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The colour, grain size,
texture, density, etc., of rocks are qualitative
attributes, whereas the proportions of the minerals making up the rock
(the MODAL composition of the rock) and the
composition of the rock in terms of element proportions, (its CHEMICAL
composition) are quantitative attributes.
The colour of a rock, while a useful descriptor in evaluating whether a
rock is generally mafic or felsic, provides very little information of
use in the determination of rock type, or the possible petrogenetic
relationship of one rock to another. On the other hand the modal
and chemical compositions of rocks permit an exact identification of rocks,
as well as allowing an evaluation of the petrogenetic relationships of
rock suites. In the following text, three simple examples are provided
of how modal and chemical compositions are used to establish petrogenetic
relationships.
In the first case it is assumed that a
basaltic magma intruded into continental crust has melted the crust and
subsequently mixed with the granitic melt so produced. Knowing the modal
mineral composition of the two end member rock types and their mixing proportions,
an estimate is made of the modal composition of the resulting mixture.
In the second case, the chemical composition of a rock is calculated from
the known modal composition of the rock. And in the third case, the operation
is reversed with the calculation of the mineral proportions of a rock from
its chemical composition.
Modal Compositions and Mass Balance Calculations
Modal composition, Mixing, and Triangular Plots
Exercise 1) Assuming the modal mineral compositions (molar %) of a Granite and Gabbro are as follows:
Granite Gabbro Kspar 30 0 Qtz 20 0 Ol 0 30 Plag 50 47 Cpx 0 23 Total 100 100Calculate the mineral composition of of a 70:30 mixture of these two rocks, respectively. According to a simple mass balance calculation, the mineral proportions would be:
Kspar - 21; Qtz - 14; Olivine - 9; Plag - (35 + 14) = 49; Cpx - 7; TOTAL = 100
However, olivine and quartz cannot coexist in the same rock, and consequently the latter two minerals must be converted to orthopyroxene according to the reaction relationship
1Olivine + 1 Qtz = 1 Opx
and the more correct mineral composition would therefore be:
Kspar - 21; Qtz - 5; opx - 9; Plag - 49; Cpx - 7; TOTAL = 91
which recalculates in terms of percentages as:
Kspar - 23; Qtz - 5; opx - 10; Plag - 54; Cpx - 8; TOTAL = 100
What plutonic igneous rock would the mixture represent?
Calculate the relative % of Quartz, Plagioclase,
and K-feldspar
Kspar - 23; Qtz - 5; Plag - 54; TOTAL = 82
And the proportions in percent are:
Kspar - 28; Qtz - 6; Plag - 66; TOTAL = 100
Use the Streckeisen triangular
plot to determine the rock type.
Exercise 2). If sufficient water was present to allow the formation of Amphibole (Ca2Mg4Al2Si7O22(0H)2), what would be the composition of the mixture?
In this case you will need to use the following reaction relationship to determine the proportion of Amphibole that might form:
2CaAl2Si2O8 + 2CaMgSi2O6 + 3Mg2Si2O6 + 2H2O = 2Ca2Mg4Al2Si7O22(OH)2
The reaction of 9 moles of Opx with 6 moles of Clinopyroxene and 6 moles of Anorthite would form 6 moles of Amphibole, and the mineral proportions would therefore now be:
Kspar - 21; Qtz - 5; Plag - 43; Cpx - 1; Amph - 6; TOTAL = 76
What plutonic igneous rock would this mixture represent?
Again, calculate the relative % of Quartz, Plagioclase,
and K-feldspar.
Kspar - 21; Qtz - 5; Plag - 43; TOTAL = 69
And the proportions in percent are:
Kspar - 30; Qtz - 7; Plag - 63; TOTAL = 100
Use the Streckeisen triangular plot
to determine the rock type.
Conversion of a Modal Mineral Composition to a Chemical composition
Exercise
1). Make a rough estimate of the chemical
composition of a rock from the proportion and
composition of the minerals it contains.
The rock is made of:
25 % Anorthite (CaAl2Si2O8)
(CaAl2Si2O8 = CaO Al2O3 2SiO2 = 4 oxide moles; therefore per
mole of Anorthite, CaO =1/4 An, Al2O3=1/4 An, SiO2 = 1/2 An)
and 75 % Forsterite (Mg2SiO4),
(Mg2SiO4 = 2MgO SiO2 = 3 oxide moles; therefore per mole of
Forsterite, MgO = 2/3 Fo,
SiO2 = 1/3 Fo).
The oxide mole fractional composition of the rock would therefore be:
CaO = 25% of 1/4
= .0625
Al2O3 = 25% of 1/4
= .0625 ,
MgO = 75% of 2/3 = .5,
SiO2 = 25% of 1/2 + 75% of 1/3 = .375.
Exercise
2) If a rock is made up of 25 % Quartz, 30 % Anorthite, 10 %
Albite, 25 % K-feldspar,
and 10 % Enstatite, what would be the oxide
chemical composition of the rock?
CaO = 30% of 1/4 = ?
Al2O3 = 30% of 1/4 + 10% of 1/8 + 25% of 1/8 = ? ,
Na2O = 10% of 1/8 = ?
K2O = 25% of 1/8 = ?
MgO = 10% of 2/3 = ?
SiO2 = 30% of 1/2 + 10% of 3/4 + 25% of 3/4 + 10% of
1/2 + 25% of 1 = ?
Conversion of a Rock composition to a Normative Mineral Composition.
Exercise
1) If a rock is composed of 1 mole of Na2O, 1 mole of Al2O3,
2 moles of MgO, and 10 moles of SiO2,
what would be the mineralogical composition
of the rock?
1 Na2O + 1 Al2O3 + 2 MgO + 10 SiO2
= 2NaAlSi3O8
+ 1 Mg2Si2O6
+ 2 SiO2
2 Albite
1 Opx
2 Quartz
Exercise
2). If a rock is composed of 1 mole of Na2O, 1 mole of Al2O3,
2 moles of MgO, and 5 moles of SiO2,
what would be the mineralogical composition
of the rock?
1 Na2O + 1 Al2O3 + 2 MgO + 5 SiO2
=
1 NaAlSi3O8 +
1 Mg2SiO4 +
1 NaAlSiO4
1 Albite
1 Olivine
1 Nepheline
FIGURES:
Structural Provinces of North America.
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