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Mineral Chemistry and Crystallography
Definition of a Mineral
All minerals:
1) Occur naturally
2) Are inorganic solids
3) Have a definitive
chemical formula
4) Have a crystalline
structure
Mineral Chemistry
An understanding of minerals require a knowledge of
basic chemistry, most specifically chemical bonding.
As a result we will review some basic chemistry:
Bonding occurs when atoms and ions share electrons. The
4 basic types of bonding are:
1) Ionic bonds - electron(s)
completely transferred - e.g., Halite
(NaCl) - tend to be brittle bonds
2) Covalent bonds - electron(s) are
more evenly shared - e.g., Diamond
(C) - strong bonds (diamond is
strongest))
Mineral Chemistry
3) Metallic bonds - transition metal
nuclei swimming in a sea of shared
electrons - e.g., Silver (Ag) malleable
4) Intramolecular bonds - including
hydrogen bonds and Van der Waals
bonds - generally weak (often feels
greasy) - e.g., graphite (C) or talc
(Mg3AlSi3O10(OH)2)
* Since this course does not require that you have any Chemistry courses
as a prerequisite , please ask the teacher for help learning some of these
basic chemistry ideas if you have not taken Chem. (Good Video)
Crystal Chemistry
Crystal Systems - mineral groupings
based on internal symmetry sometimes reflected in ideal crystal
shapes - e.g. quartz belongs to the
hexagonal crystal system, halite is cubic
 Unit Cell - smallest building block
that has all of a mineral's structural and
chemical characteristics. Unit cells fall
into one of the 6 crystal systems.
Crystal Chemistry
Bond types and crystalline structure are directly related to
physical properties (see below) - e.g. Diamond is a colourless,
very hard, cubic due to its arrangement of carbon atoms.
Graphite is a silver, very soft, hexagonal mineral based on its
arrangement of carbon atoms.
Diamond:
octahedral crystal
(Cubic Crystal System)
Graphite:
Hexagonal prism
(Hexagonal Crystal System)
Crystal Chemistry
 The majority of the world’s minerals are silicates. 95% of
the crust is made up of just 9 minerals – ALL silicates –
known as the rock forming minerals (ex. quartz, feldspar,
amphiboles, pyroxenes, micas, etc.)
Silicate minerals, containing covalently bonded SiO4
tetrahedra are very common in the Earth's crust and mantle.
 Silicates can be sub-classified based on linkages of the
SiO4 tetrahedra.
Mineral Formation
Minerals form:
1) As magma or lava cools, minerals form inside the earth
through crystallization. (Ex. Most rock forming silicates)
2) When materials dissolved in water crystallize through
evaporation. (ex. Halite (NaCl), Calcite (CaCO3) and Gypsum
(CaSO4))
3) From solutions heated by magma (ex. Igneous intrusions,
the mid-ocean ridge). Under great pressure water can exist at
400°C or greater. Water dissolves minerals, travels through
fissures and minerals are deposited as the temperature
decreases. Solutions are also heated at great depths in the
Earth’s crust (ex. Gold, Quartz, Pyrite)
4) From the chemical and physical alteration due to
metamorphism (ex. Garnet or Kyanite)
Mineral Collection
1
Quartz
11
Barite
21
Galena
2
Pyroxene
12
Gypsum
22
Chalcopyrite
3
Amphibole
13
Apatite
23
Pyrrhotite
4
Talc
14
Calcite
24
Bornite
5
Biotite
15
Garnet
25
Graphite
6
Muscovite
16
Hematite
26
Fluorite
7
Phlogopite
17
Magnetite
27
Halite
8
Orthoclase
18
Corundum
28
Malachite
9
Plagioclase
19
Sphalerite
29
Chromite
10
Olivine
20
Pyrite
30
Sodalite
Chemical Composition of the
Earth’s Crust
Element
Atom (Ion)
Percent
Oxygen
Silicon
Aluminum
Iron
Magnesium
Calcium
Potassium
Sodium
O2Si4+
Al3+
Fe2+ or Fe3+
Mg2+
Ca2+
K1+
Na1+
46
28
8
6
4
2.4
2.3
2.1
All others
(by weight)
<1
The abundance of elements
in the Earth's crust show
that silicon and oxygen
dominate. As a result, the
vast majority of rocks are
silicates. Pretty much all
minerals found in igneous
and metamorphic rocks are
silicates. Since
sedimentary rocks are
weathered and eroded
igneous or metamorphic
rocks, they are commonly
made of silicate minerals as
well.
Silicates
mineral
formula
mineral
formula
Quartz
(1)
SiO2
Orthoclase (8)
(Feldspar)
KAlSi3O8
(framework silicate)
(3-D framework structure)
Pyroxene
(2)
CaMgSi2O6
(single chain silicate)
Plagioclase (9)
(Feldspar)
CaAl2Si2O8
(3-D framework structure)
Amphibole
(3)
A2Z5Si8O22(OH)2 or
Olivine (10)
(double chain silicate)
(isolated silicon
tetrahedrons)
Talc
(4)
Mg3Si4O10(OH)2
(sheet silicate)
Garnet (15)
(ring structure)
A3Z2Si3O12
A2+ B3+
Biotite
(5)
mafic mica
Sodalite (30)
Na8Al6Si6O24Cl2
(sheet silicate)
Feldpathoid – feldspar like
structure
Muscovite
(6)
Phlogopite
(7)
felsic mica
(sheet silicate)
intemediate mica
(sheet silicate)
(Mg,Fe)2SiO4
Silicate Structural Diagrams
Since the common rock forming minerals
are all silicates it is worthwhile showing
how the silicon tetrahedron is formed.
The smaller Si4+ cation fits almost perfectly
in the middle of a tetrahedron formed of
larger O2- anions.
Three ways of drawing the silica tetrahedron:
a) At left, a ball & stick model, showing
the silicon cation in orange surrounded
by 4 oxygen anions in blue
b) At center, a space filling model
Silicates are network covalent solids that
are very stable and have high melting
points. Within silicate structures are metal
cations – so ionic bonds are also found.
The more ionic bonds in the structure, the
more easily the mineral is broken down
through chemical erosional processes.
c) At right, a geometric shorthand model.
This is the model favoured by geologists
Mineralogists and Crystallographers find it
because of their simplicity.
easier to display silicate structures by using
geometric diagrams.
Silicates: Olivine Structure
Isolated Silica Tetrahedrons
The diagram shows Olivine with isolated
silica tetrahedra. As a result the Si:O
ratio is 1:4. The blue dots represented
by M1 and M2 are the locations of the
2+ cations. Olivines have the structural
formula M2SiO4. Typically the M cations
are Mg2+ and Fe2+. Pure Mg2SiO4 is
known as Forsterite and Fe2SiO4 is
known as Fayalite. Thus Olivine is a
solid solution between the two “end
members”. Most olivines are about 90%
Mg. Olivine is a common constituent of
basalts and is common in the mantle.
Silicates: Olivine Structure
Isolated Silica Tetrahedrons
This diagram shows the M sites as
octahedra since the cations are
surrounded by 6 oxygen atoms. Note that
the blue silica terahedra point upwards
and the pinks ones point down. The large
number of cations are the reason that
olivine erodes easily into other minerals.
Look at Mr. Snyder’s Olivine
model to see the hexagonal
closest packing of oxygen
atoms.
Silicates: Olivine Structure
Metal cations
Time for a personal plug – In his groundbreaking, epic work “High Temperature
Cation Ordering in Nickel Magnesium Olivines”, Mr. Snyder studied how Ni2+
would preferentially take the M1 location while Mg2+ would choose the M2
location. This “ordering” would decrease as temperature increased. I have a
copy of my masters thesis for anybody who is exceptionally bored and wishes
to read it.
Silicates: Pyroxene Structure
Single Chain Silicate
Since silica tetrahedra share
oxygens with adjacent
tetrahedra, the Si:O ratio is
1:3 for pyroxenes.
Pyroxenes have the formula
M2SiO3 or ABSi2O6 . M
cations are typically Mg, Fe
and Ca.
Like Olivines, pyroxenes represent a family of
minerals. Pyroxenes are common in mafic rocks
and are very common in the mantle. Here are a
few common pyroxenes:
Diopside: CaMgSi2O6
Enstatite: MgSiO3
Hedenbergite: CaFeSi2O6
Wollastonite: CaSiO3
Silicates: Amphibole Structure
Double Chain Silicate
Amphiboles have very complicated
structures (as seen in the diagram
below), but it is simplest to
understand that they are double
chain silicates (see top diagram).
Amphiboles have a Si:O ratio of 8:22.
Common amphiboles include:
Hornblende: Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2
Actinolite: Ca2(Mg,Fe)5Si8O22(OH)2
Amphiboles are more common in
metamorphic than igneous rocks.
Silicates: Mica Structure
Sheet Silicate
Chemically, micas can be given the general
formula
XY2–3Z4O10(OH,F)2 in which:
X is K, Na, or Ca or less commonly Ba, Rb, or Cs;
Y is Al, Mg or Fe or less commonly Mn, Cr, Ti, Li;
Z is Si or Al but also may include Fe3+ or Ti.
Micas have 2-dimensional sheets of
silica tetrahedra. Layers of these
silica tetrahedra sheets are
separated by hydroxyl ions. (see top
diagram). Sheet silicates have a Si:O
ratio of 2:5.
Common micas include:
Biotite: K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2
Phlogopite: KMg3AlSi3O10(F,OH)2
Muscovite: KAl2(Si3Al)O10(OH,F)2
Talc: Mg3Si4O10(OH)2 and Chlorite have
similar sheet silicate structures.
Micas are more common in
metamorphic than igneous rocks.
Silicates: Sheet Silicates
This diagram shows how sheet silicates
look in side view. Note the silica
tetrahedra in light blue and the Mg
octahedra in dark blue.
In between the sheets of silica
tetrahedra are hydroxyl (OH-) ions.
The covalent bonds in the mica sheets
are very strong but the ionic bonds
formed by the hydroxyl ions are very
weak.
Hence the bonds between layers of mica
are very weak which explains why layers
of mica can be peeled apart.
Silicates: Feldspar Structure
Framework Silicate
Feldspar has a 3 dimensional silica
structure with a Si:O ratio of 3:8.
Common feldspars include:
Orthoclase: KAlSi3O8
Albite: NaAlSi3O8
Anorthite: CaAl2Si2O8
Feldspars are the most
common mineral in the
Earth’s crust!
Albite and Anorthite form a solid
solution of feldspars called
Plagioclase.
Silicates: Feldspar Composition
Orthoclase: KAlSi3O8
Albite: NaAlSi3O8
Anorthite: CaAl2Si2O8
Albite and Anorthite form a solid
solution of feldspars called
Plagioclase. Feldspars fall in the
range seen in the diagram.
Orthoclase
Anorthite
Quartz (SiO2)
 Quartz is composed of silica tetrahedra
linked at their corners in a hexagonal
crystal structure
 Quartz (SiO2) is very common in
igneous (especially felsic) and
metamorphic rock. Sandstone is
essentially pure quartz.
 When silicate mineral erode (weather),
they break down chemically. Ionic
bonds break first and metallic minerals
are removed and oxidized leaving the
tough silica tetrahedra (Quartz)(which
are held together with covalent bonds).
Quartz (SiO2)
 Quartz has a vast variety of colours due
to chemical impurities
clear – quartz
black - smoky
purple – amethyst
yellow – citrine
pink – rose
 Quartz is often deposited by hot
solutions into hard, layered rocks that
are often very colourful (agate,
chalcedony, red jasper, black flint)
 These quartz solutions often form veins
that host a variety of important
minerals – gold, copper, molybdenum
Other Silicates:
Garnets, Tourmalines, Ring Silicates