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Lecture 11

EPSC 201 Lecture Notes - Lecture 11: Diopside, Ice Crystals, Pyroxene


Department
Earth & Planetary Sciences
Course Code
EPSC 201
Professor
Anthony Williams- Jones
Lecture
11

Page:
of 3
EPSC201 - Lecture 11 Notes
We established that minerals are chemical com-
pounds that occur naturally. Minerals are expressed
as crystals. Minerals have a set of arranged atomic
arrangement which makes them different from just
random atomic arrangement
Anion is a negative charged atom
Cation is a positive charged atom
The size of an ion has a big effect on the geometry of
elements.
Ionic Bonds – the transfer of electrons from
one atom to another, which forms charged ions.
Covalent Bonds – the bond shares electrons
between the atoms, to fill the outer shell of each atom, making it stable. The bonds are much
stronger then ionic bonds.
Taking halite as an example, the ionic bond created between sodium and chlorine allows for the transfer
of one electron which helps make sodium and chlorine charged ions but also stable. There is an equal
number of sodium and chloride in halite which through ionic bonds, starts building up its molecular struc-
ture by organizing atoms through relative sizes.
Relative Sizes of Ions:
Cations with a high charge are small, if they have
a small charge, they are larger in size.
For anions, there are variation in sizes.
In ionic bonding, we want to pack the anions in
and around the cations, based on the size of both
the cation and anion. We refer to this as ion coordination.
Diamonds:
We started talking about diamonds last class; are purely carbon. Every carbon atom is bonding to every
other carbon atom. They are short and strong bonds (homogenous), which results in a super hard struc-
ture. When we say it’s hard, it’s not that we cannot break it with a hammer (we will smash it), what it
means is that it cannot scratch easily.
What makes diamond super strong is the fact that the carbons are cova-
lently bonded which makes it super hard.
Diamonds are ONLY formed under high temperature and high pressure
(need depth of at 150-200km). If diamonds were formed on shallow depths,
it would be graphite instead and not diamond. Diamonds reach the surface
of the earth through volcanos; specifically magma liquid breaking pieces of
rocks/mantle as it shoots up through the earth.
Atomic Structure of Graphite:
Graphite is made up of purely carbon atoms as well, but in a layered sheet (plane) formation through co-
valent bonding. The atomic arrangement is the same as in diamond, but the plane sheets are held togeth-
er weakly by Van der Waal’s bonding.
Graphite and diamonds are not forever. But the amount of energy it takes to convert diamonds to graphite
is extraordinarily high and it will take a few hundred/thousands of years before it becomes graphite. How-
ever, if you put diamond in the oven, it will fuse with oxygen and pro- duce car-
bon dioxide.
Hydrogen Bonding:
Ice crystals are a perfect example of hydrogen
bonding (in water molecules). Oxygen is negative-
ly charged and hydrogen is positively charged
(water is a polar molecule). Water molecules will ar-
range themselves so that the slightly positive side
to link with the slightly negative side of another wa-
ter molecule.
Silicate Mineral:
Most rocks are formed through silicate mineral in
which silicon bonding to oxygen is one of the key
building blocks.
Granite for example: Around 99.9% of the minerals in granite are silicate minerals
The mantle of the earth is typically made up of peridotite and olivine.
In the simplest form, silicon is bonded to four oxygens in the form of a tetrahedron through covalent bond-
ing. What we see in the diagram is an arrangement of silicon tetrahedron. If you add up all the charges of
silicon and oxygen, you end up with -4 (SiO4). (Si has charge of +4, oxygen has charge of -2)
There are a variety of structures you can form through silicate minerals, but the Key Point is that when
you have SiO2 which has a net charge of 0 and the oxygens are all covalently shared, you get the three-
dimensional framework silicate, quartz.
Nesosilicate (Olivine):
Olivine Fe2SiO4 or Mg2SiO4
By adding in iron or magnesium, you are using those metals as glue to neutralize the charge and to pro-
vide the molecule with three-dimensional structure which net charge of 0.
Inosilicate (Single/Double Chain):
Pyroxene: two types (Diopside or Hypersthene)
Pyroxene are chains of silicate tetrahedron glued together by cations through ionic bonds. Key Point is to
understand how chain silicate tetrahedrons need cations to balance the charge in order to form neutral
structures.
Sheet Silicate:
Sheet silicate have three oxygen atoms sharing through covalent bonds.