What is isomorphous substitution and how does it affect the charge of layer-silicate
clay minerals? what is the dominant type of charge (permanent or variable) on layer-
silicate clay minerals and why?

Isomorphous substitution has a net negative charge on clay. This charge needs to be balanced as to produce a positively charged ions from the solution.

Explanation:

As the word isomorphous closely related to the same shape and thus atoms must contain the physical and chemical properties. Clay minerals have silicate layers as a result of the usually a product of chemical weathering. Most of them are found in shales a common type of sedimentary rock. As they act as chemical sponges they hold water and dissolved pale matter from other chemicals. This results in unbalanced electric charges,  clay minerals that can attract molecules as an example of mica. Soil containing 45% of minerals, 5% of organic matter, 25% of both air and water make up the sand, silt, and clay, etc. Kaolinite, Vermiculite, Illite, and Sodium Smectite along with Calcium smectite are generally considered as the soil clay forming minerals.

Answer (datum point);