.28 (ave)    4.30 (high)

Average                 4.16                                4.05

Refractive Index              1.789 (low)  1.800 (ave)  1.820 (high)

Average                 1.810                              1.790

Note “low” indicates the lowest value, “high” the highest value and “ave.” the average value for the isomorph.

Since pure endmembers are seldom found in Nature the figures above can vary depending on the composition of the isomorph.

In deciding what name to give a gemstone of the above the general rule is if it is 70% or more of an endmember then it is named that endmember, if between 30% and 70% of an endmember then it is given the midmember name.

Other isomorphic groups are the Spinels and the Tourmalines.

Spinel

 The Spinel group’s general  formula is A B2 O4  where again A is di-positive and B tri-positive

A is Mg, Zn or Fe while B is Al for what are called the aluminium spinels, The endmembers of the “A” isomorphic group are Spinel (MgAl2O4), Gahnite (ZnAl2O4)and Hercynite (FeAl2O4).  Midmembers are given a joint name e.g. Gahnospinel (ZnMg)Al2O4.

Other dipositive ions found in Nature are Titanium, Manganese and Nickel while tripositive ions in the “B” situation are Iron, Manganese, Chromium and Titanium.  However none of these lead to gemstones.

Tourmaline

In the Tourmaline isomorphic series Lithium, Iron, magnesium etc replace each other giving the resulting wide variation in colour of these gemstones.

Tourmaline provides the ultimate is complexity with its midmember general formula of

(Ca,K,Na,Li)(Al,Fe,Li,Mg,Mn)3(Al,Cr,Fe,V)6(BO3)3(Si,Al,B)6O18(OH,F)4

Where the ions inside each pair of brackets vary in their proportion in the final gemstone.

 

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