In a rock these general properties are determined by averaging the relative properties and sometimes orientations of the various grains or crystals.
As a result, some properties that are anisotropic ( differ with direction) on a submicroscopic or crystalline scale are fairly isotropic for a large bulk volume of the rock.
Rocks are commonly divided into three major classes according to the processes that resulted in their formation.
These classes are (1) igneous rocks, which have solidified from molten material called magma; (2) sedimentary rocks, those consisting of fragments derived from preexisting rocks or of materials precipitated from solutions; and (3) metamorphic rocks, which have been derived from either igneous or sedimentary rocks under conditions that caused changes in mineralogical composition, texture, and internal structure.
That is to say, porosity is the ratio of void volume to the bulk volume (grains plus void space).Rock, in geology, naturally occurring and coherent aggregate of one or more minerals.Such aggregates constitute the basic unit of which the solid Earth is comprised and typically form recognizable and mappable volumes.The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water.Metamorphism often produces apparent layering, or banding, because of the segregation of minerals into separate bands.Depending on the type of close-packing of the grains, porosity can be substantial.It should be noted that in engineering usage— Total porosity encompasses all the void space, including those pores that are interconnected to the surface of the sample as well as those that are sealed off by natural cement or other obstructions.geotechnical engineers examine the nature and behaviour of the materials on, in, or of which such structures as buildings, dams, tunnels, bridges, and underground storage vaults are to be constructed; reservoir engineers analyze the response measured on well logs or in the processes of deep drilling at elevated temperature and pressure.Since rocks are aggregates of mineral grains or crystals, their properties are determined in large part by the properties of their various constituent minerals.The changes can be chemical (compositional) and physical (textural) in character.Metamorphic rocks are often formed by processes deep within the Earth that produce new minerals, textures, and crystal structures.