Issues in Earth Science
“Eww, There’s Some Geology in my Fiction!”
Issue 6, May 2016
Suggestions for Activities and Discussions to accompany a Reading of
In the Rough by T. S. Brothers
How do Minerals Form?
Learning to identify and classify minerals is a common exercise in 8th grade earth science classes. Students often test minerals for hardness, streak, and density, and examine the character of visible crystal faces. However, understanding minerals is about much more than being able to identify them and give them names. Science is about how things work and how we know they work that way.
Rather than simply encourage your students to think about the names of the minerals in the story, you might instead encourage them to consider how much of this story might be real science--how blue spinel actually forms--and how much is simply magic.
For example, you might encourage your students to do some online library research to identify key processes in the formation of blue spinel and compare these to the processes in the story. Are they alike or different? In what ways are they alike and different?
Depending on your students' prior knowledge, you might prompt them with questions. For example, What is the significance of pressure? What is the significance of the magma? What is the significance of the limestone? What is the significance of olivine? What is the significance of garnet?
Other questions that come to mind include: What is the source of Al for the spinel (not limestone or olivine!)? Why is the spinel blue as opposed to its more common colors?
The text below is written by T. S. Brothers in support of the story, giving insight into the research done for the story and possibly giving your students some ideas to consider.
In The Rough: Geologic Supplement
By T. S. Brothers
This story centers around the formation of blue spinel. Much of the specifics are drawn from the article "Blue Spinel of the Luc Yen District of Vietnam" by Chauviré et al., published in the Spring 2015 edition of Gems and Gemology. This article is freely accessible on the Gemlogical Institute of America’s website: http://www.gia.edu/gems-gemology/spring-2015-blue-spinel-luc-yen-district-vietnam. Though the term ‘spinel’ is often used by geologists to describe an entire group of minerals sharing a particular crystal structure, it refers in specific to the mineral spinel, MgAl2O4. In antiquity spinel was often confused with ruby or sapphire because of its similar hardness, durability and range of colors, and because the two often form in the same manner and are found in the same places. One notable example is the “Black Prince’s Ruby” in the British crown jewels, which is actually an enormous red spinel. In the modern era it has finally been recognized and valued as its own unique gemstone.
Gem-grade spinel is formed during the metamorphism of limestone into marble. Though limestone is largely composed of biogenic calcite, it is never entirely pure. Over time it can be mixed with evaporite minerals, organic material or continental sediments, and can also be partially converted to dolomite by the addition of magnesium to its crystal structure. These various components are generally mixed evenly within the limestone, and explain why limestone tends to be darker in color than pure calcite.
On its own, limestone is perfectly happy with these impurities. However, when it's subjected to metamorphic conditions, the components separate out of the mixture and begin to recrystallize. Just like how sea ice is composed of pure water, excluding the salt as it freezes, when the calcite crystals reform the impurities are also removed. Depending on what the initial limestone contained, the concentrated impurities can form into a wide range of minerals.
For example, high concentrations of iron tend to favor formation of non-gem minerals. High aluminum, on the other hand, is more favorable to ruby, sapphire, and spinel. While the major components tend to determine the type of gem formed, trace elements can have a huge impact upon coloration and quality. In lower concentrations, for instance, iron can cause gems to be murky or dark. Traces of chromium can create rich red rubies or spinel, while very finest blue spinel are colored by cobalt.
In the Luc Yen stones, the formation of spinel and their electric blue color was the result of starting with very little iron, but in the story Simone has to actively adjust the mineralogy of the rock to achieve these conditions. Luc Yen spinels do occur with olivine, but it is near-end-member forsterite containing very little iron.
Growing your Own Crystals:
Making crystals of your own can be fun. When crystals grow quickly, they tend to form patterns that do not emphasize crystals faces. Beautiful crystals require time for the atoms to arrange themselves in the lowest-energy patterns, which brings out the crystal faces. If you have a few months, you and your students can grow some crystals of sugar.
Warm a quart of water on the stove and dissolve as much sugar in it as you can (when the sugar dissolves it will not appear as white material in the water, but will be part of the water and clear).
Pour the sugar water carefully into a cleaned quart canning jar. Cover the jar loosely with plastic wrap and set in a location where it will not be bumped, jarred, or disturbed in any way.
Wait a few months for your crystals to grow! The picture below shows crystals of sugar that were grown in this way. Notice the interesting pattern of the crystal faces, like little roofs. This crystallographic form is called a sphenoid.
The picture below shows sugar crystals that were grown over many years as water slowly evaporated from a closed jar of maple syrup. You can see that the crystals had to grow inside the bottle since they are too big to get through the top.
Planning for Instruction:
The article below, from NSTA's middle school journal, Science Scope, explores how you might shift your instruction from 'classifying and identifying' to guiding students to make sense of why things are the way they are. The article is free, although you do have to register with NSTA to get the article.
The Teacher Resources for in the Rough are written by Russ and Mary Colson.
Photo Credits: Russ Colson
In the Rough by T. S. Brothers
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