How Do Silica Tetrahedra Work? – A #UREES101 #GoodQuestion

Most common rock-forming minerals on Earth belong to a group of minerals called silicates. Silicates are distinguished from other minerals by the silica tetrahedron (sometimes called the silicate tetrahedron), a structural unit composed of one silicon atom surrounded by four oxygen atoms that bond directly to the silicon. This gives it the chemical formula of SiO4.

The silica tetrahedron is a four-sided pyramid-like structure, where the faces of the pyramid are all equilateral triangles and the corners (or vertices) are where the oxygen atoms are. The silicon atom is in the very center of the tetrahedron.

The silica tetrahedron, as a molecular diagram and as a solid. CREDIT: Hbf878 Public Domain

The silica tetrahedron looks a little different when the individual sizes of the atoms are considered.

A space-filling atomic model of the silica tetrahedron.
CREDIT: Helgi CC By-SA 3.0

A space-filling atomic model of the silica tetrahedron, with the atoms labeled.
CREDIT: Helgi CC By-SA 3.0

The question came up in class today: How does a silica tetrahedron thing bond? How does it work?

Sadly, I had no answer. In the nearly 30 years I’ve been studying geology, it never occurred to me to ask that simple question. How – thermodynamically, chemically, physically – is it possible for a silica tetrahedron to exist. It’s always just simply been. The tetrahedron is. Just like air. It just is.

Well, that’s about as satisfying of an answer as “because I told you so,” or “because it’s always been that way.” Useless.

So, I looked for answers.

Those of us with a chemistry background agreed that at a first pass, since silicon lies just below carbon on the periodic table, it will behave in roughly the same way. Carbon is able to form four covalent bonds at once (Methane, CH4 being the simplest example of this) which results in the tetrahedral shape. Methane is tetrahedral, with a carbon atom in the middle and hydrogen atoms on the four corners.

Methane shown three different ways. Upper left: molecular sketch; Upper right: stick drawing; Bottom: space-filling model. Blue is carbon, white is hydrogen.
CREDIT Effeietsanders CC By 2.5 nl

The tetrahedral shape works great for methane, because each hydrogen atom “wants” another electron, and the carbon atom “wants” four more electrons. By sharing electrons (covalent bonding), the carbon and the hydrogen are “happy” and methane is a stable molecule.

Covalently bonded hydrogen and carbon in a molecule of methane.
CREDIT DynaBlast CC By-SA 2.5

Does this work with the silicate tetrahedron? No. Not quite. Like carbon, the silicon in the center of the silica tetrahedron “wants” four more electrons. However, the oxygens (unlike hydrogen in methane), each “want” two electrons.

The result is that the silica tetrahedron (SiO4) has a strong negative charge and should properly be written SiO44-. This little detail is often glossed over when silicates are introduced in introductory classes (just like mine, oops). But it’s because of this charge that silicates come in so many varieties and forms.

Silica tetrahedra may remain independent in a mineral (as in the nesosilicates) or they may bond to each other in pairs (sorosilicates), rings (cyclosilicates), chains (inosilicates), sheets (phyllosilicates), or as a complex three-dimensional network (tectosilicates). When the tetrahedra bond to one another the charge is then reduced. The remaining charge (or the entire 4- charge, in the case of nesosilicates) is taken up with anions (atoms with positive charges) such as magnesium, iron, potassium, calcium, and aluminum.

The details of how the various silicates form etc. would be a different blog post. But I hope that this one at least satisfies our collective curiosity about how the silica tetrahedron can even be a thing.

Long Distance Prospecting – #365papers – 2018 – 61

Conroy, Emerson, Anemone, and Townsend, 2012, Let your fingers do the walking: A simple spectral signature model for “remote” fossil prospecting: Journal of Human Evolution, v. 63, p. 79-84.

What’s it about?

The authors demonstrate the utility of satellite imagery combined with surface observations and GIS software to make predictions about where fossil localities may be located. Continue reading

Bringing Up Baby (Mountains) in Western North America – #365papers – 2018 – 59

Yonkee and Weil, 2015, Tectonic evolution of the Sevier and Laramide belts within the North American Cordillera orogenic system: Earth-Science Reviews: v. 150, p. 531-593

What’s it about?

This paper is a wonderful, yet highly technical, summary of the tectonic events leading to the Rocky Mountains as we know them today. Continue reading

Migrating Marsupials of the Pleistocene – #365papers – 2018 – 44

Price, Ferguson, Webb, Feng, Higgins, Nguyen, Zhao, Joannes-Boyau, and Louys, 2017, Seasonal migration of marsupial megafauna in Pleistocene Sahul (Australia-New Guinea): Proceedings of the Royal Society B, v. 284: 20170785

What’s it about?

Seasonal migrations are seen in many large mammals. In modern animals, however, such migrations are not observed in marsupials. The authors put together geochemical data from rocks and fossil to show that the massive wombat-like extinct marsupial Diprotodon migrated seasonally as far as 100 km each way. Continue reading

Did the Chixulub Impact Make the Oceans Erupt More? – #365papers – 2018 – 43

Byrnes and Karlstrom, 2018, Anomalous K-Pg-aged seafloor attributed to impact-induced mid-ocean ridge magmatism: Science Advances, v. 4: eaao2994

What’s it about?

The Chixulub Impact is the event linked to the extinction of the dinosaurs. The authors here show that at the same time as the impact, ocean floor spreading increased for just a little while. They hypothesize that the seismic waves caused by the impact resulted in the mobilization of molten rock, leading to this increase in volcanic activity. Continue reading

Using Glass to Estimate Altitude – #365papers – 2018 – 37

Dettinger and Quade, 2015, Testing the analytical protocols and calibration of volcanic glass for the reconstruction of hydrogen isotopes in paleoprecipitation, in DeCelles, Ducea, Carrapa, and Kapp, eds., Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir 212, p. 261-276.

What’s it about?

Isotopes of oxygen and hydrogen from water can give us insights into the altitude at which that water fell to the ground as rain. Some of this water can become incorporated into volcanic glass (in ash), preserving the isotopic values of the original water. Continue reading

Did Bolide Bombardment Kill Life on Earth More Than Once? – #365papers – 2018 – 36

Grimm and Marchi, 2018, Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability: Earth and Planetary Science Letters, v. 485, p. 1-8.

What’s it about?

The first billion years or so of Earth’s existence was marked by repeated bombardment of the planet by various asteroids, and even planetessimals. It is thought that this bombardment would superheat the Earth’s surface and kill any life that may have started to develop there. This study shows that, while the heating was extreme, there were still places that were protected from life-killing heat. Continue reading

Fossil Mammals and the Rocks that Contain them at Fossil Butte, Wyoming – #UREES270 – 2018

Gunnell, Zonneveld, and Bartels, 2016, Stratigraphy, mammalian paleontology, paleoecology, and age correlation of the Wasatch Formation, Fossil Butte National Monument, Wyoming: Journal of Paleontology, v. 90, p. 981-1011

What’s it about?

This paper contains a discussion of the mammalian paleontology at Fossil Butte National Monument, Wyoming. The authors examined and identified at least 46 species of fossil mammals from 29 localities within rocks of the Wasatch Formation at Fossil Butte. Using techniques of stratigraphy, the authors correlated all the localities in order from oldest to youngest. Further, they used the species present and clues from the rocks themselves to interpret the ancient environment in which the mammals lived. Continue reading