Question

Which one of the following has a two dimensional layer structure? (a) Tremolite asbestos, $$ \mathrm{Ca}_{2} \mathrm{Mg}_{5}\left(\mathrm{Si}_{4} \mathrm{O}_{11}\right)(\mathrm{OH})_{2} $$ (b) Zircon, \(\mathrm{ZrSiO}_{4}\) (c) Silica, \(\mathrm{SiO}_{2}\) (d) Talc, \(\mathrm{Mg}_{3}(\mathrm{OH})_{2}\left(\mathrm{Si}_{4} \mathrm{O}_{10}\right)\)

Short answer

Talc has a two-dimensional layer structure.

Step-by-step solution

Identify the compounds

Let's list the given chemical compounds and their general functions: 1. Tremolite asbestos, which is a type of silicate mineral. 2. Zircon, often used as a gemstone. 3. Silica, commonly known in forms such as quartz. 4. Talc, known for its use in talcum powder and as a mineral.

Understand structure types

Before examining the compounds, note that a two-dimensional layer structure implies that the compound forms sheets or layers held together by weaker forces, allowing them to slide over each other. Common examples include graphite and certain silicate minerals.

Analyze potential 2D layer structures

For each compound: - **Tremolite asbestos**: It typically forms fibrous crystals, not layers. - **Zircon**: This is a crystalline mineral with a three-dimensional network structure. - **Silica (SiO2)**: Exists in various forms (quartz, cristobalite) with three-dimensional network structures. - **Talc**: Composed of magnesium silicate, known for a sheet-like structure allowing easy layer sliding.

Identify the compound with a layered structure

Reviewing the compounds, talc is recognized for its layered, sheet-like structure due to its silicate layers bonded weakly to each other. This allows it to have a soft, greasy feel, which is characteristic of substances with two-dimensional layer structures.

Conclusion

Talc, \(\mathrm{Mg}_{3}(\mathrm{OH})_{2}\left(\mathrm{Si}_{4} \mathrm{O}_{10}\right)\), has a two-dimensional layered structure. The magnesium silicate sheets in talc are linked to each other with weak forces, allowing them to slide, confirming its classification as having a layer structure.

Key concepts

Silicate Minerals

Silicate minerals are a large family of minerals composed mainly of silicon (Si) and oxygen (O), the two most abundant elements in the Earth's crust. These minerals are essential constituents of rocks and play a critical role in geology. The basic building block of silicate minerals is the silica tetrahedron, which is a pyramidally shaped structure consisting of four oxygen atoms surrounding a single silicon atom. This tetrahedron can link in various ways, forming a diversity of silicate structures.

• Nesosilicates: Isolated tetrahedra.
• Sorosilicates: Double tetrahedra.
• Cyclic silicates: Form rings.
• Inosilicates: Chain structures.
• Phyllosilicates: Sheet or layered structures.
• Tectosilicates: Framework structures.

Phyllosilicates, which include talc, are particularly noteworthy for their two-dimensional sheet-like character. This characteristic results from the way silica tetrahedra are linked, creating layers that can slide over one another easily.

Talc Structure

Talc is a silicate mineral, famous for its softness and greasy feel. It is commonly used in talcum powder but also plays a role in various industrial applications due to its distinctive physical and chemical properties. Talc's chemical formula is \(\mathrm{Mg}_{3}(\mathrm{OH})_{2}(\mathrm{Si}_{4}\mathrm{O}_{10})\).
The structure of talc is composed of magnesium \((\mathrm{Mg})\) and hydroxyl \((\mathrm{OH})\)groups sandwiched between sheets of silicate. The silicate layers consist of silica tetrahedra that share three of their oxygen atoms with adjacent tetrahedra, forming an extended two-dimensional network. The layers are held together by weak van der Waals forces, allowing them to slip past each other with ease.
This is why talc feels smooth and slippery to the touch. Such a structure also categorizes talc as a phyllosilicate, emphasizing its layered characteristics.

Crystal Structures

Crystal structures describe the orderly, repeating arrangement of atoms, molecules, or ions in a crystalline material. The type of arrangement and how the structural units are connected to each other define the physical properties of the mineral.
For example:

• 3D Framework: Like silica (\(\mathrm{SiO}_2\)) and zircon (\(\mathrm{ZrSiO}_4\)), these have a complex, interconnected network.
• Sheet or 2D Structures: Found in minerals like talc, characterized by layered arrangements where layers can slide easily.
• Fibrous Structures: Such as those in asbestos, showcasing long thread-like formations.

Understanding these structures help in classifying minerals and predicting their behavior in natural settings. For instance, the layered structure of talc explains its softness, making it distinct from harder, more rigid structures like those of quartz.

Layered Compounds

Layered compounds are materials with structural units arranged in layers, held together by relatively weak forces between them. These layers enable properties that can be leveraged in various applications, prominently visible in both natural minerals and synthetic materials.
Some common characteristics of layered compounds include:

• Low Hardness: Due to slippage between layers.
• Greasy or Slippery Texture: Like in talc, where layers easily slide past each other.
• Thermal Stability: Better insulation properties due to layer arrangement.

Such compounds are useful in lubrication (like graphite), as additives in ceramics, or other industrial roles. The distinct sheet-like arrangement gives rise to optical, mechanical, and chemical diversity, making layered compounds crucial in both geological and material science fields.