igneous rocks are generally hard and water parcolate in them with great difficulty give reason
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Answer:
In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however, could have high porosity
Secondary porosity is porosity that developed after rock formation. Processes such as fracturing, faulting, and dissolution can create secondary porosity.
Permeability is a measure of the degree to which the pore spaces are interconnected, and the size of the interconnections. Low porosity usually results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with little or no interconnections between pores. A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.
A thin layer of water will always be attracted to mineral grains due to the unsatisfied ionic charge on the surface. This is called the force of molecular attraction. If the size of interconnections is not as large as the zone of molecular attraction, the water can't move.
Thus, coarse-grained rocks are usually more permeable than fine-grained rocks, and sands are more permeable than clays.