The Silica Tetrahedron

The molecular structure of silica tetrahedron (SiO₄) is the basic building block of the silicate minerals, which make up the vast majority of the minerals in the Earth’s crust.

The silica tetrahedron is a combination of one silicon and four oxygen atoms that form a four-sided pyramid shape, with the silicon atom in the center and an oxygen at each corner of the pyramid.

This low-tech "marshmallow model"shows the basic design of the silicon tetrahedron.

In a more explicit model of a silica tetrahedron (see below), the element silicon is a cation with a charge of +4. Each oxygen atom is an anion, with a -2 charge; thus four oxygen atoms make a -8 charge. The combination of +4 and -8 makes -4. The silicon tetrahedron molecule has therefore has a net -4 charge, which means it readily combines with other elements besides oxygen in order to balance out the additional -4 negative charge and form a net-zero charge (i.e., nature abhors a molecule with a net negative charge; just like nature “abhors a vacuum”).

There are six main configurations (e.g., rings, chains, frameworks etc.) of linkage among silicon tetrahedra. There are many internet sites that show beautiful reconstructions of these configurations, some of which are quite complicated. One of the most useful sites is: <openeduationalberta.ca/practicalgeology/chapter 3>  You will have to access it yourself because Google no longer allows external links within blogs.

The mineral quartz (SiO₂), which is the most abundant mineral in the Earth’s crust is an  example of the framework configuration known as the tectosilicates. In quartz, in order to neutralize the ionic charge difference, the four oxygen atoms of the silica tetrahedron are “shared” by adjacent silica tetrahedra. Each corner of the pyramidal tetrahedron is bonded to another tetrahedra (with an oxygen shared at each corner of each). As a result, the ratio of silica to oxygen atoms is 1:2, and the atomic charge is neutral (zero). 

A cluster of clear quartz crystals (6.5 cm wide, 6 cm high).

A single crystal of clear quartz (2.4 cm wide, 5.7 cm high).

One of the simplest silicate minerals is olivine, which is made up a single tetrahedron bonded to divalent iron (+2) and/or magnesium (+2), thereby creating Fe₂SiO4 or Mg₂SiO4, or some combination of the two (Fe, Mg)₂SiO4. The combination can occur because both iron and magnesium are divalent (+2) [thus they balance the charge of the silica tetrahedron] and their atomic sizes are similar; thus they can substitute for one another.

The above image is a cluster of tiny olivine crystals (hand specimen 3.6 cm wide, 2.7 cm high).

Terminology Hints

The words silicon and silica, etc., get used a lot in our modern vocabulary. The following list might be of some help to you in understanding these similar terms:

silicon (Si) = the 14^th element on the “Periodic Table of The Elements.”

silica = a solid material made out of SiO₂ (but not necessarily a mineral; e.g., opal---see my previous post on chalcedony).

silica tetrahedron = a combination of one silicon atom and four oxygen atoms that form a tetrahedron (= a four-sided pyramidal shape).

silicate = a mineral that contains a silica tetrahedron or many silica tetrahedra. 

silicone = a flexible synthethic material made up of Si-O chains with added organic molecules.