Fix the mistake: Mineralogy and Petrology

Rock-Forming Minerals (Structure and Classification of Silicate Minerals)

Minerals that make up rocks are called rock-forming minerals. Most of the crust consists of silicate minerals (minerals whose main components are Si and O), and understanding their structure and classification is fundamental to petrology.

Basic Knowledge

A mineral is “a naturally occurring solid with a definite chemical composition and crystal structure.” The basic unit of silicate minerals is the SiO₄ tetrahedron (one Si atom surrounded by four O atoms). Silicate minerals are classified according to how these tetrahedra are bonded.

The six especially important rock-forming minerals are: quartz (SiO₂, a three-dimensional network of tetrahedra, colorless to white), feldspar (potassium feldspar and plagioclase, a three-dimensional network, white to gray or pink), mica (sheet structure, perfect cleavage, biotite and muscovite), amphibole (double-chain structure, dark-colored), pyroxene (single-chain structure, dark-colored), and olivine (isolated tetrahedra, green, stable under high-temperature conditions).

Physical properties of minerals, such as hardness (Mohs hardness: talc 1 to diamond 10; quartz = 7), cleavage (the tendency of a crystal to split easily in certain directions: mica has perfect cleavage in one direction), luster, color, and specific gravity, are used for identification.

Example Problem
State the basic structural unit of silicate minerals and explain the difference between the bonding patterns of the SiO₄ tetrahedra in quartz and mica.
Answer: The basic unit is the SiO₄ tetrahedron (a regular tetrahedron in which one Si atom is surrounded by four O atoms). In quartz, SiO₄ tetrahedra form a continuous three-dimensional network by sharing oxygen atoms (all oxygen atoms are shared). In mica, SiO₄ tetrahedra share oxygen atoms to form sheets (silicate sheets), which are bonded together by K, Al, and other elements.
Key Points
  • Basic unit of silicate minerals: SiO₄ tetrahedron
  • Quartz: three-dimensional network structure, colorless to white, hardness 7, no cleavage
  • Feldspar: three-dimensional network (with partial Al substitution), white to gray, cleavage in two directions
  • Mica: sheet structure, perfect cleavage in one direction (easy to peel)
  • Amphibole: double-chain structure, dark-colored, cleavage in two directions (60°/120°)
  • Pyroxene: single-chain structure, dark-colored, cleavage in two directions (90°)
  • Olivine: isolated tetrahedra, green, a major mineral of the mantle

Points to Note

① Pyroxene and amphibole are both dark-colored minerals, but their cleavage angles differ (pyroxene ≈ 90°, amphibole ≈ 120°). ② Quartz has no cleavage and shows conchoidal fracture (feldspar has cleavage). ③ Olivine is stable at high temperatures, so it is a major constituent mineral of the mantle (but weathers rapidly at Earth's surface).

Practice

  1. State the name and composition of the basic structural unit of silicate minerals.
  2. Explain, based on its crystal structure, why mica peels off easily into thin sheets.
  3. Which rock-forming mineral has a Mohs hardness of 7 and no cleavage?

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