Download 1 The Periodic Table, Distribution and Association of elements. 1

The Periodic Table, Distribution and Association of elements.
1- The periodic Table: (Fig. 1)
Periods and Groups
 Period # = Principal quantum # (n), or # of energy levels that are being filled with
electrons.
 Group #: # of electrons in the outermost shell (valence electrons?)
Metallic character: decreases from groups I to VIII, but increases going down each
group (i.e. from one period to the next), although the latter is less pronounced!
Ionic Radii: The largest ions occur at the beginning and end of each period; ionic radii
increase systematically down each group.
Transition elements:
 incompletely filled d orbitals
 All are metals
 Many oxidation states; d- electrons often serving as valence electrons.
 properties not easy to predict from the periodic table.
Lanthanides and Actinides:
 All have 3 valence electrons.
 Incompletely filled f orbitals; filling of the f orbitals 2 orbitals below the ultimate
orbital or shell with increasing Z.
 Lanthanide contraction
 Outer electronic structure for these elements is so similar that they have almost
identical chemical properties.
 Eu & Yb
2- Goldschmidt's classification of elements: (Fig. 2; Table 1)

A qualitative classification based on element associations, and assumes that the
earth was initially molten.

In a molten earth, elements would be distributed between four phases: Fe rich liquid,
sulphide liquid, slag (silicate – rich), and a gas phase.

Siderophile elements: dominantly noble metals with low electrode potentials that
occur in the middle of the periodic table (mostly transition elements).

Chalcophile elements: intermediate electrode potentials

Lithophile elements: high electrode potentials; occur at both ends of the periodic
table.

Atmophile elements: H, N, and the inert gases.

Overlaps between the different groups, with some elements belonging to more than
one group  different physicochemical conditions of formation of different rock
types govern this behavior to some extent.
3- Isomorphism & Goldschmidt's Rules of Ionic substitution:
Is the substitution of one cation (or anion) for another in a mineral or compound. For such
substitution to occur, the substituting ions have to be of similar sizes, and the structure of
the mineral after substitution has to be electrically neutral (see Goldschmidt's rules).
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Complete isomorphism, as in the case of olivine, occurs when the substituting ions are of
similar sizes and charges. If that is the case, the two end-members may be considered
completely miscible in the solid state, a phenomenon also known as solid solution. When
the two ions differ in size, the miscibility between the two end-members may not be
complete (except perhaps at very high temperatures) and we are left with two minerals
each rich in one end-member, instead of a single mineral. Because the charge (Z) and
radius of ions or elements are so important, it is useful to define the ionic potential as
Z/r.
Goldschmidt's Rules of Ionic substitution: Are important for predicting which
elements can enter or substitute for other elements in mineral structures, and therefore for
understanding the distribution and association of elements.
(1) Extensive substitution if differences in r+ is  15%
(2) Electrical neutrality and coupled substitutions: if difference in Z is > 1,
substitution is slight or difficult.
(3) Higher Z/r is favoured.
(4) Two substituting ions should also have similar electronegativities.
Substitution of trace elements for major ones in rock forming minerals:
 Capture: Z/r of trace element > Z/r of substituted major element
 Camouflage: similar Z/r
 Admission: Z/r of trace element < Z/r of major one. Will not occur if difference in
radius between two ions is > 15%!
Distribution Coefficients:
Kd = CA/CB
4- Distribution of elements in igneous rocks:
Magmatic differentiation (Fig. 3)
Separation of immiscible liquids
Variation diagrams (Fig. 4)
Trace elements in igneous rocks:
 Distribution coefficients (Kd) and bulk distribution coefficients "D" (Table 2).
 Factors affecting the values of Kd
 Types of trace elements (according to their behaviour in an igneous system):
(i) Compatible elements
(ii) Incompatible elements:
High field strength elements
Large ion lithophile elements
 Closely associated elements:
Rb & K, Ga & Al, Hf & Zr, Cd & Zn.
 Empirical statements:
Pb2+ & Tl+ for K+, Zn2+ for Fe2+, Bi3+ for Ca2+.
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Rare Earth Elements in igneous petrology (Figs. 6 & 7)
Applications of trace and rare earth element abundance to igneous petrogenesis
(i)
Testing models of magmatic differentiation: fractional crystallization vs.
partial melting (Figs. 5a & b)
(ii)
Depth of generation of primary magma (Fig. 6)
(iii)
Oxidation states of magmas (Fig. 8)
(iv)
Tectonic and paleotectonic settings of magmas (Fig. 9)
5- Distribution of elements in sedimentary rocks:
Factors:
(i) differences in solubility of different compounds in H2O
(ii) adsorption
(iii) organic activity and reactions with organic matter:
(iv) ease of hydration: elements with higher Z/r will hydrate more easily. Note that
after hydration, the radii of the hydrated ions will be larger for elements with a
smaller radius!
(v) oxidation: important for Mn, As & Sb.
(vi) reduction: U and V
Observations: (Table 3)
 Most trace elements are more abundant in shales than in sandstones or limestones 
sorption
 Sr & Mn in limestones  ionic size!
 Zr and REE in sandstones
6- Distribution of elements in metamorphic rocks:

Metamorphism is a closed system process, with the exception of H2O and CO2.

Na, Mg, Ca, Cl, and S in fluid inclusions

Trace elements: Are they really mobilized at extremely high pressures? Key thing is
which elements can be fractionated into the fluid phase at the specified P-T
conditions (Fig. 12).

Metasomatism: A different ball game!

Ca metasomatism, Na metasomatism, K- metasomatism, sea floor metamorphism
and addition of Na and Mg (albitization and spilitization)! (Figs. 10 & 11)

Importance of water : rock ratios.