Mineralogy of Phyllosilicates and Tectosilicates

Crystal Chemistry of Phyllosilicates and Tectosilicates

Phyllosilicates and tectosilicates are comprised of numerous mineral groups and important mineral species. The most well-known phyllosilicates are talc, micas, serpentines, and clays. Tectosilicates include quartz and numerous quartz modifications, as well as the extensive feldspar family, feldspathoids, and zeolites.

Introduction to Phyllosilicates

Most rock-forming phyllosilicates are made up of infinite two-dimensional layers. Phyllosilicates are sometimes called “sheet” or “layered silicates” in reference to their layered structure and their sheet or scale-like mineral forms. The structural unit of phyllosilicates is [Si₄O₁₀]^4- (sometimes written as [Si₂O₅]^2- — as the structural unit divided by 2).

Phyllosilicate sheets or layers are not identical within one mineral. Rather, they alternate between layers of octahedral and tetrahedral atomic structure. As a result, phyllosilicates are further subdivided into:

• 1-layer silicates, where all layers are similar (for example, apophyllite (KCa₄[Si₄O₁₀]₂(F,OH)₂⋅8H₂O))
• 2-layer silicates (for example, kaolinite (Al₄[Si₄O