Chapter 7 basic mineralogy

Chapter 7 Basic Mineralogy


Mineral: a
naturally occurring,
inorganic
substance with a
characteristic
internal structure
and a chemical
composition that is
either fixed or
varies within
certain limits.

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Ta ble 7-1. Mineral cl asses
Class

Chemical characteristics

Exa mples

Borates

Vario us ele ments in co mbination with boron

Bora x [Na2 B 4 O 7 10H 2 O]

Carb onates

Metals in co mbination with carb onate
2
( CO 3 )

Calcite [CaCO 3 ]
Cerrusite [Pb CO 3 ]

Ha lides

Alkali metals or alkaline earths in
co mbination with halogens (F, Cl, Br, I)

Halite [NaCl]
Fluorite [CaF 2]

Hydro xides

Metals in co mbination with hyd ro xyls (OH -)

Brucite [Mg(OH) 2]

Native elements

Pure co mpound of a meta llic or non metallic
ele ment

Gold [Au]
Graphite [C]

O xid es

Metals in co mbination with o xygen

Hematite [Fe 3O 4 ]

Phosphates, arsenates,
vanadates, chro mates,
tungstates & molybdates

Vario us ele ments in co mbination with the
ZO 4 radical where Z = P, As, V, Cr, W , Mo

Apatite [Ca5 (PO 4 )3 (F,Cl,OH)]
Carnotite [K 2(UO 2 (VO 4) 2 3H 2 O]
Scheelite [CaWO 4 ]

Silicates

Metals in co mbination with silic a tetrahedra
4
( SiO4 ) for ming thre e d imensional
networks, sheets, chains and isolated
tetrahedra

Quartz [SiO 2 ]
Forsterite [MgSiO 4]
Orthoclase [KAlSi 3O 8 ]

Sulfates

Alkaline earths or metals in c o mbinatio n with Barite [BaSO 4 ]
2
sulfate ( SO 4 )
Epso mite [MgSO 4 7 H 2O]

Sulfides

One or more meta ls in co mbination with
Pyrite [FeS2 ]
reduced sulfur or che mically similar elements Galena [PbS]
(As, Se, Te )
Skutterudite [CoAs3 ]


Ionization potential: a measure of the energy required to
remove an electron from an atom and place it at an infinite
distance from the nucleus.
Electronegativity: a measure of the ability of an atom to
attract electrons. (The smaller the electronegativity, the less
likely the atom will attract electrons—it will most likely donate
them instead.)


A Measure of electronegativity of elements as seen
in the periodic table.


Ta ble 7-2 . Electroneg ati vities
Ion

Electronegativity

Z

Ion

Electronegativity

1

H+

2.2 0

33

As5 +

2.18

3

+

Li

0.9 8

34

Se

4

Be2+

1.5 7

35

Br -

5

B

3+

C

4+

7

N

5+

3.0 4

8

O2 -

3.4 4

9

-

Z

6

11

F

2.0 4
2.5 5

3.9 8
+

Na

2+

12

Mg

13

Al 3+

14

4+

15
16

Si
P

5+

S

2-

0.9 3

3.1 6
0.8 2
1.0 0

21

Sc

3+

1.3 6

22

Ti4 +

1.5 4

23

3+

24

1.6 3

Cr

2+

25

Mn

26

Fe 2+

27
28
29

Co
Ni

2+

2+

Cu

+

1.6 6

42

Nb

Mo

6+

2+

46

Rh
Pd

2+

Ag

+

48

Cd

2+

49

In 3 +

47

Sn

2+

51

Sb

5+

52

Te 2-

50

53
55

-

I

Zn

1.6 5

Ga 3+

1.8 1

32

4+

2.0 1

Pr

3+

Nd

3+

70

1.25

3+

---

Tm
Yb

71

Lu

3+

1.0

72

Hf 4+

1.3

73

5+

1.5

6+

1.7

7+

1.9

74

Ta
W

75

Re

2.2

76

Os 6 +

2.2

77

6+

2.2

2.28
2.20
1.93

78
79

Ir

4+

2.2

Au

+

2.4

2+

1.9

Pt

1.69

80

Hg

1.78

81

Tl3 +

1.8

82

2+

1.8

3+

1.9

1.96

Pb

2.05

83

Bi

2.1

84

Po 4+

2.0

85

5+

2.2

+

0.7

1.10

Ce

69

1.24

3+

2.10

0.89

58
60

2.16

2+

3+

59

1.6

Cs

La 3+

31

0.95

2.66

Ba

1.9 0

0.82

+

57

30

Ge

41

56

2+

1.23

Er3 +

1.33

1.8 3
1.9 1

Ho

68

1.22

1.5 5
1.8 8

67

2.96

Y

45

Cl

2.55

Zr 4+

2+

K+

1.22

3+

40

Ru 2 +

1.9 0

Dy 3+

39

5+

Electronegativity

Ion

65

3+

Tc

19

3+

Sr

44

2.5 8

V

2+

43

17

Ca

Rb

1.6 1

-

20

38

+

1.3 1

2.1 9

2+

37

2-

Z

0.79

1.12
1.13
1.14

87

At
Fr

88

Ra

2+

0.9

89

Ac3+

1.1

90

4+

1.3

4+

1.5

91
92

Th
Pa
U

6+

62

Sm

3+

3+

1.17

93

Np

64

Gd 3 +

1.20

94

Pu 4+

1.7
1.3
1.3


Ta ble 7-3. Percent ionic character of a single chemical bon d
Difference in
electronegativity

Ionic
character, %

Difference in
electronegativity

Io nic
character, %

0.1

0.5

1.7

51

0.2

1

1.8

55

0.3

2

1.9

59

0.4

4

2.0

63

0.5

6

2.1

67

0.6

9

2.2

70

0.7

12

2.3

74

0.8

15

2.4

76

0.9

19

2.5

79

1.0

22

2.6

82

1.1

26

2.7

84

1.2

30

2.8

86

1.3

34

2.9

88

1.4

39

3.0

89

1.5

43

3.1

91

1.6

47

3.2

92


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Example 7-1
The mineral fluorite has the chemical composition CaF2.
Calculate the ionic character of the bond between Ca-F.
From Table 7-2, the difference in electronegativity
= 3.98 (F-) -1.00(Ca2+) = 2.98
From table 7-3, the bond is ~89% ionic.


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Coordination number: the number of
anions that surround a cation in an
ionic crystal.
Radius ratio: the radius of the cation
divided by the radius of the anion.

So, we seem to think that silica (SiO44-) has a coordination
number of 4. Let’s test this.
From appendix III, the ionic radius of Si4+ = 0.48 & O2- = 1.32.
Then Rc/Ra = 0.48/1.32 = 0.36. If we were to check the
corresponding radius ratios from figure 7-2, we would see that it
fits nicely in the tetrahedral arrangement with a coordination
number of 4. Of course, we already knew that one!
Forsterite: Mg2SiO4

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The Unit cell is the basic building block for a crystal. In order to
understand this concept, think of the unit cell as being like a
brick in a wall (if the wall is built by stacking bricks directly
upon one another).


X-ray Crystallography: the science of determining the
arrangement of atoms within a crystal from the manner in which
a beam of X-rays is scattered from the electrons within the
crystal. The method produces a three-dimensional picture of the
density of electrons within the crystal, from which the mean
atomic positions, their chemical bonds, their disorder and sundry
other information can be derived.


Bragg’s Law describes
the relationship between

the angle of the incident
monochromatic x-ray
beam and the diffracted
ray as a result of the
crystalline structure and
interplanar spacing.
nλ = 2dsinθ
A-C is the interplanar
spacing and is equal to d.
λ is the wavelength of the
x-ray and θ is the angle of
incidence and diffraction.


X-ray Diffraction Pattern for Forsterite Mg2SiO4.

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& Fluorite CaF2

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Silica
(SiO4)
O

O

Si

O

O


olivine

Examples of silicate minerals
epidote

augite

beryl
hornblende

quartz

muscovite
Mineral pictures from: mindat.org


Quartz Varieties
Pink (Rose) : due to traces of iron, manganese or titanium.
Amethyst : Maybe be manganese but some believe it could be
organic, iron or even aluminum.
Citrine : iron
Aventurine : inclusion of green mica (fushite)
Tiger's eye : inclusion of fiber of silicified crocidolite (variety of
asbestos)
Prasiolite : Iron or copper
Milk quartz : gas and liquid inclusions

Smoky : Radioactivity on quartz containing aluminium
Blue : pressure.
Chalcedony is a variety of quartz with micro-crystals. Agate is a
multicolor variety of chalcedony and onyx is a variety of agate
with parallel strips of various nuances of black.


Ionic Substitutions
When minerals crystallize, certain minor or trace elements that
are present in the environment can enter the structure of
the crystallizing mineral. There are four rules that predict,
with many exceptions, the uptake of trace elements by
crystallizing minerals.
1. Ions of one element can substitute for those of another in a crystal
structure if their radii differ by less than ~15%.
2. Ions that differ by one charge unit substitute readily for each other as
long as charge neutrality is maintained.
3. When two ions occupy the same site in a crystal structure, the ion with
the higher ionic potential preferentially enters the site.
4. Even if the size and charge of the minor and major ion are similar,
substitution may be limited for the minor ion if it has a very different
electronegativity and forms a bond of very different character from that
of the major ion.


Clay Minerals and Surface Ion Exchange
Clay mineral – fine-grained hydrous silicate composed of layers of
tetrahedrally and octahedrally coordinated cations

Figure 7-5. Structure of the octahedral and tetrahedral layer.

Mg2+ in the octahedral layer = brucite. Al3+ in the octahedral
layer = gibbsite. Al3+ can substitute for Si4+ in the tetrahedral
layer.

Clays – any particle less than 2 microns in size. May or may not be clay mineral


General clay types
Kaolinite, illites, smectites, vermiculite

Kaolinite – 1 tetrahedral and 1 octachedral layer (1:1)
-Limited subsitution of Al in the basic formula (Al 2Si2O5(OH)4)
-net surface charge minimal, negligible CEC
Illite – 2 tetrahedral and 1 octachedral layer (2:1) ….the octahedral sandwich
-Al substitution for Si in tetrahedral layer
-marginal net surface charge minimal, low CEC
Smectites – also a 2:1 clay
-lots of Fe and Mg substitutions for Al in octahedral layer
-lots of Al substitution for Si in the tetrahedral layer
-swelling clay
-significant net surface charge, high CEC
Vermiculites – also 2:1 clay
-higher net surface charge
-high CEC


1:1 Clays: consist of tetrahedral layer and an octahedral layer;
substitutions are limited and the net charge is minimal (have a
low CEC.)

kaolinite


2:1 clays: consists of two tetrahedral layers with an intervening
octahedral layer. The octahedral layer can be either di- or trioctahedral and a large variety of substitutions are possible. 2:1
clays have a greater variation with net charge possibilities and
generally have a greater C.E.C.

montmorillonite


The octahedral and tetrahedral layers are arranged in different ways with
different amounts of elemental substitutions to produce different clay
minerals.
Table 7-5. Su mmary of the principal characteristics of the layered clay mineral grou ps*
Ka olinites

Illites

S mectites

Ve rmiculites

Structure
T etrahed ra l:
Octa hedral

1:1

2:1

2:1

2:1

Octa hedral layer

Di-octahe dra l

M ostly diocta hedral

Di- or triocta hedral

M ostly triocta hedral

Inte rlayer c ations

Nil

K

Ca , Na

Mg

Inte rlayer water

Only in ha lloysite

So me in

hydro muscovite

Ca , two la yers
Na, o ne to many
layers

Ca , two la yers
K, one la yer to nil

Basal spacing

7.1 

10 

Va ria ble
most ~ 15 

Va ria ble
14.4  whe n fully
hydrated

Ethylene glycol

Only taken up by
halloys ite

No effect

T wo glyc ol laye rs ,

17 

One glycol layer,
14 

Ca tion e xchange
ca pacity (CE C) in
meq/100 g c lay

Nil
3 - 15

Low
10 - 40

High
80 - 150

High
100 - 150

Formula

Al 2Si 2 O 5(OH) 2,
little variation

K 0 .5-0.7 5Al 2(Si,A l)2
O 10 (OH) 2

M +0 .7( Y3+ , Y2+ ) 4-6

(S i,Al) 8O 2 0(OH) 4 n
H2 O

M 2+ 0.66 (Y 2+ , Y3+ ) 6
(Si,Al) 8O 2 0(OH) 4 8
H2 O

Dilute ac ids

Scarce ly soluble

Re adily a ttacke d

Attac ked

Re adily a ttacke d

E xcept halloysite,
uncha nged

No ma rke d c hange

Colla pse to
appro xi mate ly 10

E xfolia tion,
s hrinka ge of layer
s pacing

Hea ting 200

o

C


E xa mples

Ka olinite, dickite ,
nac rite , ha lloysite

*M odified fro m Dee r et al. (1992)

Illite , hydrous
micas , phengite ,
bra mmallite,
glauc onite,
ce ladonite

M ontmo rillonite ,
beide llite,
nontronite,
hec torite , saponite,
sauconite

Ve rmiculite


Clays will have negative net surface charge caused by:
1) Subsitutions ….Al3+ for Si4+ in tetrahedral layer, Mg2+ for Al3+ in octahedral layer

2) Imperfections in crystal structure (e.g. missing cations)
3) Broken bonds at edges of crystals (exposing O2- or OH- ions)
For the 2:1 clays surface charge arises mostly from substitutions and imperfections
For 1:1 clays surface charge arises mostly from broken bonds at crystal edges
Table 7-7. Per manent negative surface charge of 2:1 clay minerals11
M ineral group

1
2

Charge ( mol sites kg -1) 2

Kaolinite

0.02 - 0.2

Illites

0.1 - 0.9

S mectites

0.7 - 1.7

Vermiculites

1.6 - 2.5

Data fro m Sposito (1989), Lang muir (1997)
Charge in moles of monovalent sites per kg of clay

What is Cation Exchange Capacity (CEC) and why is it important?

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