The description and interpretation of thin sections should be kept separate as far as possible. A full description of a thin section must precede interpretation of the processes that have taken place. This Handbook is concerned with description - not interpretation.

The descriptive side of micromorphology is vital for a number of reasons:

1. To record in detail the characteristics of soils whether at the level of a quartz grain, a structural arrangement or a feature due to a particular process;
2. To form a basis for the reconstruction of processes that have taken place or are taking place in the soil;
3. to provide a sound basis for the classification of soils either generally or for specific purposes; and
4. To compare the properties of different soils within an order, suborder, great group, or at lower levels of classification.

The major purpose of a micromorphological description is to summarize observed objects in the form of written text, tables, graphs figures, or pictures. The summary needs to be clear, simple, logical, and systematic so that other specialist with little knowledge of soil micromorphology can use the information. A uniform terminology should be used so that others can understand the description.

the Handbook was not written to define a rigid system for describing thin sections. It is simply a reference source of terms and concepts. How those are used is left up to the individual micormorphologist.

This section descriptions should begin with an examination of the section with the naked eye (1X magnification). Homogeneous zones can then be identified which will be examined separately under the microscope. Alternatively, it is sometimes useful to examine the block from which the thin section was obtained to see if there were any areas that were not impregnated or to examine the macrostructure. If the impregnating solution conatains a dye, the macropore structure is often better illustrated on the polished block. At this stage it is sometimes useful to study the field description to see if items noted earlier can be seen in the thin section. I f features observed in the field description to see if items noted earlier can be seen in the thin section. If features observed in the field are not present in the thin section, the thin section may not be representative of the soi material. Some micromorphologists even complete a macromorphological description of the thin section first. This approach helps bridge the gap between the field relationships and micromorphology.

Frior to starting to describe the thin section under the microscope, it is useful to first scan the entire section using different magnifications to see the variation in microstructure, groundmass, and pedofeatures which are present. When soil profiles have been sampled, it is also useful to quicklyscan the thin sections from adjacent horizons to hain an overall view of the material.

The approach to describing thin sections depends on the purpose of the study. There are two main approaches:

1. Start with the identification and description of the simplest units, and examine the way in which they combine through various stages of complex units to the fabric level. This goes from the microscopic towards the macroscopic level.
2. Proceed from the most complex units and examine how these break down into less complex units and eventually into the simplest units. This is from th ecamcaroscopic toward the microscopic level.

SIZE:
The identification and measurement of constituents in thin section are strongly dependent on the resolving power of the microscope used. It is difficult to measure the size of individual compoonents in thin section for a number of reasons, but primarily because the size measured is a funtion of the orientation of the components and the direction in which they are cut. The sizes measured are generally smaller than their actual sizes as the particles or features are generally smaller then their actual sizes as the particles or features are generally not cut through their longest dimension. The following table lists a general guide for the difinition sizes.

FREQUENCY:
Frequency refers to the proportionate area of the thin section occupied by the constituent, or their total number. For most purposes a visual estimate is sufficient. For general purposes, the following frequency classes are recommended:

Very dominant	>70%
Dominant	50-70%
Common	30-50%
Frequent	15-30%
Few	5-15%
Very Few	<5%

SORTING:
the degree of sorting of components in a thin section is an expression of the variation in particle size. It is the extent to which particles spread on either side of the average. Generally, a rough estimate of sorting by eye is satisfactory using the following classes:

Perfectly sorted:	normally only one size fraction is present
Well sorted:	5-10% of fractions other than those stated
Moderately sorted:	10-30% of fractions other than those stated
Poorly sorted:	the sorted component is not the dominant one
Unsorted:	the particles are of a variety of sizes with no fractions appearing more sorted than others

SHAPE:
the shape of a component in thin section is only a two dimensional respresentation of a three dimensional object. However, it is sometimes possible to infer or even deduce the three dimensional form but extreme care is necessary to do so because the shape in two dimensions will be dependent on the orientation of the section throught the object.

Equidimensional (or compact): The three axes are of the same order of magnitude. An example would be quartz and calcite.
Elongate: One of the axes is considerable longer than the others. Extreme cases are needle shaped or acicular.
Platy: One of the axes is considerably shorter than the others. An intermediate case is tabular.
Lathlike: One dimension is much longer and the second much shorter than the third one. Mica is an example.
Spherical: Like a sphere.
Lenticular: Shaped like a lens. Gypsum is often lenticular.

Shapes of Common Particles

In the case of mineral grains it is necessary to specify the development of the crystalline forms. The following are recommended:

Euhedral: Grains are competely bounded by crystal faces.
Subhedral: Grains are only incompletely bounded by crystal faces.
Anhedral: Grains not bounded by crystal faces.

Crystalline particles are also characterized by a particular habit. the following habits are for three-dimensional bodies:

platy, fibrous, lenticualr, columnar, acicular, and tabular

ROUNDNESS:

Roundness is defined as description of the relative sharpness of the particle corners.

Angular: Strongly developed faces with sharp edges and corners. Secondary corners are numerous and sharp.
Subangular: Stronly developed faces with somewhat rounded edges and corners. Secondary corners are numerous.
Sub-rounded:Poorly developed flat faces with corners well rounded. Secondary corners are greatly subdued and few.
Well-rounded: The entire surface consists of broad curves. No secondary corners.

SURFACE ROUGHNESS/SMOOTHNESS:
An important property of particles, aggregates, voids and some pedofeatures is their surface character. Roughness can arise from a number of processess, e.g. weathering of mineral particles, mastication of plant fragments by soil animals, projection of coarse and fine mineral particles along void walls and crystallization onvoid walls. Processes that give rise to smoothness include translocation of particles, some forms of chemical precipitation, depostion of clay at pore and aggregate edges and movement of soil fauna through soil material. the degree to which roughness or smoothness is apparent is often a function of magnification; the higher the magnification, the more irregular the surface may appear.

The descriptions of surface roughness/smoothness should be at a magnification of 20-60x, but observations can be reported for higher magnifications, provided the magnification is given. three categories of surface roughness/smoothness are used:

Rough: The surface has indentations deeper than they are wide.
Undulating: The surface is dominated by broad shallow undulations.
Smooth: there are few or no irregularities on the surface.

BOUNDARY:

Boundary is strongly dependent on the magnification used; what may have a sharp boundary at a lower magnification may appear diffuse at a higher magnification. It is thus important to specify the magnification at which the boundary conditions are observed. Contrast refers to the degree to which the feature being described is clearly differentiable from other features and the matrix.

Prominent: The individual is conspicuous and stands out from other individuals in terms of color, particle size distribution, birefringence and other morphological properties.
Distinct: Although not striking, the individual can clearly be seen. It has some morphological properties in comon with other individuals. In terms of color, the individual usually has the same hue as the one with which it is compared but differs in chroma.
Faint: The individual is only evident on close examination. In terms of color, the individual and the feature with which it is compared have the same hue and differ only slightly in chroma. Similarly, there will be only a small difference inparticle size distribution between the two features>

Sharpness refers to the transition between the particular feature and the matrix and other features. It is described at a specified magnification both between crossed and uncrossed polarized light.

Low: Knife edge boundaries between colors and/or particle size distribution.
Medium: Color transition and /or particle size transition <60µm wide.
Diffuse: Color transition and/or particle size transition >60µm wide.

ORIENTATION PATTERNS:
Orientation patterns of anisotropic individuals of a size above the resolution of the optical microscope can be usually redily identified. The orientation patterns of both coarse and fine material should be described where appropriate.

Basic: this refers to the orientation pattern of like components with respect to each other. Description of the orientation of clay-sized particles below the resolution of the optical microscope depends on identifying the way in which they are orgainzed into domains within which the particles are oriented with respect to each other. There are essentially tow types of orientation, random and parallel (see the next section for definitions).

*Degree of orientation refers to the extent to which the material below the resolution of the microscope is oriented into domains, the size of the domains and "goodness" of orientation within each domain. The pattern of basic orientation is a measure of the degree of preferred orientation of the compound anisotropic units. The extent to which these units are oriented with respcet to each other is reflected in the overall appearance of the material.

Strongly oriented: More than 60% of the individuals are oriented with their principal axes within 30° of each other.
Moderately oriented: 40-60% of the individuals are oriented with their principal axes within 30° of each other.
Locally oriented: 20-40% of the individuals are oriented with their principal axes within 30° of each other.
Unoriented: ther is no preferred orientation.

Referred: The orientation pattern of the like components with regard to a specific reference feature (see distribution patterns for term definitions).

Unrelated:
Perpendicular: Calcite crystals growing perpendicular to the edge of a rock fragment.
Parallel: Clay particles in a clay coating parallel to the wall of a void.
Inclined: Calcite fibers (lublinite) grown inclined to walls of voids.
Bow-like: Semi-elliptical arrangements of soil constituents between sub-parallel walls.

Related: The orientation pattern of like components with respect to groups of components of a different kind (see definitions for referred distribution patterns).

DISTRIBUTION PATTERNS: See Brewer (1976) for a detailed discussion of concepts

Basic: Concerned with the distribution of individuals with respcet to other individuals of the same type. There are essentially two types, random and parallel, although from a theoretical point of view, a perpendicular basic orientaion pattern is also possible. Description of the orientation of clay-sized particles belo the resolution of the optical microscope depends on identifying the way in which they are organized into domains within which the particles are oriented with respcet to each other.

Random: Indivuals are distributed randomly thoughout the thin section.
Clustered: Indivuals are concentrated in clusters.
Linear: Indivuals have a linear arrangement.
Banded: Indivuals are concentrated in bands. The distance between individuals is smaller than the distance between bands.
Fan-like: Indivuals have a fan-like arrangement. Interlaced: Indivuals are interlaced with each other.

Referred: The distribution of individuals with respect to specific reference features e.g. the surface, a planar void, etc.

Unreferred: The pattern of distribution in unrelated to any reference feature.
Perpendicular: Individuals are distributed perpendicular to the reference feature.
Parallel: Individuals are parallel to the reference feature.
Inclined: Individuals are at an angle to the reference feature.
Radial: Individuals are grouped along radiating lines.
Concentric: Individuals are grouped along approximately concentric lines or surfaces.

Related: the distribution of individuals with respect to individuals of a different type. The distribution patterns of components whether they be primary mineral grains, organic particles or units composed of a number of constituents are generally complex. Related distribution are more fundamental to fabric analysis.

Monic: Only fabric units of one size group, or amorphous material, are present (e.g. pebbles, sand, clay).
Gefuric: The coarser units are linked by braces of finer material (e.g. clay or organic matter bridges between coarser material).
Chitonic: The coarser units are partly or wholly surrounded by a cover of smaller units (e.g. sand-sized grains or aggregates coated with clay).
Enaulic: There is a skeleton of larger fabric units with aggregates of smaller units in the interstitial spaces. These aggreates do not competely fill the interstitial sapaces, and the larger units support each other.
Porphyric: the large fabric units occur in a dense groundmass of smaller unites. Interstitial pores at the scale of the distribution pattern are absent.

Close prophyric: the larger units or grains have points of contact with each other.
Single spaced porphyric: the distance between the larger units or grains is less than their mean diameter.
Double spaced prophyric: the distance between grains or units is about 2 times the mean diameter.
Open porphyric: the distance between units or grains is more than 2 times the mean diameter.

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This page (http://www.ces.ncsu.edu/plymouth/programs/gen.html) created by
Vera MacConnell, Research Technician, I on January 23, 1998.
Last Updated on January 26, 1998.