EPSC 201 Section 5.pdf

Mineralogy
Minerals and crystals

World’s largest crystals:
A cave in the Naica Lead Zinc mine, Mexico

Miners in Cueva de los Crystals:
the mineral gypsum
Crystals of a variety of gypsum
(CaSO 4 .2H 2 O) precipitated from
hot water at 60 degrees celsius
in a limestone cave in the Naica
mine

Minerals
Tourmaline on Feldspar
Emerald
Beryl
(Be 3 Al 2 (SiO 3 ) 6

Crystal Shapes
Halite
(NaCl)
Diamond
(C)
Quartz
(SiO 2 )
Garnet
(Fe 3 Al 2 Si 3 O 12 )
Stibnite
(Sb 2 S 3 )
Calcite
(CaCO 3 )

Pyrite (FeS 2 ) crystals

Feldspars – two of the most important
rock-forming minerals
Plagioclase
Albite – NaAlSi 3 O 8
Anorthite CaAl 2 Si 2 O 8
K-feldspar
Sanidine
Orthoclase
Microcline
KAlSi 3 O 8

Colours and forms of quartz
Citrine
Amethyst
Milky quartz
Rose quartz

Quartz crystals
displaying
prism and
pyramid faces

Pyramid
Quartz Crystal
Faces display consistent
angular relationships
Angles measured with a
goniometer
Prism

Crystals have ordered arrangements of atoms
Order
Disorder

X-ray diffraction
X-ray beam splits into numerous smaller beams. Interference of
waves of different beams produces a diffraction pattern on a
screen or film. The pattern indicates the spacing and
arrangement of atoms.

Surface of galena (PbS) imaged
With an atomic force microscope
Atomic structure of galena
(PbS)
Sulphur
Lead

The atomic structure of halite (NaCl)
Ionic bonding

Ionic bonding – transfer of electrons
Cation +
Anion -

Relative sizes of ions
Ion co-ordination

The atomic structure of diamond
Covalent bonding

The source of Diamonds
The ‘Big Hole’ at Kimberly, South Africa
Kimberlite containing diamond

Cutting diamonds

Covalent bonding – sharing of electrons

Atomic structure of graphite
Covalent bonding within
sheet
Van der Waal’s bonding
between sheets

Ice Crystals
Hydrogen bonding

The silicon tetrahedron

Silicate Structures
Nesosilicate Inosilicate Inosilicate
(Single chain) (Double chain)
SiO 4
4 -
Si 4 O 11
6 -
SiO 3
2-
Sheet silicate
Framework silicate
Si 4 O 10
4 -
SiO 2

Nesosilicate
Olivine
(Fe,Mg) 2 SiO 4

Inosilicate
(Single Chain)
Pyroxene
Diopside
CaMgSi 2 O 6
Hypersthene
(Fe,Mg)SiO 3

Inosilicate
(Double Chain)
Amphibole
Tremolite
Ca 2 Mg 5 Si 8 O 22 (OH) 2
Riebeckite
Na 2 Fe 5 Si 8 O 22( OH) 2
Tigers Eye

Asbestos
Chrysotile (Mg 3 Si 2 O 5 (OH) 4
Uses and risks
(Insulation, heat,
fire resistance)
Serpentine
(Mg 3 Si 2 O 5 (OH) 4

Sheet Silicate
Biotite
KAl(Fe,Mg) 3 Si 3 O 10 (OH) 2
Muscovite
KAl 3 Si 3 O 10 (OH) 2

Framework Silicate
Quartz SiO 2
Plagioclase Albite NaAlSi 3O 8
Anorthite CaAl 2 Si 2 O 8
K-feldspar KAlSi 3 O 8
Amethyst SiO 2

Relationship of Quartz structure
to Feldspar Structure
Imagine four SiO 2 molecules 4 x SiO 2 = Si 4 O 8
Substitute Al 3+ for Si 4+ in one of these molecules
SiO 2 + AlO 2 + SiO 2 + SiO 2 = AlSi 3 O 8
-
Add Na + or K + to supply missing charge = NaAlSi 3 O 8 or KAlSi 3 O 8
Albite
K-Feldspar
SiO 2 + AlO 2 + AlO 2 + SiO 2 = Al 2 Si 2 O 8
2-
Add Ca 2+ to supply missing charge = CaAl 2 Si 2 O 8
Anorthite