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Crystallography in the Classroom—Modeling Silicates without Silicate Models Alyson Ponomarenko Department of Earth and Environmental Science, University of Texas at San Antonio, San Antonio, TX 78249, [email protected] ABSTRACT I propose two alternatives to the traditional ball-and-stick model for teaching silicate chemistry and crystallography in the classroom. The first model uses fresh fruits of varying sizes to demonstrate the basic silicon-oxygen tetrahedral structure, and to show how covalent and metallic bonding can reduce the negative charge balance, ultimately creating viable silicate minerals. The second uses students to represent large mobile silicon tetrahedra in a truly hands-on approach to understanding silicate minerals and magmas. The two methods used together and supplemented with active in-class discussion provide for optimal learning, even in large classes. INTRODUCTION In an ideal world, students would begin to gain a working understanding of minerals and rocks and their processes of formation as early as middle school and high school, so that they’ll know of geology and want to continue with it in college. In an ideal world, this knowledge would be expanded in introductory college geology courses, and cemented in semester-long mineralogy and petrology classes for geology majors. Of course this is not an ideal world—most secondary schools across the country offer no courses at all in earth sciences, many college-level courses in geology brush quickly by minerals and rocks in favor of more exciting material, and some universities now combine major courses in mineralogy and petrology into a single semester. One of the reasons why minerals and rocks are not treated with very much depth even in college courses in physical geology may be that while it is easy enough to describe the physical properties of a mineral (cleavage, hardness, etc), it is not always so easy to explain why a mineral has these properties. At the first mention in class of the chemistry of minerals or first glance at a ball-and-stick model of a mineral, students often pull back and stop even trying to understand. Because this response is immediate and obvious, many educators have chosen simply to cover different material in physical geology. Certainly there is more than enough material to fill a semester of geology without ever even mentioning a rock or mineral. Hence mineral chemistry, properties, and processes of formation are slowly being lost from introductory courses. This leaves more information that must be covered later in mineralogy and petrology, not less. The problem with this trend away from teaching rocks in “Rocks for Jocks” is that minerals and rocks are a large part of the foundation of our lives today, from the copper in our wiring to the silicon in our computers to the steel in our SUVs. An understanding of the way minerals are formed and destroyed can help to shape an understanding of the processes that are ever changing and forming the earth today. For example, knowledge of Ponomarenko - Crystallography in the Classroom the structural and physical properties of quartz helps to explain why basaltic volcanoes, while the least explosive, are the most common, why so many plutons are granitic in composition, and why most sandy beaches are made up primarily of quartz. Taken a step further, such knowledge can also explain why quartz is in such demand in our modern economy, for use in silicon chips, solar panels, lighting, and glass. I have found that it can be not only easy but also a lot of fun to teach mineralogy, and from that foundation petrology, in physical geology classes. Where traditionally ball-and-stick models or triangle diagrams have been used to teach silicate crystalline structures (and, consequently, Bowen’s reaction series), I prefer to opt for oranges and apples and grapes—and lots of audience participation. I present two different but complementary models to explain the progressive crystallization of silicate minerals from a melt and to keep students actively involved and learning. Both can be used well for students as young as 8th grade, and both work exceptionally well in very large classrooms where there is some open space at the front of the room. The first model, which uses oranges and red grapes to represent O-2 and Si+4 respectively, is incredibly powerful in demonstrating the basics of silicate structure and chemistry. Other fruits, such as apples, plums, nectarines, etc., (in season) can be used to represent the metal cations Mg+2, Fe+2, Ca+2, Na+1, K+1, and Al+3. For the sake of simplicity it is probably best to ignore Fe+3 for now. These fruits should be chosen for their relative sizes, to reflect differences in atomic radii of the different elements. The element and its most common charge can be written directly on the fruit or on a label beside the fruit. Because a single grape (Si+4) nestles perfectly between four large oranges (O-2), it is easy for students to see the basic structural stability of the negatively charged silicon tetrahedron. It is also easy to take the structure apart and count the charges with the class to see the electrical instability—the imbalance in charge—of that same tetrahedron. To begin with, all the fruits can be tossed randomly into a crate to show the major elements loose in a liquid magma. A key indicating the element and associated charge represented by each fruit can be drawn on the board, directly on the fruit, or handed out with any other supplementary material that may be appropriate to the class. The relative proportions of each element in the Earth’s crust can also be listed, to help students see why most igneous magmas, and therefore igneous minerals, are silicon and oxygen based, and to help them decide which cations to use first in making silicate minerals. The oranges are then arranged in pyramids with grapes in the center to form two or more silicon tetrahedra. The two isolated silicon tetrahedra have a combined charge of –8, and need two Fe+2 or Mg+2 cations each to form two stable olivine minerals—the nesosilicates. This is typically the first mineral to form from a molten magma at high temperatures, in part because individual silicon tetrahedra form easily, and in part because metal cations are widely available for ionic bonding. One of the corner oranges can be taken away 31 Figure 1. Several silicon tetrahedra linked by covalent bonding to form the basic structure of single-chain inosilicate minerals (e.g. the pyroxene group). For the sake of simplicity, fruits representing metal cations have not been added yet in this photograph. and two tetrahedra moved together to represent covalent bonding between silicon tetrahedra to begin to form the sorosilicates. Two tetrahedra sharing one oxygen (sorosilicates) have a combined charge of only –6, and need only three metal cations to balance the charge. Removal of oranges can be continued to reduce the negative charge of the basic structure through single-chain and double-chain inosilicates such as the pyroxenes (Figure 1), and sheet (phylo-) silicates such as micas and clays. Metal cations (e.g. plums or nectarines) are added as needed to further reduce the charge to zero. Students by now will have noticed the trends of increasing silicon and oxygen content and decreasing metal content in each new mineral all on their own. They may also have noticed that each mineral group is stronger than the last, based on the increasing ratio of covalent to ionic bonds. While there is no easy way to show tectosilicates, a whole crate of oranges, real or imagined, might best represent the framework silicates. A few green grapes thrown in to represent Al+3 can show the difference between quartz and feldspars. Here you will need to note the small difference in charge between Si+4 and Al+3 which substitute for one another in the tetrahedral structure, and have the class explain how the charge imbalance can be accounted for in nature. They should also be able to guess which is more stable at surface conditions, quartz or feldspars, and to explain why. Now you have painlessly introduced the concept of complex solid solution series. The simple solid solution series will be easy after this. In this way you will have explained all the major silicate mineral groups, the general chemical composition of each, the relative temperatures of formation, and their relative strengths (which can be used to explain resistance to weathering and the formation of clays in a later section on sedimentary minerals and rocks). Of course this may have gotten a little messy as oranges tend to roll away unbidden, but handled appropriately the class will still be fully engaged and learning. The next step is to trade the produce for living breathing subjects, and to modify the discussion to include Bowen’s reaction series and a 32 discussion of magma viscosity (and therefore perhaps also gas content and the types of volcanoes associated with each step along Bowen’s). Whole humans are in some sense better representatives for silicon tetrahedra than are oranges and grapes, in as much as silicon tetrahedra, once formed, behave chemically as a unit. That cohesiveness is less obvious with loose fruit. Additionally, this will go much faster both because students are much less likely to roll away, and because the class will be able to build on their experience with the fruit models. Human silicon tetrahedra are easier to see and usually much more cooperative. Begin with several students at the front of the room, standing with their arms spread wide to the left and right and their legs spread forward and back, forming a slightly off-scale tetrahedron with the hands and feet as oxygen and the torso as silicon. It is because this “structure” is so off-scale that it helps to either work with fruits before this demonstration, or to have an overhead projection of the actual relative sizes of the elements in question clearly displayed at the front of the room. Each student, standing alone and holding a representative metal cation in each hand (leftover fruits if readily available, or soccer balls if not), represents the individual silicon tetrahedron that forms the basic structure of olivine. Each of these “olivine” minerals can be made to move randomly around the front of the room. The only difficulty here is in maintaining the tetrahedral stance, characterized by a forward waddle that always brings laughter, but there is little real difficulty involved in moving as individuals. This ultimately relates to the viscosity of the magma, which will become important as talk of mineral structures turns to talk of igneous rock formation. The instructor can assume the role of a water molecule at this point to demonstrate how easily and quickly olivine is weathered, easily knocking the metal cations from each student’s hands. If you are following Bowen’s reaction series, you may wish to skip sorosilicates and cyclosilicates, and jump straight to the single-chain inosilicates. Six or seven students holding hands can represent inosilicates. Note that grasped hands represent one (not two) oxygen atoms, covalently bonding two silicon tetrahedra together. Ideally you can have students alternate facing forward and back, but this is often more complicated than necessary and doesn’t add much to the discussion. A second row of students lined up parallel with the first and touching feet with the first row represents the double-chain inosilicate amphibole. At this point students generally understand the concept, and can be asked what they would have to do to create sheet or framework silicates (micas or quartz) without actually risking injury. By now Bowen’s reaction series is assembled on the board, and the class can discuss the difference between the discontinuous series and continuous series minerals, and whichever trends down Bowen’s the students have already discovered for themselves. Increasing mineral stability down Bowen’s can be demonstrated by again assuming the role of water and attempting to push through grasped hands to break the structure. Increasing viscosity of the magma can be shown by asking the students in a double chain to make their way out the (impossibly narrow) door without ever disconnecting either hands or feet. This task is of course very difficult for the students to do quickly or well, but will clearly show that flow is slowed with increasing silica content (and decreasing temperature). Journal of Geoscience Education, v. 52, n. 1, January, 2004, p. 31-33 The whole demonstration has proven to work well in classes from 8 students to 80, and can be easily modified to slip back and forth between humans and fruits and, if possible, actual mineral examples. The single most useful mineral example for this lesson is definitely one of the micas, because weak Van der Waals forces between silicate sheets is easily demonstrated using the actual mineral, and because biotite or muscovite can provide an excellent example of crystal habit and cleavage. Understanding is greatly facilitated if the students are given an outline to follow and fill in at the beginning of class. A sample outline, including a basic discussion of silicon tetrahedra, the six major silicate structures, common silicate minerals, and Bowen’s reaction series, is posted online at http://www.utsa.edu/eps/programs/geology/silicate_crystal_structures.htm. This is especially necessary for the “volunteers”, who will be unable to take notes while they are at the front of the room. Discussion of igneous environments and igneous rocks can follow this demonstration directly. The discussion can stay focused on igneous minerals alone or can incorporate the properties of the magma and the associated volcanism. Depending on the level of the class and the desired degree of detail, there is room in this lesson for a discussion of network-forming and network-modifying ions in a discussion of magma properties. While Bowen’s reaction series is still on display, with the trends of decreasing temperature and Ponomarenko - Crystallography in the Classroom increasing viscosity, Si and Al content, and resistance to weathering listed to one side, the other side can be filled in with names for the intrusive and extrusive rocks associated with different combinations of minerals. Using their understanding of viscosity, students should be able to estimate the gas content related to each type of magma, and therefore the explosivity of the associated volcanoes. From here they may also be able to recognize why basaltic volcanoes are the most common, what type of materials to expect with basaltic volcanism, as well as why granitic plutons are the most common, and their daughter volcanoes the most explosive. It is also possible to expand into a discussion of the silicate sedimentary minerals (particularly quartz and clays), and of the silicate metamorphic minerals. In my own classes I refer back often to this lesson in my discussions of both sedimentary and metamorphic environments, but do not run the demonstration again during class. Students are encouraged in this exercise to think for themselves while at the same time getting truly involved in class discussion. They recognize that geology IS fun, and typically come to class more consistently afterwards, if only to not risk missing any more adventures. Students that major in geology have a stronger base when they begin mineralogy. And rocks and minerals still have a place in the introductory classroom. 33