Chapter 6: Crystal chemistry and chemical composition of minerals
6.1 Crystal chemistry
Atom: the smallest subdivision of matter that retains characteristics of an
element. It contains proton and neutron in the massive nucleus and surrounded
by electron. Electron mass is 1/1837 that of proton.
Atomic number (Z): number of proton,
positive charge, that equal to number of
electron in uncharged atom.
Mass
number
or
characteristic
mass:
summation of proton and neutron of an
element.
Elements that contain different
numbers of neutrons called isotopes.
Periodic table: arrangement of elements in order of increasing atomic numbers
that clearly displays chemical and physical properties of elements
Electron configuration: allocation of electrons in energy orbitals of atoms.
Electrons tend to completely occupy lower energy orbitals then higher energy
levels.
Ion: atom that lost or gains electron(s) i.e. cation and anion. Metal elements
tend to give electron and nonmetal elements tend to receive electron. Energy
require for removing first electron from neutral atom is “first ionisation
potential”
Bonding force in crystals
- Ionic bond (electrostatic bond): the attraction
between oppositely charged ions. Resulting in
exchanging of metal atom (forming cation) to
nonmetal atom (forming anion).
-
Covalent bond: or electron
sharing
bond
is
the
strongest
chemical bond. Electrons in outer
orbital are filled as in stable inert
gas configuration.
Hence ionic
bonding also shares some electrons and covalent bond often contain
electrostatic charge, therefore, the proportion of ionic to covalent character
can be assessed by “Electronegativity”: a measure of ability of an atom to
attract electron to itself. The greater different of electronegativity, the more
ionic bonding
-
Metallic bond: atomic nuclei and nonvalence
electron orbitals bound together by the aggregate
electrical charge of a cloud of valence electron that
surrounds the nuclei.
Electron(s) belong to no
particular nucleus and freely mobile through the
structure or even out of it.
-
van der Waals bond: a weak dipole
attraction that forms by synchronizing of
electrons’ motions in order to avoid each
other as much as possible. This weak dipole,
therefore, induce a similar effect on the
adjacent atoms. It is the weakest chemical bond.
- Crystal with more than one bond
type: natural crystals possess more than
one bond type are common. They share
different properties of different bond
type e.g. graphite; thin sheets bound by
strong covalent bonding where excellent
cleavage between the sheets indicate
the weak van der Waals.
6.2 Chemical composition of the earth’s crust
U Earth’s interior: crust, mantle, and core
U 10-mile thick crust: 95% igneous rock (metamorphic equivalent)
: 4% shale
: 0.75% sandstone and 0.25% limestone
U Abundance of elements in crust: O, Si, Al, Fe, Ca, Na, K, and Mg
U Oxygen compounds: silicates, oxides, and carbonates
Î ‘rock forming minerals’
U Abundance of economic elements (ppm) e.g. Cu 55, Pb 13, Pt 0.01
Î Ore deposits (elements concentrate)
U Dispersed elements: never form compounds but associate with other element
in mineral e.g. Rb with K-rich minerals.
U Some elements strongly concentrate in certain minerals e.g. Zr in zircon
(ZrSiO4), Ti in rutile (TiO2), and ilmenite (FeTiO3)
6.3 Compositional variation in minerals
Solid solution: a mineral structure in which specific atom site(s) is occupied
invariable proportions by two or more different chemical elements, which is
controlled by substitution factors;
1. Comparative sizes of ions, atoms, or ionic groups
2. Charges of ions involved in substitution
3. Temperature
Types of solid solution
1. Substitutional solid solution
- Simple cationic or anionic substitution
Olivine (Mg, Fe)2SiO4: forsterite ÍÎ fayalite
Mg2SiO4
(Mg,Fe)CO3
Fe2SiO4
rhodochrosite ÍÎ siderite
MnCO3
FeCO3
- Coupled substitution
Corundum Al2O3 Fe2+ and Ti4+ Î 2Al3+
Plagioclase
albite NaAlSi3O8 ÍÎ anorthite CaAl2Si2O8
Na1+ Si4+ ÍÎ Ca2+ Al3
2. Interstitial solid solution:
ions or atoms located in structural
voids e.g. channel-like cavities in
beryl (Be3Al2Si6O18), ring silicate,
occupied by K+, Rb+, Cs+, H2O,
and CO2.
3. Omission solid solution: occurs when more highly charged cation
replaces two or more other cations for charged balance e.g.
Amazonite, blue-green microcline (KAlSi3O8) results from Pb2+
substitutes K+;
K+ + K+ ÍÎ Pb2+ +
Create lattice vacancy and become colour centre.
Pyrrhotite, (Fe(1-x)S), occurs in hexagonal closest packing, where Fe in 6coordination with sulfur. If Fe2+ occupies all octahedral sites, pyrrhotite would
be FeS. Percentage of variation in octahedral site causing composition ranges
from Fe6S7 to Fe11S12 close to FeS.
Maghemite, γ-Fe2O3, is an oxidation product of magnetite (Fe3O4 or
(Fe3+)2 Fe2+O4). Fe-deficient can be stated as Fe3+ (Fe3+)1.67
0.33O4
Exsolution: a process whereby an initially homogeneous solid solution
separates into two (or more) distinct crystalline minerals without the additional
or removal of material to or from the system