They represent the soils' porous space, occupied by air and water.
In thin sections they are colourless with only the polarizer and when the analyser is placed, they are black (they include the isotope resin used to obtain the hardened block of the soil sample).
There are several different types:
Simple packing voids. Equidimensional to elongated, interconnected, which are found between skeleton grains. They are the result of simple packing.
Compound packing voids. equidimensional to elongated, interconnected, which are found between aggregates, whose faces do not accommodate each other. They are the result of packing the aggregates.
Complex packing voids. They are the voids between single grains and between the aggregates.
Vughs and vesicles
Channels and chambers
There is a description of: shape, size, abundance and wall smoothness / roughness.
Key to voids