Problem 27 - Chemical Principles

Question

Describe the structures of a silicate in which the silicate tetrahedra share (a) one \(\mathrm{O}\) atom; (b) two \(\mathrm{O}\) atoms.

Step-by-Step Solution

Answer

In silicate structures, when tetrahedra share one oxygen atom, they form chains or rings; when they share two oxygen atoms, they form double chains or sheets.

Step 1: Identify the Silicate Tetrahedron

A silicate tetrahedron is the fundamental building block of silicate minerals, consisting of one silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron.

Step 2: Describe the Sharing of One Oxygen Atom

When silicate tetrahedra share one oxygen atom, they are linked together at a single point. This means the tetrahedra are not directly bonded to each other, but rather are connected through a common oxygen atom. This leads to the formation of chains or rings, depending on how the tetrahedra are configured.

Step 3: Illustrate the Sharing of Two Oxygen Atoms

When two oxygen atoms are shared, the silicate tetrahedra form double chains or sheets. In this structure, each tetrahedron is linked by two of its oxygen atoms to two neighboring tetrahedra. The sharing of two oxygen atoms allows for a more continuous and stable structure than when only one oxygen is shared.

Key Concepts

Silicate Minerals

Silicate minerals form the largest and most complex family of minerals that make up over 90% of the Earth's crust. Imagine the silicate minerals as the building blocks of rocks; they are composed of silicate tetrahedra, which are four-sided pyramids with silicon at the center and oxygen at each corner.

These tetrahedra link together in various arrangements to create the diversity of silicate minerals we see. Simple silicates contain single tetrahedra, while more complex silicates can have these tetrahedra sharing oxygen atoms, thus joining in chains, sheets, or three-dimensional frameworks. Each arrangement significantly alters the properties and characteristics of the resulting mineral, from its hardness and cleavage to its melting point. For example, the mineral olivine consists of isolated tetrahedra, whereas minerals like quartz have a highly interconnected three-dimensional framework.

Oxygen Sharing in Silicates

The concept of oxygen sharing in silicates is crucial to understanding the complexity of these minerals. Each oxygen atom in a silicate tetrahedron can be linked to another silicon atom, thus creating a bond between two tetrahedra. When one oxygen atom is shared, it forms the simplest connection known as a single chain or single ring, depending on the orientation of the tetrahedra.

Miners and geologists find these patterns in minerals like pyroxenes, which exhibit a single chain structure. When two oxygens are shared, it allows for stronger and more complex connections, leading to the formation of double chains, as seen in amphiboles, or expansive sheets, like those found in micas. These sheets can stack on top of each other, held together by weak van der Waals forces, allowing for the characteristic flaky texture of minerals such as biotite.

Chemical Bonding in Silicates

In silicate minerals, chemical bonding is responsible for the silicate tetrahedra's ability to form different structures. The silicon-oxygen bond in a tetrahedron is very strong due to the high electronegativity of oxygen and the small size of silicon. This results in a stable tetrahedral structure.

However, beyond the individual tetrahedra, these structures are connected together by weaker bonds, which can be ionic or covalent, depending on the presence of metallic cations or additional shared oxygen atoms. For example, when tetrahedra share oxygen atoms, partial covalent bonding occurs, creating the diverse silicate structures we observe. The presence and type of chemical bonding influence the physical properties such as hardness, cleavage, and melting point of the silicate minerals. Additionally, the types of cations present (such as magnesium or iron) can further alter the characteristics of the mineral, leading to an extensive variety of silicate minerals found in nature.