Talc particles characterization

Figure 35-15 Talc retinopathy characterized by numerous yellow-white intra-arteriolar particles scattered throughout perimacular area.

Pyrophyllite is the simplest layer aluminosilicate in which two tetrahedral Si04 layers are condensed on to the octahedral A106 layer to produce a three-sheet layer, the composition of the unit cell being [Al2(OH)2(Si205)2]2. Another ideal structure in which A1 is replaced by Mg is that of talc. In both cases the three-sheet layer is electrically neutral and the layers are stacked in the ABAB... sequence. Because of the cohesive strength of this ideal structure, neither pyrophyllite nor talc occurs in the form of the very fine particles which generally characterize clay minerals. 

In the above, <7C is the equivalent volume diameter, i.e. the diameter of a sphere having the same volume as an irregular particle. As Hinds (1982) points out The equivalent volume diameter can be thought of as the diameter of the sphere that would result if an irregular particle were melted to form a droplet <7C is calculated from microscopic measurement of the actual particles being considered, while x is the dynamic shape factor which is included to allow for the effects of shape on terminal velocity. For example, talc dust is characterized by a dynamic shape factor (/ ) of 1.88, sand particles by 1.57, etc. Spheres have a dynamic shape factor of 1.0 while cubes have a dynamic shape factor of 1.08. 

The layers are characterized by a thickness of about 1 nm, and the other dimensions vary from 30 mn to several micrometres or more. Several layers are stacked in clay particles, kept together by weak van der Waals forces. The performance of polymer-clay nanocomposites strongly depends on the breaking-up of clay particles in the polymer matrix. However, the most frequently used layered silicate, MMT, is a naturally occurring 2 1 phyllosilicate, which has the same layered and crystalline structure as talc and mica but a different layer charge. A central octahedral sheet of alumina fused between two external silica tetrahedral sheets (the oxygens from the octahedral sheet also belong to the silica tetrahedral) forms the layers of the... 

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