Modeling Silicates and the Chemistry of Earth's Crust

In this activity, students will use marshmallows to model the formation of silicates – the family of minerals that make up more than 90% of Earth’s crust – then analyze their chemical and physical structures.

• Consider having students construct their silicates on a sheet of paper or a paper towel instead of on their desks and tables.
• Remind students of the expectation that their materials are not to be consumed unless it is safe to do so at the end of the activity.

Igneous rocks, which form from magma, have diverse characteristics that provide geologists with clues about the environment in which these rocks formed. We can gather these clues, such as a rock’s chemical composition or crystal structure, and use Bowen’s reaction series to also understand the conditions under which they formed.

By now, students have learned that our Earth is composed of a solid inner core, a liquid outer core, a mostly solid mantle, and a rigid crust that floats on the mantle and moves around over geological time due to the activity of tectonic plates. In the mantle, magma, a heated molten rock, is formed and rises through the process of convection. The transfer of heat to cooler rocks near the mantle-crust border, combined with changes in pressure, can cause cooler rocks to be assimilated into the magma being forced upward. This process can produce a wide range of igneous rocks as young material breaches the surface to form new rock structures.

Magmas (and the igneous rocks they form) are generally made up of the following eight elements, in order of decreasing abundance: oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, and potassium. Those that are relatively higher in magnesium (Mg) and iron (Fe) are called mafic (e.g., gabbro or basalt), while those that are relatively poor in magnesium and iron are called felsic (e.g., granite or rhyolite).

Magma's composition is ultimately determined by where it forms, including:

• Mid-ocean ridges, such as the Mid-Atlantic Ridg­e, where oceanic crust spreads, allowing for lava to move through thin sections of crust.
• Mantle plumes­, such as the Hawaiian islands and Yellowstone National Park, ­where localized hot spots in the mantle bring heat toward the crust through thick crust regions.
• Subduction zones, like the Cascadia subduction zone, where dense oceanic crust is forced below less dense continental crust, forcing surface rock deep into the earth to be melted.

Silicon has a charge of +4, while oxygen carries a charge of -2. That means that a single silicate tetrahedron (SiO4-4) requires two iron cations (Fe+2) to become stable.

Credit: NASA/JPL-Caltech

The scarcity of iron forces silicates to form more and more complex frameworks to help share the charge.

Credit: NASA/JPL-Caltech

As a result, the type of magma found at a given location provides clues as to the tectonic borders or tectonic movement taking place. Generally, magma formed at ridges travels a shorter distance, and is much more iron rich (mafic), while rising magma that travels through thicker continental crust, such as at subduction zones, usually consumes its iron first, leaving a more silicate-rich material by the time it reaches the surface (felsic).

To understand the conditions that result in different types of magma, we turn to chemistry. As magma cools, it crystallizes into networks of iron, silica, and oxygen in arrangements that reflect the temperature and rate at which it cooled as well as other elements present in the process. As you may remember from your chemistry class, silicon has a charge of +4, while oxygen carries a charge of -2. That means that different arrangements of silicon and oxygen will result in different charges, and will require other positively charged metals, or cations, to balance the charge. When iron-rich magma flows up from the mantle, a single silicate tetrahedron (SiO₄^-4) requires two iron cations (Fe⁺²) to become stable.

As the magma continues to cool, iron’s high melting point causes it to crystalize earlier than other elements. The scarcity of iron forces the silicates to form more and more complex frameworks to help share the charge.

To determine how silicates and positively charged ions will arrange, we turn to Bowen’s reaction series. Bowen's reaction series defines the process by which silicate structures form as a result of crystallization as they cool from iron-rich magma – where cations are plentiful, forming minerals such as Olivine – through to when all the iron is consumed, or crystalized, forming Quartz, which needs no iron at all.

Geologists use this process of analyzing the properties of rocks not just here on Earth, but also on the Moon and Mars to provide us a snapshot of the conditions that lead to their formation. Heavily cratered areas of our Moon, for example, are rich in basaltic material that is believed to have reached the surface from upwelling magma flows. Conversely, the lunar highlands contain more felsic-type materials, indicating that they were created late in the crystallization process, forming the exterior of the Moon during its cooling process.

Cleavage plains can be seen in this side view of a sample of muscovite.

Credit: NASA/JPL-Caltech

Cleavage plains can be seen in this side view of a sample of biotite.

Credit: NASA/JPL-Caltech

• What can be determined about the ratio of silicon, oxygen, and iron as you move from a single tetrahedral to more complex networks? Based on what you know about Bowen's reaction series, what does this suggest about where iron is located at various depths inside Earth? Where would you expect to see each type of structure at different depths, from Earth's core to its surface?
• Looking at the structures you formed, which would you expect to create cleavage planes, and why?
• Not all forms of iron or metal need a +2 charge. What would you expect to result from the crystallization of silicates in the presence of other common cations, such as sodium (Na+1) or Aluminum (Al+3)?
• Based on their compositions, which minerals or mineral groups would you not expect to find together? Possible answer: Quartz and olivine.

• Students will be able to correctly represent the silicate structures created using their marshmallows and toothpicks on their guided notes.
• Students should be able to use knowledge of ions and charges to determine how many iron cations are needed to balance the charge.
• The most challenging component will be having students determine, likely by performing an internet search, the type of silicate structure created and representative rock structures to capture in their guided notes. Some suggested responses can be found on the teacher key.

• Have groups team up to make a three-dimensional network from their sheet silicates. Can they build in more directions than side to side? When the sheets are stacked, have students determine the final charge of each tetrahedron and what that suggests about where in Earth’s crust we would find this material?
• Have students research other tetrahedron configurations that can form in nature?

• Facts and Figures: Mars Geology
• Website: NASA Mars Exploration
• Website: NASA Moon
• Website: NASA Solar System Exploration