Problem 22 - Chemistry A Molecular Approach

Question

How are the silica tetrahedrons linked for CaSiO \({ }_{3}\) ? Which class of silicates does this compound belong to?

Step-by-Step Solution

Answer

In CaSiO3, silica tetrahedrons are linked by corner-sharing. It belongs to the class of inosilicates.

Step 1: Identify the Basic Unit of the Silicate Structure

The basic structural unit of silicates is the silica tetrahedron, which consists of a silicon atom covalently bonded to four oxygen atoms. This forms a tetrahedral shape.

Step 2: Determine the Linkage of the Tetrahedrons

In calcium silicate (CaSiO3), each silica tetrahedron is linked to other tetrahedra by sharing oxygen atoms. All oxygens are shared with adjacent tetrahedra, meaning the linkage is by corner-sharing.

Step 3: Identify the Class of Silicates

Calcium silicate (CaSiO3) belongs to the class of inosilicates because its structure is composed of single chains of silica tetrahedra linked together.

Key Concepts

Silica Tetrahedron

The silica tetrahedron is the fundamental building block of silicate minerals, which are the most widespread minerals in the Earth's crust. Imagine a three-dimensional pyramid with a silicon atom at its center, bonded to four oxygen atoms located at the corners. This tetrahedral structure (
) is defined by the chemical formula SiO4^4-, with silicon being tetravalent and surrounded symmetrically by the negatively charged oxygen ions. Silica tetrahedra can link together in various ways, leading to diverse mineral structures. Understanding this geometric unit is crucial for grasping the complexity and variety of silicate minerals found in nature. (
) These tetrahedra can connect via their corners, edges, or faces, but the most common linkage in silicates, as in the case of CaSiO3, is the corner-sharing linkage.

Inosilicates

In the realm of silicate minerals, inosilicates or chain silicates are characterized by their structure of extended chains made up of interconnected silica tetrahedra. There are two main types: single chains and double chains. (
)In single-chain structures, each tetrahedron shares two of its oxygen atoms with adjacent tetrahedra, forming a repetitive SiO3 unit. The formula for a repeating single-chain segment is often written as SiO₃^2-, which reflects the two negative charges per silicon accounted for in shared oxygens. (
)Calcium silicate, CaSiO3, belongs to this group and exhibits a single-chain structure. The chains extend indefinitely in one direction, resembling a 'beaded necklace'. This linear configuration contributes to the distinct physical properties of inosilicates, such as their directional cleavage patterns and fibrous or columnar crystal habits, which are observed in minerals like pyroxenes and amphiboles.

Corner-Sharing Linkage

The corner-sharing linkage is a specific way in which silica tetrahedra can connect. In this mode of bonding, each tetrahedron shares one of its four oxygen atoms with an adjacent tetrahedron, linking the tetrahedra by their corners. The significance of corner-sharing lies in how it contributes to the stability and diversity of silicate mineral structures. (
)As tetrahedra share an oxygen atom, the resulting bond strength and the angle between tetrahedra maintain the overall integrity of the mineral structure. This type of linkage can lead to a variety of silicate configurations, from three-dimensional frameworks like in quartz and feldspars, to the single-chain structures found in inosilicates like the mineral wollastonite, represented by the formula CaSiO3. (
)It's important to note that in the corner-sharing linkages, the negative charge of the tetrahedra usually requires the presence of cations, such as calcium in CaSiO3, to balance out the structure, making the overall mineral neutral.