Lab 6: Minerals and metamorphic rocks

Transcription

1 Geology 115/History 150 Name(s): Lab 6: Minerals and metamorphic rocks Minerals A mineral is a naturally-occurring, solid, usually inorganic element or compound with a definite crystal structure and chemical composition which varies only within specific limits. Rocks are merely aggregates of minerals. The mineralogical composition of a rock depends on the conditions under which that rock formed. Igneous rocks tend to have minerals that form at high temperatures; sedimentary rocks contain minerals that are stable at Earthsurface conditions. Metamorphic rocks consist of minerals that form under a range of pressure and temperature conditions within the Earth. Common rock-forming minerals: Minerals in igneous rocks Quartz Orthoclase Plagioclase Biotite Muscovite Amphibole Pyroxene Olivine Minerals in metamorphic rocks Quartz Biotite Muscovite Amphibole Garnet Talc Chlorite Staurolite Kyanite Orthoclase Plagioclase Minerals in sedimentary rocks Quartz Orthoclase Biotite Muscovite Calcite Halite Gypsum Clay minerals Mineral identification The first part of this lab is to identify mineral specimens, using the charts provided in the Geology Lab Manual. Note that most rock samples will not have minerals as large as the ones you will see in this part of the lab, so take notice of diagnostic characteristics that do not depend on mineral size. 1. For instance, consider color, a seeming obvious choice: find the quartz display in the cabinet in the back of the lab room. Quartz is a mineral that also happens to be a gem in some of its forms (e.g., opal, tiger s eye, amethyst). Consider all the different varieties of quartz; is there are unique color for quartz?

2 2. Now consider shape, another fairly obvious characteristic. Again examine the quartz display in the cabinet. Is the shape uniform for both specimens of the same mineral? In fact, if a mineral is left undisturbed as it precipitates, it can develop its crystal growth habit. The shape of such minerals is generally attractive, or, as mineralogists call it, euhedral. Needed: Mineral testing kit (located in the Tub 1 space) and mineral samples M- 1 through M-14 (Tub 2). Please label the minerals with their M-numbers (use the lab tape and a pen) so that they can be returned to their rightful box. Using the charts: So what characteristics are actually useful in identification? It turns out that the chemical composition of a mineral (which distinguishes one mineral from another, usually) manifests itself in certain ways. The most apparent of these is the mineral's luster, which can be metallic or non-metallic. Luster refers to how the mineral reflects light; a metallic luster is how a piece of steel or bronze or copper would reflect light. Compare a piece of metal's luster to the luster of a piece of glass; the glass' luster (vitreous) is not a metallic luster. Of course, if the mineral has a dull or pearly luster, it is a nonmetallic luster. 3. a. Look at minerals M-1, M-3 and M-7. Only one of these samples has a metallic luster. Which one? b. Now examine minerals M-2 and M-10; again, only one of these has a metallic luster. Which one? Hint: you may need to look at different specimens of the same mineral. Why was this question harder to answer than part a? Next, recall how color was problematic. However, we can still use it appropriately in mineral identification by simply determining if the mineral is dark-colored (black or one of the cool colors) or light-colored (white or one of the warm colors). The difference is due to the particular chemical elements that make up the mineral.

3 4. Look at samples M-11 and M-13. Which one is dark-colored? Which is light? Now use the mineral identification charts found in the colored pages in the lab manuals located in the tub labeled Lab Manuals. There are three sheets: yellow, red and blue and they are divided according to the two useful characteristics already mentioned. Since you ll be eventually filling out the table on the next to last page of this lab, you can now enter the two useful characteristics for all of the minerals in this lab (M-1 through M-14) into the table: luster and color. To identify a mineral s name, once you have determined a mineral s luster and color, note that you will be using one of the three colored sheets. Follow the headings of the sheet left to right on the sheet to determine what test to do next. For all minerals, the relative hardness of the mineral may be determined by scratching a corner of the mineral on a piece of glass (or scratching a corner of the glass plate on the mineral). Hardness is the mineral's ability to resist scratching or abrasion. A mineral will scratch all softer minerals and will be scratched by all harder minerals. Certain index minerals define the Mohs Hardness Scale, so you can get a numerical value for hardness. Rather than finding the exact numerical value, you will simply need to determine whether the mineral is harder than glass.

4 5. Use the corner of a glass plate and scratch minerals M-1, M-6 and M-10 and record the results below. Then scratch a corner of each mineral on the flat surface and record the results. Be sure to brush off any flakes of mineral to make sure that you ve actually left a scratch! Then combine the information to draw a conclusion. Hint: there s one of each type. Mineral Does the glass scratch the mineral? Does the mineral scratch the glass? The mineral is harder than, softer than or the same hardness as the glass? M-1 M-8 M-10 Again for all minerals. cleavage is another property that helps narrow down the identity of the mineral. Cleavage is the ability of the mineral to split along closely spaced parallel planes. The planes along which a mineral cleaves (when hit with a hammer, for instance) are the planes where all the weak atomic bonds in the crystal structure exist. Notice that if all bonds are uniformly strong (like in a piece of quartz), the mineral will not cleave along a plane; instead, it will break unevenly and roughly...it will fracture. Cleavage is sometimes confusing because some minerals have good cleavage, some have poor cleavage and still others have no cleavage (they fracture). The table below should help identify different types of cleavages, but ask if this concept is confusing! Also note that we discounted shape a while ago. Cleavage is not the same thing as shape shape is how the mineral crystal grew, whereas cleavage is how the crystal broke. 6. a. How many cleavages does M-7 have? Hint: It s called a sheet silicate for a good reason! b. How many cleavages does M-10 have? Remember not to count parallel faces twice. What angle separates each distinct cleavage? c. Look at the display of the quartz spar crystal. Note that the top of the crystal has a nearly perfect six-sided symmetry; then examine the bottom of the crystal where it was broken off. How many cleavages does this chunk of quartz have? So what cleavage-related property does quartz have?

5 Then there are more specialized tests that are not applicable to all minerals, but for the minerals that the test works on, they are diagnostic! For instance, if the mineral has a metallic luster, determine the mineral's streak color. Streak refers to the color of the powderized mineral, most easily accomplished by rubbing a corner of the mineral sample against the porcelain streak plate provided. 7. a. Use the porcelain streak plate on sample M-4; what color does the streak turn out to be? Is it the same as the color of the mineral? b. Try streaking a few of the nonmetallic luster minerals. What seems to be the problem with the streak test and nonmetallic minerals? For minerals with metallic luster, magnetism may be used to identify the mineral. 8. Use the magnet to determine if M-2, M-3 or M-4 is magnetic. Weakly magnetic, strongly magnetic and not magnetic are acceptable answers. In addition to writing your answer here, enter the information under other properties on the appropriate row of the table.

6 9. Another test is a mineral s reaction to weak acid. Obtain an acid dropper bottle and place one drop of (hydrochloric) acid on a sample of M-1 and M-8. Which one reacts? How can you tell? 10. All right, now put it all together and identify the rest of the mineral samples. To make this simpler, here are the mineral names in alphabetic order: amphibole, biotite, calcite, chalcopyrite, galena, hematite, magnetite, muscovite, orthoclase (potassium feldspar or K-spar), plagioclase feldspar, pyroxene, pyrite, quartz. Mineral ID Chart Sample Luster M1 M2 M3 M4 M7 M8 M9 M10 M11 M13 M14 M17 M41 M42 M43 Streak (if it is useful) Hardness (relative to glass) Cleavage/ Fracture Other Properties Mineral Name

7 11. Mineral summary: Name three physical characteristics you can use to distinguish quartz from calcite. Mining economics Consider the following selected assay results for layers of the Homestake breccia deposits, as reported by Noranda Exploration and Crown Butte Mines (Van Gosen, 2005). Layer number Thickness (meters) Gold tenor (opt) Silver tenor (opt) Copper tenor (opt) Given that the tenor (concentration) of the various metals in each layer is report in troy ounces per ton of rock mined, for layer 1, how many tons of rock would have to be mined to yield 1 troy ounce of gold? The fixed costs of excavation, hauling, crushing and refining run about $480/ton of rock mined; is this layer economically feasible to mine for gold? Why or why not? 13. As of May 9, 2016, gold s spot price in New York is $1266/troy ounce, silver is $17/troy ounce and copper is $0.13/troy ounce. Per ton of rock mined, which layer number is the most economic to mine? Prove it with a calculation. Is the silver and copper ever economic to mine here?

8 Metamorphic rocks Metamorphic rocks have been subjected to sufficient heat and/or pressure to melt some of their constituent minerals, but not all of them. As a result of this selective mobilization of chemicals, only certain chemical reactions can occur, and so a whole new set of metamorphic minerals are crystallized. Throw in the presence of fluids such as water and carbon dioxide (yes, at these pressures, even carbon dioxide can be a liquid), and nature has the means to create even more metamorphic minerals and therefore metamorphic rocks. Note that metamorphic rocks must be formed at depth; metamorphism is not a surface process, and so is distinguishable from mere sedimentation. Rocks that have foliation (a sort of wavy layering, though it can resemble horizontal layering) are metamorphic rocks; the foliation indicates that directional pressure was applied to the rock while the mineralogical changes were occurring. On the other hand, some metamorphic rocks are not foliated; they appear crystalline, like coarse-grained igneous rocks. These metamorphic rocks were subjected to isotropic, or nondirected, pressure. Because there are so many metamorphic minerals (of which you have seen but a few), there are all sorts of ways to name metamorphic rocks. We will concentrate on naming rocks by their metamorphic grade (that is, by the maximum degree of heat and pressure they were subjected to, and not their mineral composition), or, in some unusual cases, by their apparent composition (for instance, rocks like marble, quartzite or metaconglomerate, from which you cannot determine the metamorphic grade). The parent rock of a metamorphic rock is the original rock that was metamorphosed into what you see today. As you can see from Table 6.1, the parent rock s minerals really do determine the resulting metamorphic rock s composition. Note the differences in mineralogy even at the same grade. Table 6.1 Mineralogy of metamorphic rocks related to parent rock and grade Metamor- Facies Parent rock phic grade Basalt Shale Low Zeolite Calcite, chlorite, zeolite Zeolite, sodium-rich micas Greenschist Chlorite, amphibole, plagioclase, epidote Chlorite, muscovite, plagioclase, quartz Medium Amphibolite Amphibole, garnet, plagioclase, quartz Garnet, biotite, muscovite, quartz High Granulite Pyroxene, plagioclase, garnet Biotite, orthoclase, quartz, andalusite

9 A metamorphic facies is a name of a set of metamorphic minerals which is uniquely created at a particular pressure and temperature. So, in addition to a metamorphic grade, a rock can belong to a particular metamorphic facies as well! Confused? You bet! However, realize that these terms all have their uses. Note that not all minerals in a given cell in the table above will show up in every specimen of that grade/facies/parent rock, but all minerals in the specimen will be named in the cell! One other consideration: there are three different types of metamorphism, related to the particular tectonic setting of the metamorphism. As you are aware, the deeper rocks are drawn into the lithosphere, the higher the temperatures and pressures the rocks are subjected to. This is called regional metamorphism. However, there are two other sets of conditions. Blueschist-type metamorphism occurs under high-pressure but lowtemperature (high P, low T) conditions. Contact metamorphism occurs under high-temperature but low-pressure (high T, low P) conditions. This means that, depending on the tectonic setting, three different metamorphic rocks could arise from the same parent rock. Table 6.2 summarizes these types. Table 6.2 Mineralogy of metamorphic rocks related to parent rock and grade Meta. Facies Parent rock type Basalt Shale Regional See table 6.1 Dynamic (low grade) Blueschist Blue amphibole, chlorite, Ca-silicates Blue amphibole, chlorite, quartz Dynamic Eclogite Pyroxene, garnet, not observed (high grade) kyanite Contact Hornfels Pyroxene, plagioclase Andalusite, biotite, orthoclase, quartz Needed: Samples M18 and M 19 (Tub 37), R34 through 45 (Tubs 38 49) 14. Some minerals are made under metamorphic conditions. You have seen some of them previously in this lab. Two other metamorphic minerals are kyanite and chlorite; write their characteristics below (similar to terms in the tables) Mineral Characteristics (luster, color, hardness, cleavage, other features) kyanite chlorite

10 15. a. Look at rock sample R34, a regionally-metamorphosed shale. Name two minerals that are in this rock. Hint: there s a dark one and a light one. b. Given that muscovite is present in R34 but hard to see, what grade of metamorphism does this mineralogy imply (use table 6.1)? c. Still using that table, what metamorphic facies is R34? d. So what is the name of the rock? To find this, use the diagram below. One way that metamorphic petrologists try to quantify the conditions of metamorphism for various rocks is to draw a pressure/temperature (P/T) diagram as shown in the figure on the next page. The field of the graph shows the ranges of various metamorphic facies. The vertical axis shows the depth of the metamorphism and the equivalent pressure in kilobars (kb). 1 bar is approximately 1 atmosphere of pressure, and therefore 1 kb is about 1000

11 atmospheres of pressure. The horizontal axis shows the temperature of the metamorphism in degrees Celsius. 16. a. Use the facies from question 4 to determine the range of possible maximum pressures and the range of possible maximum temperatures at which R34 formed. Use units of C for temperature and kbar for pressure. b. Suppose another area where the parent rock was found was subjected to less than 1 kbar of pressure but the same temperature range during metamorphism. Name one other mineral (besides the ones you named in the previous question) you would expect to find. As you have seen, some minerals are quite useful in determining the grade or type of metamorphism because they can only form under certain metamorphic conditions. These are called index minerals.

12 17. You are given the following information about a metamorphic rock: Mineral composition: pyroxene, garnet, kyanite Chemical composition: silicon dioxide 50.24%, aluminum oxide 13.32%, calcium oxide 10.84%, iron oxide 9.85%, magnesium oxide 8.39% Which type of composition is more useful in determining the grade and parent rock of metamorphism and why? Or do both lists give equivalent information? 18. a. Now look at R35, which is the same metamorphic grade as R34. What are the mineralogical differences? (In other words, what minerals show up in R34 but not R35? In R35 but not R34?) b. But what is the name of this rock, anyway? Hint: kind of a trick question. 19. In fact, for many metamorphic rocks, the most common mineral in the rock is used as an adjective in front of the rock name. Fill in the appropriate mineral name for the samples below, using the suggested test given: Sample # Test Rock name R34 Cleavage schist R35 Obvious mineral schist R36 Color schist R37 Scratch schist

13 Parent rock Intensity of metamorphism Low grade grade High shale slate phyllite rhyolite granite basalt limestone sandstone conglom. metaconglomerate schist marble quartzite gneiss amphibolite 20. What changes in foliation thickness and mineral grain size would you expect to see in a shale as it is subjected to greater temperatures and pressures during metamorphism? (Hint: compare, in order, R38, R39, R34, R40) 21. So fill in the following rock names, using your answer to the previous question and the fact that each sample represents a different metamorphic grade: Sample # Metamorphic grade Rock name R38 R39 R R41 and R42 are nonfoliated metamorphic rocks (they are sometimes called granoblastic rocks ); both of these rocks achieved the same grade of regional metamorphism as R34 and R35 did. Identify the rock names using the hints suggested in the characterization column; identify their parent rocks from the table above. Sample # Characterization Rock name Rock parent rock R41 Glass plate R42 Acid bottle

14 Plate Tectonics and Metamorphic Rocks 23. R43 is blueschist, a unique type of metamorphic rock that forms under conditions of high pressure and low temperature. Label the area on the crosssection below where you might expect blueschist to crystallize. 24. So, if you were to find blueschist as you walked along the Appalachian Trail in North Carolina, what could you infer about the history of the East Coast of the US? 25. R44 is serpentinite, which blueschist often becomes over time. A key mineral in blueschist is forsterite, a form of olivine, with the chemical formula Mg2SiO4. A key mineral in serpentinite is (surprise) serpentine (chemical formula: Mg3Si2O5(OH)4). How does serpentinite form from blueschist? (Hint: consider readily available simple molecules at metamorphic depths and the difference between the two chemical formulae) 26. R45 is hornfels, a unique type of metamorphic rock that forms under conditions of low pressure and high temperature. Label the area on the crosssection below where you might expect hornfels to crystallize. 27. What is hornfels' parent rock? Or is there a unique parent rock? 28. Why is contact metamorphism such an appropriate term for this type of metamorphism?

15 Geologic Map of Wyoming (1985) 29. Yellowstone NP is located in the northwest corner of the map; find a chunk of Ti rock (it s a magenta color) located at about 44.8 N, W. Write the description of the Ti rock, and, even though the explanation does not explicitly state it, determine what the radiating lines of that Ti color are, geologically. Finally, explain why contact metamorphism is likely in this area. 30. Recall that replacement of the skarn minerals is where the gold and other metals will end up. To produce skarn, you need some carbonate rocks nearby rocks like limestone or dolomite. Look around at nearby formations from the Ti outcrop, and, using the explanation sheet, determine the name(s) of the potential carbonate formation(s) and the name(s) of the rock(s) they contain that will metamorphose to skarn.