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Mineralogy re-exam 2015
1. Silicates
What is the basis of silicate classification? Name different classes of silicates and give examples.
(5 pts.)
Silicates are classified based on the structure of their silicate anion. In almost all silicates, Si has
a tetrahedral coordination and is surrounded by 4 oxygens. To what extend the free electron of
each oxygen is connected to other Si cations determines the architecture of the crystal lattice
and thus macroscopic properties.
Neso ( single tetrahedra ) Olivine, garnet
Soro ( double tetrahedral ) Epidote
Cyclosilicate ( rings ) Tourmaline, Beryl
Ino ( single chain ) Pyroxenes
Ino ( Double chain ) Amphiboler
Phyllo ( sheets ) Micas, chlorite
Tector ( framework) Quartz, feldspars
2. Aluminum
Explain the terms “crystal lattice” and “coordination number”.
What are the two typical coordination-numbers for Aluminum? What is the typical oxidization
state of Aluminum?
Which other elements have one or the other of these coordination numbers (1 example for each
coordination number)? What are the typical oxidization states of the other elements that can
substitute for Aluminum.
Name 2 minerals in which Aluminum can exchange with elements with a different oxidization
state and explain how this works (8pts.)
Basic conception of ordered structure, translation and symmetry. Coordination refers to the
number of nearest neighbors in crystal lattice. Al can have a tetrahedral ( 4 neighboring oxygen
ions ) or octahedral coordination ( 6 neighboring oxygen ions ). Oxidization state of Al is 3+.
Other elements with tetrahedral coordination: Si
Other elements with octahedral coordination: Fe, Mg, Ca, Ti
Oxidization state of substituting elements varies (2+, 3+, 4+)
If Al is replaced with an ion with a different oxidation state, there has to be a coupled other
exchange balancing the loss/gain of charge. Examples are: Jadeite - Diopside: Ca2+ mg2+ <->
Na1+ AL3+.
Tschermak substitution: Al3+Al3+ <-> Se4+Mg2+
3. Pyroxenes
What do you know about pyroxenes?
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Explain the lattice structure. Name at least 5 chemical endmembers of pyroxene - at least one
should be an ortho- and one a clinopyroxene endmember. Explain how elements in these
endmembers occupy crystallographic sites. In which rocks do you find pyroxenes. (5pts.)
Common mineral in magmatic rocks ( e.g. basalt, gabbro, peridotite ) and high-grade
metamorphic rocks ( e.g. mafic granulites, eclogites or calc-silicate rocks. ) Single chain silicate
with formula ABSi2O6. Monocline or rhombic. Good cleavage parallel c, almost perpendicular
to each other. Great chemical variety, hence varying colours and optical properties.
A site slightly larger than B site ( position in chain )
A site: Ca, Na, Mg, Fe
B site: Mg, Fe, Al, Ti
If large ion occupies A site pyroxene is monocline.
Clinopyroxenes. Diopsides ( CaMgSi2O6), hedenbergite ( CaFeSi2O6), Jadeite (NaAlSi2O6),
acmite ( NaFe30Si2O6)
Orthopyroxenes: Enstatite ( MgMgSi2O6), Ferrosilite ( FeFesi2O6)
4. Reaction balancing
Draw a ternary chemographic diagram with the endmembers Al2O2, SiO2, and CaO. Plot the 5
minerals/endmembers SiO2, Ca2Si2O6, Ca3Al2Si3O12, Al2SiO5, and CaAl2SiO6 in the diagram. Do
you know these minerals? The last one is especially difficult – can you guess the mineral group?
Find all reactions that are stoichiometrically possible between these endmembers? (5pts.)
5. Beryl
What is Beryl? In what kind of rocks do you find Beryl? What other minerals are found together
with Beryl? Explain why Beryl occurs in these environments. (3pts.)
2
6. Mineral formulas
Which of these Mineral formulas are very likely not correct? (Elements are given in mol)
Which minerals correspond to the correct analyses? (3pts.)
No.
1
2
3
4
5
6
7
8
Si
2.7
2.6
3
3
3
2
2
6.2
Ti
Al
0.2
1.3
1.4
1.9
1.8
1.6
0.8
0.4
2.6
Fe
1.8
1.9
0.7
0.1
0.4
2.9
Mn
0.3
0.4
Mg
0.7
0.4
0.2
0.1
0.2
1.1
0.2
0.5
2.5
0.8
0.4
1.7
0.2
0.6
0.6
Ca
0.1
0.4
Na
0.2
0.6
K
0.7
Cr
0
0.1
H
O
2
8
8
12
12
12
6
6
23
7. Spinel vs. garnet
List the members of the spinel group minerals and compare them to the garnet group
minerals (similarities, differences, chemistry, crystallography, occurrences). (5pts.)
8. Plagioclase vs. scapolite
How is plagioclase and scapolite related to each other? What are their occurrences? (2pts.)
9. Mineralogy of a rock
A rock contains sanidine, plagioclase with 2V = 80°, positive (see Figure 1), and hornblende.
What type of rock is it? (2.pts.)
3
Fig. 1
10. SiO2 polymorphs
Draw a P-T diagram with the stability fields of the SiO2 polymorphs. Describe the
transformations between the different polymorphs. In which rock types do the different SiO2
polymorphs occur? (5pts.)
4