Question

In \(\mathrm{SiO}_{4}^{4-}\), the tetrahedral molecule, two oxygen atoms are shared in (a) sheet silicates (b) double-chain silicates (c) chain silicates (d) three-dimensional silicates.

Short answer

In \textrm{SiO}_4^{4-}, the tetrahedral molecule with two oxygen atoms shared forms (a) sheet silicates.

Step-by-step solution

Understanding The Structure of Silicates

Silicates are minerals composed of silicon and oxygen atoms. The basic building block of all silicate minerals is the silica tetrahedron, one silicon atom bonded to four oxygen atoms. In different silicate minerals, these tetrahedra are arranged and bonded together in various ways, including isolated tetrahedra, rings, single chains, double chains, sheets, and three-dimensional frameworks.

Identify the Arrangement with Two Shared Oxygen Atoms

To find the type of silicate mineral where two oxygen atoms are shared, we look at the structural arrangement of the silica tetrahedra. Shared oxygen atoms mean that some oxygen atoms are bonded to more than one silicon atom. Single-chain silicates share two oxygen atoms to form a chain structure, double-chain silicates share two or three oxygen atoms to form a double-chain structure, sheet silicates share three oxygen atoms resulting in a two-dimensional sheet-like structure, and three-dimensional silicates share all four oxygen atoms to create a three-dimensional framework.

Match the Description to the Correct Option

According to the given descriptions and since the exercise specifies that two oxygen atoms are shared, the arrangement that best fits this description is the sheet silicates as they possess a structure where each tetrahedron is sharing three of its oxygen atoms with adjacent tetrahedra, effectively forming a two-dimensional sheet.

Key concepts

Silica Tetrahedron

The silica tetrahedron is the cornerstone of silicate minerals, a group that makes up nearly 90% of the Earth's crust. A silica tetrahedron consists of a single silicon atom surrounded by four oxygen atoms positioned at the corners of a tetrahedron—hence the name. This structure is represented chemically as \( \mathrm{SiO}_{4}^{4-} \).

The tetrahedron has a negative four charge due to the four additional electrons provided by the oxygen atoms. This charge plays a crucial role in how silica tetrahedra bond with other elements and within themselves to form various silicate minerals. Understanding the configuration and the charge of the silica tetrahedron is foundational to grasping the diversity of silicate mineral structures encountered in geology.

Each oxygen atom in the tetrahedron has the ability to form additional bonds to other silicon atoms, leading to complex structures that are the basis for the structural variety of silicate minerals. The magic of the silica tetrahedron lies in its simplicity, yet its capacity to create intricate and vast networks is unparalleled in the mineral world.

Structural Arrangement of Silicates

Silicate minerals differ from each other primarily in the way their silica tetrahedra are arranged and bonded. The structural diversity arises from the number of oxygen atoms each tetrahedron shares with its neighbors. When no oxygens are shared, the structure forms isolated tetrahedra; when two oxygens are shared, the typical structures include single chains or rings; with three shared, the minerals form sheet silicates, and when all four oxygens are shared, the silicates exhibit a three-dimensional framework.

For instance, in sheet silicates, often referred to as phyllosilicates, each silica tetrahedron shares three of its oxygen atoms with adjacent tetrahedra to create a two-dimensional sheet-like configuration. This leads to minerals such as micas and clays, which are known for their unique cleavage properties. The ability of oxygen atoms to be shared between multiple silicon atoms explains why the silicate minerals are so varied and abundant. By recognizing the pattern of oxygen sharing, one can often predict the properties and stability of the silicate minerals under different environmental conditions.

Chemical Bonding in Silicates

Chemical bonding within silicates determines their structure, stability, and many of their physical properties. The tetrahedral structure of the silica tetrahedron, along with its negative charge, is essential because it dictates how silicates will bond with other elements like aluminum, magnesium, and iron.

In silicates, the bonding between silica tetrahedra can lead to the formation of various structures, each with its own characteristics and stability. For example, in sheet silicates, the shared oxygens create strong bonds in two dimensions, which gives these minerals excellent cleavage in one direction, such as seen in micas. In contrast, the three-dimensional frameworks, such as those found in quartz, involve tetrahedra sharing all four oxygen atoms, resulting in a rigid and durable mineral structure.

The cations that balance the negative charge of the tetrahedral network can also affect the properties of the mineral. For instance, the presence of potassium or sodium can lead to different types of feldspars, which are essential in determining the mineral's hardness and weathering behavior. The vast array of silicate structures and their bonding intricacies make them a fascinating subject for study in mineralogy and geology.