Phase Relationships While rocks are compact, there are still empty spaces between the grains. These void spaces are filled with fluids and gases. Therefore, a phase relationship diagram is necessary to break down the rock's contents by weight and volume. The exact relationships in rocks vary per instance. Phase Diagram http://www.ce-ref.com/Soil%20mechanics/soil_properties.html Some key concepts of phase relationships are porosity and void ratio. Porosity is defined as the void volume divided by the total volume. Similarly, the void ratio is defined as the void volume divided by the solid volume.