About Penny

Scientist (Paleontology, Geochemistry, Geology); Writer (Speculative and Science Fiction, plus technical and non-technical Science); Mom to great boy on the Autism spectrum; possessor of too many hobbies.

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.

Lost with the National Museum of Brazil – Kamehameha’s Cloak – Luto #MuseuNacional

On September 2, 2018, the National Museum of Brazil (Museu Nacional) was gutted by fire. 2018 was the Museu Nacional’s 200th year.

This post is one of series in which I discuss an important specimens that may have been lost to science in the blaze.

In 1824, King Kamehameha II Liholiho of Hawaii gave Brazil’s emperor Dom Pedro, a feathered cloak (‘ahu ‘ula). Such cloaks were status symbols for the highest ranks of the ruling class of Hawaii.

Feathered cloaks are made from a woven netting that is decorated with brightly colored feathers. Though there remains over 100 examples of these cloaks worldwide, this particular cloak was lost to the fire at the Museu Nacional.

Read more and see pictures here:

Royal Hawaiian Feather Cloak Feared Lost in Brazil Museum Fire

Paleocene Mammals from Brazil – #365papers – 2018 – 67

de Paula Couto, 1952, Fossil Mammals from the Beginning of the Cenozoic in Brazil, Condylarthra, Litopterna, Xenungulata, and Astrapotheres: Bulletin of the American Museum of Natural History, v. 99, 355-394.

What’s it about?

This paper is a listing, with descriptions, of most of the Paleocene mammals of Brazil at the time of its writing (1952). Several new species, genera, families, and even orders are named, in many cases by the author, Carlos de Paula Couto. Continue reading

Why is Irritator so Irritating? – #365papers – 2018 – 66

Martill, Cruickshank, Frey, Small, and Clarke, 1996, A new crested maniraptoran dinosaur from the Santana Formation (Lower Cretaceous) of Brazil: Journal of the Geological Society, London, v. 153, p. 5-8.

What’s it about?

This paper is the description of a new species (and Genus and Family) of dinosaur, grouped with the tetanuran theropods, a subgroup of carnivorous dinosaurs along the lineage leading to modern birds.

The type specimen was collected from the Santana Formation in Brazil, the same unit that gave us Santanaraptor. Continue reading

A Lost Titan – #365papers – 2018 – 65

Kellner and Azevedo, 1999, A new sauropod dinosaur (Titanosauria) the the Late Cretaceous of Brazil, in Tomida, Rich, and Vickers-Rich, eds., Proceedings of the Second Gondwanan Dinosaur Symposium: National Science Museum Monographs, no. 15, p. 111-142.

What’s it about?

This paper is a detailed description of Gondwanatitan foustoi, a new species of titanosaur (a sauropod) from Brazil. This new species is based upon specimen number MN 4111-V at the Museu Nacional. The material includes several vertebrae, part of a shoulder blade, parts of the hips, upper arm and lower leg bones, and some ribs. Continue reading