1.1. planar voids are well interconnected to
form discrete peds completely surrounded by void: KEY TO NATURAL MICROSTRUCTURES
The chapter on microstructure is generally considered as a rather weak point in the "Handbook". Only a non-exhaustive list of types is given, without any grouping. Levels of structure are not taken into account.
Modifications are therefore necessary in order to fit the microstructures into a key. In a same soil material, different levels of structure and microstructure can exist. Some prisms, for example, can display an angular blocky structure, which peds have in turn a finer angular blocky structure, whereas the intrapedal microstructure is dominated by channels. Moreover it is common to observe that on a same level two or more microstructures are juxtaposed, e.g. channels and cracks. Sandy materials occupy a special place amongst microstructures, as these are determined by the related arrangement of the basic units: sand grains, fine material and associated voids. The resulting porosity is called "inherent microporosity" (Stoops, 1994).
This level is related to the c/f related distribution patterns. Taking into account the above mentioned characteristics, the list of microstructures mentioned in the "Handbook" could be organised and reduced.
1. Microstructures with planar voids (defining to a greater or lesser extend peds whose faces are generally accommodating):
1.1. planar voids are well interconnected to
form discrete peds completely surrounded by void:
1.1.1. peds have a vertical dimension considerably smaller than the horizontal :
1.1.1.1. peds have an elliptical shape: well-developed lenticular microstructure
1.1.1.2. others: well-developed platy microstructure
1.1.2. peds are approximately equant and have flat faces and angular vertices: well-developed angular blocky microstructure
1.1.3. other approximately equant peds: well-developed sub-angular blocky microstructure
1.2. Planar voids are less well interconnected
so that peds are about two third surrounded by void, but are nevertheless discernible:
1.2.1. peds have a verticaldimension considerably smaller than the horizontal :
1.2.1.1. peds have an elliptical shape: moderately-developed lenticular microstructure
1.2.1.2. others: moderately-developed platy microstructure
1.2.2. peds are approximately equant and have flat faces and angular vertices: moderately-developed angular blocky microstructure
1.2.3. other approximately equant peds: moderately-developed sub-angular blocky microstructure
1.3. Planar voids are rarely interconnected
so that peds are only a third surrounded be voids and are difficult to discern:
1.3.1. peds have a vertical dimension considerably smaller than the horizontal : weakly-developed platy microstructure
1.3.2. peds are approximately equant and have flat faces and angular vertices: weakly-developed angular blocky microstructure
1.3.3. other approximately equant peds: weakly-developed sub-angular blocky microstructure
1.4. Planar voids are few and do not interconnect
so that no peds are discernible: fissure microstructure
2. Microstructures with compound packing voids or star-like vughs defining to a greater or lesser extent peds which are larger than the coarse particles present, consist of an aggregation of smaller particles and have not accommodating faces:
2.1. microstructure withcompound packing voids
which are well interconnected to form discrete aggregates completely surrounded
by void:
2.1.1. aggregates are porous and often have a rugose outline and may consist of smaller aggregates welded together: well-developed crumb microstructure
2.1.2. aggregates are not porous or composed of smaller aggregates: well-developed granular microstructure
2.2. microstructure with compound packing voids
which are less well interconnected so that aggregates are about two third surrounded
by void, but are nevertheless readily discernible:
2.2.1. aggregates are porous and often have a rugose outline and may consist of smaller aggregates welded together: moderately-developed crumb microstructure
2.2.2. aggregates are not porous or composed of smaller aggregates: moderately-developed granular microstructure
2.3. microstructure with star-like vughs or
compound packing voids which are poorly interconnected so that aggregates are
about one third surrounded by void, and difficult to discern:
2.3.1. aggregates are porous and often have a rugose outline and may consist of smaller aggregates welded together: weakly-developed crumb microstructure
2.3.2. aggregates are not porous or composed of smaller aggregates: weakly-developed granular microstructure
3. Apedal microstructures without inherent microporosity:
3.1. Soil materials without visible voids: massive microstructure
3.2. Microstructures characterized by the dominance
of one of the following void types:
3.2.1. Microstructures characterized by the dominance of vesicles: vesicular microstructure
3.2.2. Microstructures characterized by the dominance of channels: channel microstructure
3.2.3. Microstructures characterized by the dominance of chambers: chamber microstructure
3.2.4. Microstructures characterized by the dominance of vughs
3.2.4.1. microstructures with common vughs which are sufficiently interconnected to break the continuity of the solid, but where aggregates are not discernible: spongy microstructure
3.2.4.2. microstructure with frequent vughs which are not interconnected to break the continuity of the solid, but where aggregates are not discernible: vughy microstructure
4. Microstructures with inherent porosity composed of simple or complex packing voids defined to a greater or lesser extend by the coarse (generally sand-sized) particles present and sometimes aggregates of a similar or smaller size, consisting of finer particles:
4.1. Solid material consists predominantly of
coarse particles (sand); structural units are individual, coarse particles and
simple packing voids occurring between individual particles:
4.1.1. Microstructures consisting almost entirely of sand sized grains: single grain microstructure or, monic c/f related distribution
4.1.2. Microstructures in which there are frequent bridges of finer material between the sand-sized grains: bridged grain microstructure or, gefuric c/f related distribution
4.1.3. Microstructures in which the sand-sized grains are mostly coated with finer material: pellicular grain microstructure or, chitonic c/f related distribution
4.2. Microstructures in which the solid units
are a mixture of single particles (sand grains) and small aggregates of finer
material, all separated by and defining complex packing voids: intergrain
microaggregate microstructure enaulic c/f related distribution
In the case of pedal microstructures (1 or 2), it is necessary to determine also the intrapedal microstructure. Combinations of several types of apedal microstructures (type 3). are common; therefore it is necessary to scan the key 3 first for the dominant type, and a second time for the less dominant type. Several microstructures listed in the "Handbook" (e.g. intergrain vesicular pore structure) have been omitted as they are considered to be merely combinations of other types.