Question

Suggest a plausible structure for the silicate anion in each of the following minerals: (a) Spodumene, \(\mathrm{LiAlSi}_{2} \mathrm{O}_{6}\) (b) Thortveitite, \(\mathrm{Sc}_{2} \mathrm{Si}_{2} \mathrm{O}_{7}\)

Short answer

(a) Spodumene features single chains, (Si2O6)^4-. (b) Thortveitite has disilicates, (Si2O7) groups.

Step-by-step solution

Identify Structural Units of Silicate Anion

In silicates, the basic structural unit is the silica tetrahedron, SiO4^4-. These tetrahedra can be isolated or can share oxygen atoms to form various structures such as chains, sheets, or three-dimensional frameworks. Our task is to determine the structure of this silicate unit within the given minerals.

Analyze Spodumene, LiAlSi2O6

The chemical formula shows that Spodumene contains two silicon atoms and six oxygen atoms. The ratio of silicon to oxygen (1:3) suggests a single chain silicate structure, where each silicon shares two oxygens with adjacent tetrahedra along a chain, creating SiO3 repeating units.

Determine Silicate Structure in Spodumene

Given the structural unit suggestion from Step 2 (a single chain SiO3), the silicate anion in Spodumene can be described as consisting of infinite single chains of silica tetrahedra, with each tetrahedron sharing two oxygen atoms with neighboring tetrahedra in a continuous chain, leading to (Si2O6)^4- chains.

Analyze Thortveitite, Sc2Si2O7

The chemical formula of Thortveitite contains two silicon and seven oxygen atoms. The Si:O ratio of 1:3.5 suggests a sorosilicate structure, where pairs of silica tetrahedra are linked by a shared oxygen, resulting in Si2O7 groups.

Determine Silicate Structure in Thortveitite

In Thortveitite, the silicate anion consists of double tetrahedra (sorosilicate) units, where two SiO4 tetrahedra share a single oxygen, forming a disilicate group. This leads to an (Si2O7) structure, fitting the given ratio and suggesting isolated pairs of tetrahedra.

Key concepts

Silica Tetrahedron

The silica tetrahedron is the fundamental building block of silicate minerals. It has a central silicon atom surrounded by four oxygen atoms, arranged in a shape that resembles a pyramid or a four-sided triangle. This geometric structure is noted as \[\text{SiO}_4^{4-}\] and has a distinct characteristic whereby the tetrahedron's oxygen atoms can link with other tetrahedra.
The sharing of one or more oxygen atoms with adjacent tetrahedra allows these units to combine into larger and more complex forms, such as chains, sheets, or three-dimensional frameworks.
The way these tetrahedra connect greatly influences the mineral's structure and properties. For example, whether these structures form chains or more extensive networks leads to differences in mineral hardness, cleavage, and appearance.

Chain Silicates

Chain silicates, or inosilicates, consist of silica tetrahedra linked together to form chains. In these structures, each tetrahedron typically shares two of its oxygen atoms with adjacent tetrahedra, leading to a repeating unit of \[\text{SiO}_3^{2-}\] in single chains. These chains extend infinitely in one direction, creating a continuous linear structure.
Minerals like Spodumene are examples of single chain silicates.

• In Spodumene, silicate anions form chains where each tetrahedron in the chain shares two oxygen atoms with its neighbors.
• This sequence results in a formula unit of \( \text{(Si}_2\text{O}_6\text{)}^{4-} \) within the mineral's crystal structure.

This arrangement affects the mineral's cleavage planes and how it breaks along these chains. Understanding these characteristics is crucial in identifying mineral types.

Sorosilicates

Sorosilicates present a fascinating structure known as "bow tie" or "double tetrahedra." In this arrangement, two silica tetrahedra join through a shared oxygen atom, forming \[\text{Si}_2\text{O}_7^{6-}\] groups. These uniquely connected double units don't form long chains or extensive frameworks but rather occur as distinct pairs.
A mineral like Thortveitite exemplifies this structure, characterized by isolated SiO4 pairs sharing a single oxygen atom.

• The presence of such units can be identified through the specific Si:O ratio of 1:3.5, as seen in \( \text{Sc}_2\text{Si}_2\text{O}_7 \).
• This contrasts with chain silicates, where such ratios result in extended linear structures.

These formations contribute to the mineral's physical properties, influencing how the mineral interacts with its environment and how it can be used practically.

Mineral Chemistry

Mineral chemistry is the study of the chemical composition and properties of minerals. It focuses on understanding how different elements and compounds combine to form various mineral structures. Silicate minerals, for example, offer a rich field of study due to their diverse structures and abundance in the Earth's crust.
The chemistry of minerals is largely determined by their atomic arrangements and bonding, including how silica tetrahedra can arrange and bond in different configurations. Factors such as:

• The size and charge of the ions involved in the bonding process.
• The conditions under which the mineral formed, such as temperature and pressure.

Determining these factors provides insight into how minerals form and evolve over time. A robust understanding of mineral chemistry can assist in industries like mining, geology, and environmental science, where it's crucial to identify mineral types and assess their potential uses and impacts.