Lesson 5 will take us one week to complete. This lesson is about mineralogy and forensic geology. First we'll examine a case study of sorts by reading an account about FBI agents and geologists who tracked down the origin of some soil adhering to the body of a murdered DEA agent in Mexico in the 1980s. Then you will come up with a short learning activity of your own in which students would examine local mineralogy.
By the end of Lesson 5, you should be able to:
The chart below provides an overview of the requirements for Lesson 5. For assignment details, refer to the lesson page noted.
Lesson 5 will take us one week to complete. 8 Jul - 14 Jul 2020.
Requirement | Submitted for Grading? | Due Date |
---|---|---|
Reading assignment "Death of an Agent" | No | |
Activity: Design your own forensic mineralogy activity. | Yes - submit to Canvas assignment called "Forensics Lesson". | 14 Jul 2020 |
If you have any questions, please post them to our Questions? Discussion Forum (not e-mail). I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
The reading assignment for this lesson is an account of some detective work done by geologists working for the FBI during the 1980s. They used forensic mineralogy to find the burial site of an undercover US DEA agent who had been killed in Mexico. One of the reasons I like this article is that it presents a case study. It's almost a story with a plot line, so it is a nice break from reading scientific articles. Furthermore, it puts into context why studying mineralogy and knowing how to identify certain types of rocks and minerals has important applications. Another reason I like it is that it synthesizes some of the material we've already covered in this course and in other courses in the M.Ed. program, while bringing up some new information, too. For example, we already know how to read geologic maps to get a feel for the plate tectonic structure of a region. After reading this article, we will study the tectonics of western Mexico and then extend our thinking to subduction zones, and volcanoes in particular.
McPhee, J. (1996). The gravel page. The New Yorker, 71(46), 44, pp. 60-69.
Note that this reading is an excerpt from a larger article. Your assignment begins on page 60 under the heading "Death of an Agent," and ends on page 69 before Annie Leibovitz's article about showgirls in Vegas.
Here is a quick overview of the main points of McPhee's article: Witnesses saw Enrique Camarena kidnapped in broad daylight in Guadalajara (red dot on the map of Mexico, below). His body was later "found" by the MFJP at the Bravo family ranch in Michoacán (yellow dot on the map below). However, the soil from his body didn't match the soil at the Bravo ranch, it matched the soil in Bosques la Primavera state park in Jalisco (green dot on the map below).
The story hinges upon not just being able to differentiate between the two soils, but also upon being able to locate the real source of the soil on Camarena's body. In order to appreciate how the FBI geologists were able to do this, we will step back and discuss some basic points regarding mineralogic classifications and we will think about this in the wider framework of the mineralogic products of volcanoes. Knowing why certain volcanoes produce certain types of mineral assemblages comes from putting together a tectonic history of the volcanoes of interest, so we'll do this, too.
A mineral is a homogeneous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice.
Wow, that's kind of a mouthful, but each part of the definition is easy to explain.
It is a common urban myth that glass acts like a liquid over long timescales. This myth, mostly perpetuated by tour guides in cities with old windows, is "proven" by pointing out to onlookers that the window glass in old buildings is thicker at the bottom. This is used as "evidence" that the glass flows downward over time due to gravity and has piled up at the bottom of the pane. Actually, the fact that old windows are thicker at the bottom is an artifact of an old method of glass-blowing, called the "crown glass procedure." In this method, still-molten glass was spun on a disc to flatten it as it cooled. This had the effect of making the disc fatter at the edges. Then the panes of glass were cut with an asymmetrical thickness built in to them. These panes were then usually installed with the thick end down because that is more stable. For a more detailed analysis of this topic, check out Plumb, 1989. "Antique windowpanes and the flow of supercooled liquids." J. Chem. Educ., 66, pp. 994-996.
In order to discuss the the classification of various types of minerals, let's take a quick look at the Periodic Table of the Elements and review some simple atomic chemistry. All elements in the periodic table are made of one kind of atom, with a specific number of protons in its nucleus. The number of protons in the nucleus is the atomic number of the element. Below is an image of the periodic table. I assume this is familiar to you from high school and college!
Let's take the element carbon, for example. Its atomic number is 6. That means it has 6 protons in its nucleus and 6 electrons orbiting. It can have different numbers of neutrons in its nucleus (6,7,8 are common). Two atoms with the same number of protons, but different numbers of neutrons are isotopes. Common isotopes of carbon are carbon-12 and carbon-14, denoted like this: C12 and C14. The superscripts in the examples for carbon are the atomic weights of the isotopes. C12 has 6 protons and 6 neutrons in its nucleus, so its atomic weight is 12. C14 has 6 protons and 8 neutrons in its nucleus, so its atomic weight is 14. Two minerals that have the same chemical composition but different crystal lattice structures are called polymorphs. For example, graphite and diamond are both pure carbon, but the way the carbon atoms are arranged is completely different, giving rise to their very different chemical and physical properties (see images below). In graphite, the carbon atoms are arranged in sheets that are weakly bonded to each other. In a diamond, the lattice structure involves a much stronger bond framework.
Not all of the elements in the Periodic Table are particularly common in the Earth's crust. Most minerals are formed from different arrangements of just a handful of the most commonly occurring elements. By weight, the two most abundant elements in the crust are oxygen and silicon. The figure below shows the abundance of elements in the crust as parts-per-billion by weight plotted vs. the atomic number of the element. See that oxygen (atomic number = 8, shown by one of the yellow dots) and silicon (atomic number = 14, shown by one of the red dots) are the highest.
In order to identify the unique suite of minerals found in the soil on Camarena's body, we must first acquire some knowledge about the tectonic history of the area. Then we can narrow our scope to the actual volcano that produced the minerals in question.
Below is a map of the plate boundaries near the western coast of southern Mexico. See if you can apply what you've learned about plate boundaries and map-reading from previous lessons to make a mental picture of the interactions of all the plates and faults depicted on the map, then watch my screencast to hear me explain what I see when I look at this map.
The map below zooms in on a section of the first map (note that the rectangular dotted line corresponds to the rectangular box on the first map) and gives details about the volcanic history of this region. We can see from this map that this has been quite an active region volcanically. There are two distinct episodes of volcanism mapped. The shaded areas show basalt flows that have been dated between 9 to 11 million years old. On top of that is a younger (less than 7 million years old) deposit of rhyolitic lava. Each little dot is a separate volcanic vent.
The difference between a vent and a volcano is that a vent emits volcanic products (gas, lava, ash, etc.) but is not necessarily a mountain. Vents can be minor and often occur on the flanks of active volcanoes where lava and gas have found a different weak spot to escape from other than the main crater. Another feature on the map below is the number of collapsed calderas. These are shown as ring-shaped normal faults with the dip lines pointing inwards to show a circular depression. Collapsed calderas are formed when a large explosive volcanic eruption has blown away the magma chamber as well as the entire volcano, leaving behind a giant crater-shaped depression. Many of these calderas will still have active vents or resurgent volcanoes at their centers. Follow along with my screencast to hear me describe the details of the map.
As we were just discussing, collapsed calderas are formed when a large explosive volcanic eruption has blown away the magma chamber as well as the entire volcano, leaving behind a giant crater-shaped depression. Often, calderas are associated with rhyolitic lavas because rhyolite, with its high silica content, is viscous. See the chart below for a comparison of the viscosity of different lavas. Viscosity is a measure of resistance to flow. Lavas with high viscosity tend to form steep-sided volcanoes and often the lava cools right on top of the main vent, essentially plugging it up. Then, pressure builds up in the magma chamber as gases, ash, and lava want to escape but can't. When the pressure finally exceeds the strength of the plug and the eruption happens, it may be so violent that an extremely large volume of ground is blown to smithereens. Historically, large and destructive eruptions, such as Krakatoa and Tambora, happened this way. Note on the map above the sheer size of some of the calderas in the stipple-shaded region that marks the rhyolitic lava flows. La Primavera caldera, at the heart of our story, is shown as one of the many silicic vents in this region.
Temperature and composition both affect viscosity. The same substance at higher temperature will usually be less viscous. Think of an ordinary substance like candle wax. When you heat it up it gets more runny, but it is always wax, it has not changed what it is made out of. Two different mineralogies will normally have different viscosities depending on their silica content. The higher the silica content, the higher the viscosity. But note the actual numbers in the classification chart! Geologists talk about basalt as "low silica" but that's only in comparison to other lavas. In fact basalt is about 50% silica!
Below is a photo of the piney, mountainous state park in Jalisco described in "Death of an Agent."
The photo below shows an outcrop of rhyolite tuff. A tuff is deposited as an air-fall ash layer from an explosive eruption. Up close, you'd see that each grain is full of air bubbles that were trapped in the rock particles when the ash cooled as it flew through the air. The scale of this outcrop gives an indication of the massive eruption that produced this ash layer.
The FBI geologists in "Death of Agent" were able to prove that Camarena did not die at the Bravo family ranch because the soil on his body was not at all like the soil at the Bravo ranch. Both soils derived from the weathered products of volcanoes, but the volcanoes couldn't have been the same because the mineralogy was different. The soil from the Bravo ranch was described as containing a lot of obsidian (see image below). Obsidian is a volcanic glass. The obsidian from the Bravo ranch was "globular" meaning that it was probably eroded and rounded by water.
In contrast, the soil on Camarena's body was rhyolite (see image below). It is described as an ash that probably cooled as it flew through the air from an explosive eruption. This process creates small particle sizes and highly vesiculated (full of air bubbles) grains.
What is the mineralogic composition of rhyolite, and specifically this particular rhyolite? Rhyolite is analogous to its more commonly known cousin, granite. The difference is that granite is an intrusive igneous rock that cools underground without erupting, whereas rhyolite is an extrusive igneous rock that forms a lava and cools out of the ground after an eruption. Otherwise, mineralogically speaking, the two rocks have the same composition. Rhyolite has the highest silica (SiO2) content of the extrusive igneous rocks, but it also has lesser components of some other compounds such as aluminum, potassium, sodium, magnesium and iron oxides. See the chart below for the average breakdown of compounds in each of the four most common extrusive igneous rocks.
If the soil on Camarena's body had been just a run-of-the-mill rhyolite, his original burial spot would have been impossible to locate. Handily, there were a few diagnostic minerals that were part of the assemblage, and luckily these minerals had been studied and the results published by earlier field geologists. The other minerals found on Camarena’s body were cristobalite (clear and “opalized”), bixbyite, and rose quartz.
Cristobalite is a high-temperature polymorph of quartz often found in volcanic deposits. Remember from earlier in this lesson that polymorphs have the same chemical formula but different lattice structures. Below is a phase diagram of silica showing the different pressure and temperature regimes that produce its different polymorphs.
Rose quartz also has the chemical formula SiO2. This is the same formula as regular quartz and cristobalite. So why do they look different? Pure quartz is colorless. However, quartz often contains impurities. In the case of rose quartz, the impurities are titanium and iron. The impurities are at the parts-per-million or parts-per-billion level, so they do not get written in the chemical formula, even though their presence completely changes the look of the mineral!
Bixbyite is manganese iron oxide (Mn,Fe)2O3. It is not such a common mineral, but when it is found, it is usually found in rhyolite deposits.
We know from this case study where the original burial spot of Enrique Camarena was located because of the unique mineral assemblage of the soil adhering to his body. What do we know about this soil? It was rhyolite. This rhyolite has several characteristics that helped the FBI geologists narrow down its provenance. The crystals were tiny and full of air bubbles, which meant that they probably came from a volcano that had erupted explosively because the small crystals would have cooled as they flew through the air. This knowledge led them to look at the region near the subduction zone west of Guadalajara where all those collapsed calderas indicated past explosive eruptions. They knew they needed to look in a mountainous region because the grains were still sharp and comparatively unweathered. They also knew they needed to find a rhyolite that contained some interesting minor minerals in specific percentages: cristobalite, rose quartz, and bixbyite.
In this activity, I'd like you to create a forensic mineralogy lab or lesson. Make it short and simple (just one or two class periods in length). If you have big ideas for a longer, more involved project, that is fine—why not save that for the course capstone project (Lesson 8) when your assignment is to create a longer lesson?
I made a really simple lab for an undergrad course in which students looked through a low-powered microscope at three samples of sand. One was synthetic sand from a playground sandbox, one was pure quartz with a very narrow grain size ordered from the US Silica Company, and one came from a beach in North Carolina. The students had to figure out which sample was which based on their observations. I had them make some drawings of the grains, and then make educated guesses about the mineralogy with the right reference books at their disposal.
L5_forensicslab_AccessAccountID_LastName.doc (or your file extension).
For example, former Cardinals outfielder and hall of famer Stan "The Man" Musial would name his file "L5_forensicslab_sfm6_musial.doc"
Note on Grading: I am interested in the scientific accuracy of your exercise. I am not going to base my grade on whether you have constructed a lesson plan in some special way (as long as all the components listed above are there). My assumption is that those of you who are teachers already know how to write a lesson plan. For those who are not teachers, I am not going to instruct you on correct lesson-plan making here. However, I am a scientist, so if the facts are not right, or could use clarification, I can assist with that.
Ferrari, L., Pasquaré, G., Venegas-Salgado, S., Romero-Ríos, F. (2000). Geology of the western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco block. Geological Society of America, Special Paper, 334, pp. 65-84.
Ferrari, L., Rosas-Elguera, J. (2000). Late Miocene to Quaternary extension at the northern boundary of the Jalisco block, western Mexico: The Tepic-Zacoalco rift revised. Geological Society of America, Special Paper, 334, pp. 41-64.
Murray, Raymond C. (2004). Evidence from the Earth: Forensic geology and Criminal Investigation. Missoula, MT: Mountain Press Publishing Company, p. 226
Rossotti, A., Ferrari, L., López-Martínez, M., & Rosas -Elguera, J. (2002). Geology of the boundary between the Sierra Madre Occidental and the Trans-Mexican Volcanic Belt in the Guadalajara region, western Mexico. Revista Mexicana de Ciencias Geologicas, 19, pp. 1-15.
Do you have another reading or Web site about these topics that you have found useful? Share it in our Teaching/Learning Discussion!
I chose a case study to highlight the Neat-o Interdisciplinary Idea for this lesson: Forensics. I like introducing the subject of mineralogy this way because I think it is more exciting to see a real-life example of why you might want to know how to classify rocks and minerals rather than just memorizing a list of classifications, vocabulary, and chemical formulas.
While memorization does have its place and studying science does require learning some new vocabulary, putting these ideas into the context of a true story can make learning seem less burdensome and more like a discovery process--don't you agree?
You have finished Lesson 5. Double-check the list of requirements on the Lesson 5 Overview page to make sure you have completed all of the activities listed there before beginning the next lesson.
If you have anything you'd like to comment on, or add to, the lesson materials, feel free to post your thoughts to our next Teaching/Learning discussion. For example, are you grateful that you were not unlucky enough to stumble onto some traffickers while hiking in Mexico in the 1980s?
Links
[1] https://www.e-education.psu.edu/earth520/node/1688
[2] https://www.e-education.psu.edu/earth520/node/1689
[3] http://serc.carleton.edu/images/usingdata/nasaimages/periodic-table.gif
[4] http://mrsec.wisc.edu/Edetc/nanoquest/carbon/index.html
[5] http://www.webelements.com/webelements/scholar/index.html
[6] http://www.bosquelaprimavera.com
[7] http://en.wikipedia.org/wiki/Obsidian
[8] http://en.wikipedia.org/wiki/Rhyolite
[9] http://www.quartzpage.de/cr/silica_phase_diagram.png
[10] http://www.galleries.com/Minerals/Silicate/CRISTOBA/cristoba.jpg
[11] http://canesgroup.net/canes/html/gemstones/rose%20quartz.jpg
[12] http://www.earthscienceworld.org/images
[13] http://volcanoes.usgs.gov/
[14] http://www.scienceofsand.info