Question

Draw a three-dimensional view of a single silicate tetrahedron. Draw the five arrangements of tetrahedra found in the rock-forming silicate minerals. How many oxygen ions are shared between adjacent tetrahedra in each of the five configurations?

Short answer

Isolated: 0, Single Chain: 2, Double Chain: 2-3, Sheets: 3, Framework: 4.

Step-by-step solution

Understanding the Silicate Tetrahedron

A silicate tetrahedron consists of a central silicon ion (Si) surrounded by four oxygen ions (O) positioned at the corners of a tetrahedron. This basic structure is the building block of silicate minerals.

Isolating Tetrahedra

In isolated tetrahedra, each tetrahedron stands alone without sharing any oxygen ions with neighboring tetrahedra. Therefore, the number of shared oxygen ions is 0.

Pairing in Single Chains

In single-chain structures, tetrahedra link together in a line, sharing two oxygen ions with the adjacent tetrahedra, forming a continuous chain.

Links in Double Chains

Double chains consist of two parallel chains of tetrahedra that share oxygen ions at the middle tetrahedra junction. Each tetrahedron shares an average of 2 or 3 oxygen ions.

Sheet Silicates Formation

In sheet silicates, tetrahedra arrange in two-dimensional layers, with each tetrahedron sharing three oxygen ions with adjacent tetrahedra, creating extensive sheets.

Network of Framework Silicates

In framework silicates, each tetrahedron shares all four of its oxygen ions with neighboring tetrahedra, forming a three-dimensional network.

Key concepts

Silicate Tetrahedron

At the heart of silicate minerals is the silicate tetrahedron, which forms the foundational structure. Imagine a pyramid-like shape where a single silicon ion is nestled at the center. Surrounding it are four oxygen ions placed symmetrically at the corners. This pyramid is what we refer to as a tetrahedron. The chemical formula representing this basic unit is usually \[\text{SiO}_4^{4-}\]. Each vertex of the tetrahedron corresponds to one oxygen ion, thereby giving the structure a degree of symmetry and stability. This tetrahedron is immensely significant as it forms the basic building block for all silicate minerals.

Rock-forming Minerals

Silicate minerals are a major component found in the Earth's crust, accounting for more than 90% of it. These are often called rock-forming minerals. They include familiar minerals like quartz, feldspar, and mica. The structure and formation of these minerals depend on how silicate tetrahedra connect and form extended structures. By varying the way these tetrahedra are arranged, a wide variety of minerals with different properties and appearances are created. Each unique configuration results in different types of minerals, each with its distinct characteristics.

Oxygen Ion Sharing

The sharing of oxygen ions between tetrahedra dramatically influences the type of silicate structure formed. In isolated tetrahedra, there is no sharing of oxygen ions, which means they remain separate. However, configurations change as tetrahedra begin to share oxygen ions:

• Single Chains: Two oxygens are shared.
• Double Chains: Two or three oxygens are shared.
• Sheet Silicates: Three oxygens are shared, forming extended sheets.
• Framework Silicates: All four oxygens are shared, creating a complex network.

Each of these shared arrangements leads to structures with unique traits and stability.

Three-dimensional Structures

The three-dimensional arrangement of tetrahedra defines the complexity and stability of the silicate minerals. The framework structure is the most intricate. In this network, each tetrahedron shares all four of its corners. This sharing forms a fully interconnected grid, yielding a solid and durable framework. This highly organized structure is found in minerals like quartz, which are incredibly tough and have high melting points. The variation in dimensionality—from isolated units to intricate frameworks—determines not just the stability but also the mineral's properties.

Mineral Configurations

The configurations of silicate minerals influence their physical properties, such as hardness and cleavage. These configurations include isolated tetrahedra, single chains, double chains, sheet formations, and three-dimensional frameworks.
Each configuration imparts a different set of physical characteristics, such as whether a mineral is prone to breaking along distinct planes (cleavage) or its overall durability. For example, minerals with sheet structures, such as mica, tend to cleave easily, while framework structured minerals like quartz are much more robust. Understanding these configurations provides insight into the mineral's uses and its behavior in geological processes.