Florida Atlantic University PETROLOGY -- MIDTERM ONE KEY

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1 GLY4310 Name 60 points February 7, took exam - Numbers to the left of the question number in red are the number of incorrect responses. Instructor comments are in blue. Florida Atlantic University PETROLOGY -- MIDTERM ONE KEY True-False - Print the letter T or F in the blank to indicate if each of the following statements is true or false. Illegible answers are wrong. (1 point each) 3 T 1. Although iron is by far the most heavily used metal, on an atomic per cent basis, three other metals are more common. 4 T 2. In the garnet structure, the A site is larger, and is occupied by divalent cations, while the B site is smaller, and occupied by smaller trivalent or possibly quadravalent ions. 5 F 3. All aluminosilicate minerals are either nesosilicates or tectosilicates. 4 F 4. Amphiboles are single chain inosilicate minerals. 8 F 5. Pyroxene minerals may be recognized by two directions of cleavage at approximately T 6. In the pyroxenoid structure, large cations occupy both the X and Y sites, producing the triclinic structure of the pyroxenoids, with chains parallel to the b axis. 4 F 7. Triphormic phyllosilicates always have divalent cations. 3 T 8. Kaolinite has the formula A1 4 [Si 4 O 10 ](OH) 8. It is dioctahedral. F 9. The extrapolated near-surface geotherm is a reasonable approximation of both the oceanic and continental geotherms to a depth of several hundred meters. This should have said kilometers, not meters, so the question will not be counted. 1 T 10. The geobarometric gradient increases in a nearly linear manner through the mantle. 0 F 11. In the IUGS classification of igneous rocks, F stands for feldspars. 1 T 12. An ex-solution texture in which the host mineral is albite, with lamellae of potassium feldspar showing coherent intergrowth has an antiperthitic texture. 1

2 Multiple-Choice - Choose the best response to each statement or question. Print the letter corresponding to your choice in the blank. (1 point each) 2 C 1. The most common element in the earth s crust, by any commonly accepted method of measurement, is: A. Aluminum B. Iron C. Oxygen D. Silicon 0 D 2. Silicate minerals are the most common minerals in the earth s crust, and are known as rock-forming minerals. What percentage of the earth s crustal minerals are silicates? A. Just over 50% B. About 67% C. 75% D. More than 90% 3 B 3. Which of the following types of minerals is the most common in earth s crust? A. Alkali feldspar B. Plagioclase feldspar C. Pyroxene D. Quartz 0 B 4. When the entire earth, rather than just the crust is considered, the most common mineral is probably what? This mineral is known to be the major constituent of the upper mantle. A. Hornblende B. Olivine C. Pyroxene D. Quartz 4 C 5. Terms like nesosilicates, sorosilicates, and inosilicates refer to: A. Mineral classes B. Mineral groups C. Mineral subclasses D. Mineral species 2 D 6. Iceland spar, tufa, and travertine are varieties of what? A. Mineral classes B. Mineral groups C. Mineral subclasses D. Mineral species 2

3 0 C 7. The basic building block of silicate minerals is an anionic group of silicon and oxygen ions in what shape? A. Octahedral B. Square Planar C. Tetrahedral D. Trigonal planar 0 D 8. Which of the following minerals has the composition Al 2 SiO 5? A. Andalusite B. Kyanite C. Sillimanite D. All of the above are polymorph with the composition of Al 2 SiO 5 2 D 9. The mineral hemimorphite is a secondary mineral formed in the oxidized portions of deposits of what element? As such, it is a good indicator mineral for this deposits of this element. A. Aluminum B. Copper C. Silver D. Zinc 9 A 10. Which of the following is an orthoamphibole? A. Anthophyllite B. Grunerite C. Hornblende D. Riebeckite 6 D 11. Which of the following mafic inosilicates characterizes alkali igneous rocks? A. Aegirine B. Glaucophane C. Riebeckite D. All of the above 1 C 12. Which of the following potassium feldspars crystallizes at the highest temperature? A. Microcline B. Orthoclase C. Sanidine D. These are plagioclase feldspars, not potassium feldspars 7 D 13. Which of the following minerals has an anorthite content between 50 and 70 percent, and is the dominant feldspar in gabbro, basalt and, often, in anorthosite? A. Anorthite B. Andesine C. Bytownite D. Labradorite 3

4 7 B 14. Which of the following mineral groups often contains unusual ion or ionic groups, like Cl - or SO -2 4? A. Epidote B. Feldspathoid C. Olivine D. Serpentine 6 D 15. Which of the following mineral groups if found as weathering products of mafic igneous minerals like olivine and pyroxene? A. Amphibole B. Feldspathoid C. Olivine D. Serpentine 8 C 16. In which region of the earth do seismic P and S waves have the highest velocity? A. Crust B. Upper mantle C. Lower mantle D. Core 1 B 17. A rock composed of predominantly euhedral crystals is said to be: A. Hypidomorphic B. Idiomorphic C. Massive D. Xenomorphic 5 D 18. A rock in which sub-parallel feldspar laths form sheaves and swirls around earliercrystallized minerals is said to have what kind of texture? A. Embayed B. Felty C. Sieve D. Trachytic 4 D 19. A deuteric alteration in which pyroxene is largely replaced by hornblende is an example of:: A. Biotization B. Seritization C. Serpentinization D. Uralization 8 A 20. In the chloritization process, what mineral does chlorite replace? A. Biotite B. Diopside C. Hornblende D. Muscovite 4

5 Fill-Ins - Write in the word or words which best completes each statement or answers each question. (1 point per blank) The minerals enstatite and ferrosilite are found on the bottom of a ternary diagram with enstatite, ferrosilite, and wollastonite as the end members. What silicate series are enstatite and ferrosilite members of? ORTHOPYROXENE The minerals diopside and hedenbergite on found in the middle of the ternary diagram. What silicate series are diopside and hedenbergite members of? CLINOPYROXENES The mineral wollastonite is part of what group? PYROXENOIDS 1 4. Minerals, like biotite or hornblende in granite, which may be present but are not required are known as ACCESSORY minerals A rock texture that has a bimodal size distribution, with large crystals surrounded by numerous small grains or glass, is described as PORPHYRITIC A term for rocks rich in magnesium and iron, and hence with low silica contents, is MAFIC.. Common minerals include olivine, pyroxene, hornblende, and biotite. The term can be used to modify either a mineral or a rock name Rocks with a silica content between wight percent SiO 2 are described as MAFIC The presence of aegirine, riebeckite, or glaucophane would likely mean that a rock was PERALKALINE, using a term from the alumina saturation method of classification A pyroclastic rock composed of glass shards should be described as a (two words) VITRIC TUFF, using the IUGS classification scheme for pyroclastic rocks. If either word was correct, credit was given. If both were correct, student received 1 point extra credit. Six students earned extra credit. 5

6 Matching I - Match the property in column one with the correct silicate subclass in column 2. (1 point each) Answers may be used once, more than once, or not at all. Column 1 0 A 1. Minerals of this subclass have a Si:O = 1:3. They include beryl and tourmaline. 1 B 2. Pyroxenoids, amphiboles, and pyroxenes belong to this subclass. A. Cyclosilicate B. Inosilicate C. Nesosilicate D. Phyllosilicate E. Sorosilicate F. Tectosilicate Column 2 1 E 3. Members of this subclass have Si 2 O 7 clusters. Important minerals include the epidote group, vesuvianite, and hemimorphite. 3 F 4. Mineral species in this subclass include andesine, sanidine, microcline, labradorite, and orthoclase. 0 D 5. This subclass gets its name from the Greek word for leaf. The micas and chlorites are members of the subclass. 6

7 Matching II -Match the descriptions from column 1 with the correct silicate group from column 2. Answers may be used once, more than once, or not at all. (1 point each) Column 1 3 G 1. Minerals like sodalite, leucite, and nepheline are important minerals of this group, which often indicate a silicate magma was undersaturated with silica. 1 P 2. Minerals like stilbite, natrolite and analcime are members of this group. They have very open structure, and natural and synthetic members of this group are used in the petroleum industry for catalysis. 0 O 3. Varieties of one mineral in this mineral group include amethyst. chrysoprase, citrine, jasper and milky. Column 2 A. Al 2 SiO 5 B.. Alkali feldspar C. Amphibole D. Chlorite E. Clay F. Epidote G. Feldspathoid H. Garnet I. Mica J. Olivine K. Plagioclase feldspar L. Pyroxene M. Pyroxenoid N. Serpentine O. Silica P. Zeolite 5 C 4. Minerals in this group are often elongated prisms, and possess 60 cleavage. Examples include riebeckite, cummingtonite, actinolite, and glaucophane. 1 H 5. Members of this group are often split into those with calcium, and those without. The grains are usually euhedral, and are isometric. Examples include uvarovite, almandine, andradite, and grossularite. 7 O 6. High temperatures members of this group include tridymite and cristobalite, and high pressure species include coesite and stishovite. 7

8 Short Answer - Write a complete, concise answer to all of the following three questions. Diagrams (labeled) may be used to supplement your written answers, where appropriate. 4 points each Both micas and brittle micas are layered silicates. How many layers do they have? There is one major difference in a physical property between these groups. What is it? How does the bonding of the groups differ, and how does this account for the difference in the physical property? THE ARE TRIPHORMIC PHYLLOSILICATES, WITH THREE LAYERS (T-O-T). MICAS ARE FLEXIBLE, BRITTLE MICAS, AS THE NAME IMPLIES, ARE SOMEWHAT BRITTLE. IN MICAS, THE LAYERS ARE HELD TOGETHER BY MONOVALENT POTASSIUM IONS IN XII. THESE BONDS ARE WEAK, AND CAN STRETCH OR CONTRACT AS THE MINERAL IS DEFORMED. IN BRITTLE MICAS, THE POTASSIUM IS REPLACED BY DIVALENT CALCIUM, WHICH FORMS STRONGER, MORE RIGID BONDS THAT DO NOT DEFORM WITHOUT BREAKING. 8

9 26 2. Define petrology. What is the difference between petrology and petrography? PETROLOGY IS THE BRANCH OF GEOLOGY DEALING WITH THE ORIGIN, OCCURRENCE, STRUCTURE, AND HISTORY OF ROCKS, ESPECIALLY IGNEOUS AND METAMORPHIC ROCKS. PETROGRAPHY DEALS WITH THE DESCRIPTION AND SYSTEMATIC CLASSIFICATION OF ROCKS, ESPECIALLY IGNEOUS AND METAMORPHIC ROCKS, AND ESPECIALLY BY MEANS OF MICROSCOPIC EXAMINATION OF ROCK THIN SECTIONS. 9

10 What is diffusion? In what ways does it affect the crystallization of a magma? DIFFUSION IS THE MOVEMENT OF A FLUID, PARTICLES, OR HEAT FROM AN AREA OF HIGHER CONCENTRATION TO AN AREA OF LOWER CONCENTRATION. IT CONTROLS THE MOVEMENT OF IONS IN MANY MAGMAS. THE DIFFUSION RATE THUS GOVERNS THE DELIVERY OF IONS NEEDED TO GROW A CRYSTAL TO THE CRYSTAL FACE WHERE THEY ARE NEEDED. IT IS ALSO IMPORTANT IN DISSIPATING HEAT FROM A GROWING CRYSTAL. THE LATENT HEAT OF CRYSTALLIZATION WILL QUICKLY HEAT A NEWLY FORMING CRYSTAL, AND MAY CAUSE IT TO REMELT IF NOT QUICKLY DISPERSED. 10

11 Midterm 1 Results B B 46.5 C+ Mean = 46.5 (77.5%) 43.0 C Median = D D 36.5 D % % % % % % % % % % % % % % Previous Results Tenth out of fourteen 11