Average Composition of
the Continental Crust
Si
O
Weight Percent
O
Volume Percent
Table
3.4
Olivine:
formed from
single silica tetrahedra
Phase – olivine ; Mg2SiO4
Components: Mg(2), Si, O(4)
Can be reduced to MgO(2) and SiO2
Which is 66.67 mole percent MgO and
33.33 mole percent SiO2
This can be converted to weight by:
.6667 moles MgO X 40.31g/mole = 26.875g
.3333 moles SiO2 X 60.09g/mole = 20.028g
46.903g
Giving 57.3 weight % MgO and
42.7 weight % SiO2
Given: 57.3 wt.% MgO and 42.7 wt.% SiO2
57.3g / 40.31g/mole = 1.4215 moles MgO
42.7g / 60.09g/mole = 0.7106 moles SiO2
1.4215/(1.4215 + 0.7106) x 100 = 66.67 mole % MgO
0.7106/(1.4215 + 0.7106) x 100 = 33.33 mole % SiO2
Or a mole ratio ( sometimes called “atomic”) of
2 MgO to 1 SiO2
The mineral formula would then be based on 4 oxygens
giving Mg2SiO4, which is olivine ( specifically forsterite)
Actual analysis (wt.) would give 34.5 % Mg, 20.0 % Si
(with the remaining 45.5 % assumed to be oxygen)
Mg (24.31g/mole), Si (28.09g/mole)
O ( 16.00g/mole)
34.5g / 24.31g/mole = 1.42 moles
20.0g / 28.09 g/mole = 0.71 moles
45.5g / 16 g/mole = 2.84 moles
Mg1.42 Si 0.71 O 2.84 converted to 1 mole O gives
Mg0.5 Si0.25 O 1 converted to whole numbers gives
Mg2 Si O4
Classification of Igneous Rocks
• Rock with 70% X,
20% Y, 10% Z
• Plot mineral
composition of
triangle diagram
Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on
triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Classification of Igneous Rocks
Anorthosite
Ga
bb
ro
90
Olivine
gabbro
Plagioclase-bearing ultramafic rocks
Pyroxene
(b)
(c)
lite
cto
Tro
• Mafic rocks: plag,
pyroxene, olivine
• Norite: gabbro with
more opx than cpx
• Top and bottom fields
at 10% tick marks
• Side fields at 5% tick
marks
Plagioclase
Olivine
G225 – Review For Exam 1
Models of the atom
Rutherford, Bohr,
deBroglie
Electronic configuration
Quantum numbers
Periodic Table
Valence electrons ; oxidationreduction
Z and A numbers
Isotopes ; stable and radiogenic
Bonding
Ionic
Covalent
VanderWaals
Metallic
Polar versus non-polar
Electronegativity
Atomic structures
Ionic radii – bond lengths
Coordination numbers
Hexagonal and cubic closest packing
Pauling’s Rules
Radicals
Isodesmic, anisodesmic, mesodesmic
Silicate tetrahedral linkages; subclasses;
polymerization
Mineral compositions and plotting procedures; solid
solution