The previous discussion emphasizes the minerals occurring in sand and silt sizes; minerals of size such that their optical properties can be readily observed and measured. However, the physically and chemically active component of the soil is the clay (<2 µm) fraction. The clay fraction is more mobile and can be translocated and reorganized by solutions moving through the soil or by dynamic soil pressures. Much of the emphasis in soil micromorphology has, and remains in th ephysical and geometric arrangement of clay minerals. The major micromorphic classification schemes that follow place a great deal of emphasis on the organization of the clay fraction.

Individual clay minerals are usually too small to be resolved in the optical microscope, let alone to be able to distinguish their optical properties. In addition, individual clay minerals are often superposed by other constituents because they are much smaller than the thickness of the thin section (30 µm). It is our good fortune that clay mineral sseldom occur as descrete entities randomly dispersed throughout the soil matrix. Clay minerals often occur as domains, aggregates, pore linings or infillings, coatings around stable grains, bridges between grains, and various patterns of preferred orientationl. These features form as a result of mineral weathering, internal or external soil pressures, or concentration (illuviation) during pedogenesis. Because clay minerals may concentrate or be reoriented with internal parallel oriented domains in a specific direction, their combined presence forms large enough bodies to be observed and optical properties discerned.

Yound and Warkentin (1975) list a hierarachical arrangement of clay particles: particles domains clusters peds. Domains consist of two or more clay particles acting as a unit, usually in parallel orientation. When domains within clusters are in parallel orientation they form units large enough to observe through the microscope. Thus, small flecks of clay are usually domains while larger optical features of clay may be described as cluster. On page 19 is a brief schematic of the orientation of clays into larger structural units. In (A) and (B) the illustration shows different orientation of clay particles. In (B) the domain may be large enough to see in the microscope. In (A) the clay would appear isotropic. In (C) and (D) the clay particles are arranged into individual domains. The domains may either be in random or parallel orientation. In random orientation the domains would yield a speckled b-fabric (Bullock et al, 1985). Prallel orientation of domains would result in a number of possible striated b-fabrics. Figure (E) shows either clay particles or domains arranged around a void or a mineral grain. This is viewed as an cutan, due to stress or illuviation.

It is also fortunate that clay minerals are platy. The a and b crystallographic axis are within the plane of the clay while the c axis is perpendicular to the plane. Mechanisms of clay concentration and rorientation within the matrix result in a feature that may act like one large crystal although it may be composed of hundreds or thousands of individual clay platelets. In this instance the interference colors of the clays are reinforced and sometimes it is possible to distinguish between 1:1 and 2:1 type clays.

In soils that have applied stress, release of internal stress may result in the rearrangement of clay particles producing oriented clay along shear planes, around stabel structural units, or within the soil matrix. In these features ther has been no clay movement, only in situ rearrangement of clay domains. Stress features may be due to shrink-swell forces (Vertisols), root pressure, over consolidation by ice during the Pleistocene, mass movement slump, creep, or tillage practices. In cross-polarized light the stress features are often thin and have various patterns of orientation. They may also occur along rigid bodies such as quartz grains, along root channels, or around voids. There may be numerous sets of stress features at differnt angles and all possible sets of features are best observed in circular polarized light. In plane polarized light these features are usually indistinguishable from the matrix since they are of the same mineralogical composition. This is one feature which permits differentiation from illuvial clay features.

Illuviated clay may have a different composition than the matrix if it originated in another horizon. They are often easily observed in plane polarized light. Illuviated clay is often thicker than stress features and tends to occur along present or former ped surfaces, conducting proes or channels. Illuviated clay is usually translocated by water and may show laminations indicating successive increments of deposition. It often exhibits parallel extinction if the feature is straight. If curved, such as along a root channel or a convouted proe, the feature may show band extinction; the band sweeping through the clay feature as the microscope stage is rotated. Although clay features caused by pressure of illviation have some distinguishing characteristic, caustion should be exercised in making distinctions based on visual observations. Also, consider the other characteristics of th esoil such as shrink-swell potential and the factors that may have influenced pedogenic processes.

This page (http://www.ces.ncsu.edu/plymouth/programs/clay.html) created by
Vera MacConnell, Research Technician, I on December 1, 1997.
Last Updated on December 1, 1997.