Types of Silicates Part 2: Inosilicates, Phyllosilicates, and Tectosilicates

Summary

This video script introduces various subclasses of silicate minerals, exploring their structures, compositions, and the differences between them. It covers orthosilicates, disilicates, cyclosilicates, and chain silicates, focusing on pyroxenes and amphiboles. The discussion extends to sheet silicates, including micas, talc, and chlorite, with insights into their unique bonding and structures. Finally, the script delves into framework silicates like feldspars and quartz, explaining their polymerized tetrahedra structures and solid solution relationships. This foundational knowledge paves the way for understanding how minerals form and combine to create rocks.

Takeaways

• 🔶 Silicates are the most important class of minerals, divided into several subclasses like orthosilicates, disilicates, and cyclosilicates.
• 🔷 Inosilicates are chain silicates with two subgroups: pyroxenes (single chains) and amphiboles (double chains).
• 🌀 Pyroxenes have a general formula of XY(Si,Al)2O6 and consist of silica tetrahedra in a zig-zag pattern.
• 🔗 Orthopyroxenes and clinopyroxenes differ in crystal structure, with orthopyroxenes being more symmetrical and clinopyroxenes having more diverse compositions.
• 💎 Amphiboles form double chain silicates with a silicon to oxygen ratio of 1:2.5 and often contain volatile elements like hydroxyl and chlorine.
• 🌋 Amphiboles typically form during the hydrothermal alteration of pyroxenes and dominate metamorphic rocks called amphibolite.
• 📐 Sheet silicates, or phyllosilicates, form infinite sheets of tetrahedra and have various structures like T-O and T-O-T configurations.
• 🌟 Sheet silicates include minerals like clay, micas, serpentine, and talc, with varying types of bonding holding their layers together.
• ⚒️ Framework silicates, or tectosilicates, are the most abundant subclass in the Earth's crust, with tetrahedra sharing all four oxygen atoms.
• ⛏️ Quartz and feldspar are key framework silicates, with quartz having a strong silicon-oxygen bond and feldspar forming solid solutions between endmembers.

Q & A

What are the main subclasses of silicates covered in the transcript?

The transcript covers six subclasses of silicates: orthosilicates, disilicates, cyclosilicates, inosilicates (chain silicates), phyllosilicates (sheet silicates), and tectosilicates (framework silicates).

What distinguishes single-chain silicates (pyroxenes) from double-chain silicates (amphiboles)?

Single-chain silicates, or pyroxenes, have silica tetrahedra linked in single chains, sharing two oxygens. Double-chain silicates, or amphiboles, form when every other silica tetrahedron shares three oxygen atoms, creating a ring-like structure with a silicon to oxygen ratio of 1 to 2.5.

What are some endmembers of the orthopyroxenes and clinopyroxenes?

Orthopyroxenes have ferrosilite (iron-rich) and enstatite (magnesium-rich) as endmembers. Clinopyroxenes include diopside (magnesium-rich), hedenbergite (iron-rich), jadeite (aluminum-rich), and aegirine (iron-rich).

What is the significance of amphiboles in metamorphic rocks?

Amphiboles often form during the hydrothermal alteration of pyroxenes and are key minerals in amphibolite, a class of metamorphic rocks. They are also present in blueschist, with glaucophane as the main mineral giving the rock its blue color.

How do the bonding types in sheet silicates affect their properties?

The type of bonding between the layers in sheet silicates affects their hardness and cleavage. For example, talc is held together by weak van der Waals forces, making it soft (Mohs hardness of 1), while micas have weak ionic bonds between layers, resulting in perfect basal cleavage.

What is the structure of a T-O-T sheet silicate, and how does it differ from T-O structures?

T-O-T sheet silicates consist of an octahedral sheet sandwiched between two tetrahedral sheets, with weak ionic bonds holding the layers together. In contrast, T-O structures have only one tetrahedral and one octahedral sheet, held together by hydrogen bonds.

What is the general formula of amphiboles, and why are they called 'garbage can minerals'?

The general formula of amphiboles is AB2C5(Si,Al)8O22W2. They are called 'garbage can minerals' because they can accommodate a wide variety of elements in their structure, making them chemically versatile.

What are some key minerals found in the framework silicates, and why are they significant?

Key minerals in framework silicates include feldspars and quartz. Feldspars, especially plagioclase, are the most abundant minerals in the Earth's crust. Quartz, being composed entirely of SiO2, is very hard due to its strong silicon-oxygen bonds.

How does solid solution work in feldspars, and what are the endmembers of this group?

Solid solution in feldspars occurs between similar cations. For example, there is complete solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), forming plagioclase. The endmembers of feldspars are anorthite, albite, and potassium feldspar (KAlSi3O8).

What is the relationship between the structure of quartz and its polymorphs?

Quartz has a strong, rigid structure composed of silica tetrahedra, making it hard. Its polymorphs, such as tridymite and cristobalite (high-temperature) and coesite and stishovite (high-pressure), change structure under different temperature and pressure conditions.