Question

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

Short answer

Silicate tetrahedrons share 0, 2, 2-3, 3, and 4 oxygen ions in isolated, single chains, double chains, sheets, and frameworks, respectively.

Step-by-step solution

Understanding a Silicate Tetrahedron

A silicate tetrahedron consists of one silicon ion ( Si^{4+} ) at the center, surrounded by four oxygen ions ( O^{2-} ) at the corners, arranged in a three-dimensional pyramid shape. The basic structural formula is SiO_4^{4-} .

Simple Tetrahedron Sketch

Draw a single tetrahedron by creating a triangular base with one oxygen ion at each corner and a fourth oxygen ion above or below the base, connected by lines to a central silicon ion.

Tetrahedral Arrangement - Isolated Tetrahedrons

In isolated tetrahedra, such as in olivine, each SiO_4^{4-} tetrahedron does not share any oxygen ions with adjacent tetrahedra.

Tetrahedral Arrangement - Single Chains

In single chain silicates, like pyroxenes, each tetrahedron shares two oxygen ions with adjacent tetrahedra, forming a continuous chain.

Tetrahedral Arrangement - Double Chains

In double chain silicates, like amphiboles, each tetrahedron shares two or three oxygen ions, alternating to form two parallel chains connected by shared oxygens.

Tetrahedral Arrangement - Sheets

In sheet silicates, such as micas, each tetrahedron shares three oxygen ions, creating one extensive sheet of tetrahedra.

Tetrahedral Arrangement - Frameworks

In framework silicates, like quartz and feldspar, each tetrahedron shares all four oxygen ions with adjacent tetrahedra, forming a three-dimensional network or framework.

Key concepts

Silicate Tetrahedron

The silicate tetrahedron is the fundamental building block of silicate minerals. It consists of one silicon ion, denoted as \( Si^{4+} \), centrally located and surrounded by four oxygen ions, each with a charge of \( O^{2-} \), at the corners. This configuration forms a three-dimensional, pyramid-like shape. The silicate tetrahedron's basic structural formula is \( SiO_4^{4-} \). This means the total charge is -4, a crucial detail because it determines how these tetrahedrons can bond and form more complex structures.
The way the ions are arranged allows the tetrahedron to interact with other tetrahedrons to build a diverse range of mineral structures. The arrangement forms the foundation of how various silicate minerals are categorized. Understanding the basic silicate tetrahedron helps in grasping more complex structures found in silicate minerals.

Tetrahedral Arrangements

Silicate minerals can adopt various tetrahedral arrangements, leading to different mineral properties and classifications. These arrangements determine the mineral's shape, bonding, and stability. Here’s a brief overview of the main types:

• Isolated Tetrahedra: Each tetrahedron stands alone and does not share oxygen ions with others. This occurs in minerals like olivine, where each \( SiO_4^{4-} \) tetrahedron remains separated.
• Single Chains: Tetrahedrons are linked in a linear chain, sharing two oxygen ions with adjacent tetrahedra. This is seen in pyroxene minerals.
• Double Chains: Two single chains are linked together, with some tetrahedra sharing two or three oxygen ions. Amphiboles are an example.
• Sheets: Formed when each tetrahedron shares three oxygen ions, creating extensive sheets of connected tetrahedra. Micas are a common type.
• Frameworks: All four oxygen ions in each tetrahedron are shared, resulting in a robust, interconnected network. This is typical in minerals like quartz and feldspar.

Oxygen Ion Sharing

Oxygen ion sharing is a vital aspect that defines the structure of silicate minerals. It influences the physical properties of the mineral. In silicate tetrahedra, sharing of oxygen ions between adjacent tetrahedrons leads to different structural formations.

• Isolated Tetrahedra: No oxygen ions are shared, which results in minerals with a simple, undeveloped structure.
• Single Chains: Each tetrahedron shares two oxygen ions, creating a chain-like structure. This increases the stability of the mineral.
• Double Chains: Tetrahedra share two or three oxygens, leading to a more complicated and stable chain.
• Sheets: Three oxygen ions are shared, forming a flat, planar arrangement like in clay minerals.
• Frameworks: Sharing all four oxygen ions among adjacent tetrahedra results in an intricate 3D network. This maximizes the stability and hardness, seen in minerals like quartz.

Rock-Forming Minerals

Rock-forming minerals are those that make up the majority of the Earth's crust. Silicate minerals are the most abundant among them, owing to the prevalence of silicon and oxygen in the Earth's crust. The silicate tetrahedron's ability to form extensive networks makes these minerals essential to the structure of rocks.
Silicate minerals can be categorized based on their tetrahedral arrangements. For instance, olivine is part of the isolated tetrahedra group and is a significant component of the Earth's upper mantle. Pyroxenes and amphiboles, corresponding to single and double chains respectively, are widely spread in both igneous and metamorphic rocks. Micas and clays, forming sheet-like structures, are essential in forming sedimentary rocks. Lastly, minerals like quartz and feldspar create framework silicates that dominate continental crust materials.

Three-Dimensional Structures

Three-dimensional structures in silicate minerals are a result of the interconnected nature of silica tetrahedra. The sharing of oxygen ions allows for complex 3D networks that define mineral shapes and properties. Framework silicates, like quartz and feldspar, exemplify the most complex of these structures. Each tetrahedron in a framework shares all four of its oxygen ions with neighboring tetrahedra, creating a durable and stable network. This network is not only strong but also significantly affects the mineral's physical characteristics, such as hardness and durability. The 3D arrangement allows these minerals to withstand significant environmental stress, making them crucial to the geological makeup of the Earth. Understanding these structures aids in comprehending the diversity and resilience of rock-forming minerals.