Download Crystallography in the Classroom—Modeling

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
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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
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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.
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