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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. Three out of Nesosilicates (single tetrahedra) [SiO4]4− (Olivine, Garnet) Sorosilicates (double tetrahedra) [Si2O7]6−, (Epidote) Cyclosilicates (rings) -­‐ [Si6O18]12−, (Tourmaline, Beryl) Inosilicates (single chain) -­‐ [SiO3]2-­‐ (Pyroxenes) Inosilicates (double chain) -­‐ [Si4O11]6− (Amphiboles). Phyllosilicates (sheets) -­‐ [Si2O5]2− (Micas, Chlorite) Tectosilicates (framework) [SiO2] (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 replaces with an ion with a different Ox. State, there has to be a coupled other exchange balancing the loss/gain od charge. Examples are Jadeite – Diopside: Ca2+Mg2+ <-­‐> Na1+ Al3+ Tschermak substitution: Al3+ Al3+ <-­‐> Si4+Mg2+ 3. Pyroxenes What do you know about pyroxenes? 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.) 1 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 colors 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. Cliopyroxenes: Diopside (CaMgSi2O6) Hedenbergite (CaFeSi2O6) Jadeite (NaAlSi2O6) Acmite (NaFe3+Si2O6) Orthopyroxenes: Enstatite (MgMgSi2O6), Ferrosilite (FeFeSi2O6) 4. Reaction balancing Draw a ternary chemographic diagram with the endmembers Al2O3, 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.) Quartz (Qtz), wollastonite (Wo), grossular (Grs), alumosilicates (As), and Ca-­‐Tschermak component in pyroxene (Ca-­‐Tsch). [Qtz] = [As]: Wo + Ca-­‐Tsch = Grs [Wo]: Gr + 2 As = 2 Qtz + 3 Ca-­‐Tsch [Gr]: Wo + 2 As = 2 Qtz + 2 Ca-­‐Tsch [Ca-­‐Tsch]: 3 Wo + 2 As = 2 Qtz + 2 Gr 5. Beryl 2 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.) Be-­‐Al-­‐Silicate (not necessary to know formula Be3Al2(SiO3)6. Typical for pegmatites. Be is incompatible and thus enriched in developed melts. Beryl occurs together with Qtz and Fsp and other minerals accommodating incompatible elements, e.g. Tourmalin. 3 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
1. not correct (no charge balance – looks like feldspar, but Ca, Na, K are turned around) 2. correct -­‐ plagioclase 3. correct -­‐ garnet 4. correct – garnet (some Fe3+) 5. not correct – (looks like garnet, but even if all Fe is Fe3+, no charge balance) 6. not correct – (looks like Cpx, but Jd-­‐component messed – no charge balance) 7. correct – omphazite with some acmite component 8. not correct – amphibole with a missing oxygen All answers can be found with charge balance consideration. 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.) magnetite, spinel, hercynite, chromite, ulvöspinel Cubic crystal system, solid solution between Mg-­‐Fe endmembers Magmatic and metamorphic origin Non-­‐silicate (oxides) vs. Silicate 4 8. Plagioclase vs. scapolite How is plagioclase and scapolite related to each other? What are their occurrences? (2pts.) Same solid-­‐solution between Ca and Na and chemical composition, but in addition scapolite endmebers have NaCl (Marialite) and CoCO3 (Meionite), respectively in their formula. Occurrences of scapolite in skarns, calcsilicates, metamorphic, metasomatic, rare pegmatites Plagioclase metamorphic and magmatic 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.) Fig. 1 Volcanic rock (because of sanidine Kfsp). Trachyte, trachy-­‐andesite, with An50 (or An70) 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.) Displacive transformation: without breaking bonds, little energy required, α−β transformation Reconstructive transformation: breaking of bonds, high energy, all other polymorph transformations of SiO2 Quartz: magmatic (plutonic, volcanic), metamorphic, sedimentary, hydrothermal Tridymite: volcanic rocks 5 Cristobalite: volcanic rocks Stishovite: high P rocks, shock metamorphism Coesite: high P rocks 6