Phyllosilicates

Phyllosilicates - 1 IV- Phyllosilicates Common Properties:...

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IV- Phyllosilicates Common Properties: Platy or micaceous habit, low specific gravity, perfect cleavage along {001}, soft, flexible, and elastic. Most members are monoclinic, some are triclinic, whereas a few are orthorhombic or trigonal. Different ways of stacking the sheets produces different polytypes. Types and structures of Phyllosilicates: All Phyllosilicates consist of sheets of tetrahedra (sharing 3 of their 4 oxygens) alternating with layers of either octahedra or some other type of polyehdra with higher coordination numbers. This is illustrated by the structure of kaolinite viewed looking down (001) (Fig. 1b) and along (100) (Fig. 1a). Two major groups: Dioctahedral and Trioctahedral Phyllosilicates. Dioctahedral Group: Trivalent cations (e.g. Al) in the octahedral sheet. The octahedral sheet is similar in structure to that of the mineral Gibbsite (Al(OH) 3 ). Two structural types within this group: one consisting of repeated pairs of a tetrahedral and octahedral sheet (known as a 1:1 or t – o structure) and another consisting of repeated stacks of an octahedral sheet sandwiched between 2 tetrahedral sheets (t – o – t) (Fig. 2). The different groups or stacks of sheets are joined to each other by the weak van der Waals bonds (i.e. bonds between the combined tetrahedral – octahedral pair; or t – o and another t-o pair are of the van der Waals type; Fig. 2). Trioctahedral Group: Divalent cations (e.g. Fe 2+ or Mg) in the octahedral sheet. The octahedral sheet is similar in structure to that of the mineral Brucite (Mg(OH) 2 ). As with the dioctahedral phyllosilicates, there are two structural types within this group: one consisting of repeated pairs of a tetrahedral and octahedral sheet (known as t – o) and another consisting of repeated stacks of an octahedral sheet sandwiched between 2 tetrahedral sheets (t – o – t). The different groups or stacks of sheets are joined to each other by the weak van der Waals bonds (i.e. bonds between the combined tetrahedral – octahedral pair; or t – o and another t-o pair are of the van der Waals type). Examples: Lizardite (a serpentine mineral with a t – o structure) and Talc (t-o-t).
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This note was uploaded on 02/27/2012 for the course GLY 314 taught by Professor Staff during the Spring '09 term at Marshall.

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Phyllosilicates - 1 IV- Phyllosilicates Common Properties:...

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