The Phyllosilicate Subclass
(The Sheet Structures)
A subclass of the Silicates


In this subclass, rings of tetrahedrons are linked by shared oxygens to other rings in a two dimensional plane that produces a sheet-like structure. Typically, the sheets are then connected to each other by layers of cations. These cation layers are weakly bonded and often have water molecules and other neutral atoms or molecules trapped between the sheets. This explains why this subclass produces very soft minerals such as talc, which is used in talcum powder. Some members of this subclass have the sheets rolled into tubes that produce fibers as in asbestos serpentine. The silicon to oxygen ratio is generally 1:2.5 (or 2:5) because only one oxygen is exclusively bonded to the silicon and the other three are half shared (1.5) to other silicons. The symmetry of the members of this group is controlled chiefly by the symmetry of the rings but is usually altered to a lower symmetry by other ions and other layers; but the overlying symmetry of the silicate sheets will usually still be expressed in a higher pseudo-symmetry. The typical crystal habit of this subclass is flat, platy, book-like and most all members display good basal cleavage. Although members tend to be soft, they are remarkably resilient. Members of this group are often the last to chemically breakdown in erosional and weathering processes and thus constitute a significant amount of soils and fine grained sedimentary rocks. This group is also generally tolerant of high pressures and temperatures and they make-up a large part of metamorphic rocks.
Below are the more common members of the phyllosilicates:

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Much information for this page provided by the Mineral Database.

 

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