Abstract
Color is used in describing and classifying soils. How soils
acquire color is not fully understood and the measurement of soil
color is not precise. Almost every soil contains clay minerals, which
affect soil properties. Montmorillonite and kaolinite are two common
types of such minerals. The main objective of the study was to investigate the color change of montmorillonite and kaolinite clay minerals with the addition of Fe, organic matter, and Mn, using either
NaOH or Ca(OH)2 for pH adjustment.
Further objectives were: (a) To measure the reflectances from
the clays and natural soils with particular emphasis on particle size;
(b) to convert the reflectance data into International Commission on
Illumination (I.C.I.) and Munsell color specifications; and (c) to
rationalize the color changes in montmorillonite and kaolinite with
the addition of Fe, organic matter, and Mn.
To accomplish these objectives, montmorillonite and kaolinite clay minerals were treated with different levels of Fe, organic matter,
and Mn and combinations of organic matter and Mn, organic matter and
Fe, and Fe and Mn. The pH of the suspensions were adjusted to different values using either NaOH or Ca(OH)2. After adjusting the pH
the suspensions were washed with water, filtered, air dried and passed
through a 0.25 mm sieve. Five natural soils were passed through different sized sieves to study the effect of particle size on
reflectance.
Reflectances from the treated clay samples and natural soils were
measured at 30 different wavelengths in the visible spectrum, specific
wavelengths for ten selected ordinates for illuminant C, used in color
determinations, using a Beckman DU spectrophotometer with reflectance
attachment. The reflectance data were converted to I.C.I. and Munsell
color specifications. Several samples of contrasting colors were subjected to D.T.A., x-ray diffraction, and infrared analysis.
A given level of Fe produced a much redder color with montmorillonite than with kaolinite and when the pH was adjusted with NaOH the
iron oxide-clay mixture gave a stronger red color than with Ca(OH)2-
A given level of organic matter gave a much darker color with montmorillonite than with kaolinite. Above pH 6, Mn had a darkening
effect. At pH 8, Mn darkened the clay color due to Fe, changing the
colors of montmorillonite and kaolinite to dark brown and light brown,
respectively. A combination of Fe and organic matter gave a dark
brown color to montmorillonite and light brown to kaolinite. A high level of Fe supressed the darkening effect of organic matter in the
case of montmorillonite but not in the case of kaolinite.
A small difference in the color of soil and clay could be detected
and defined by the spectrophotometer. As the particle size of some
natural soils used in the study decreased, reflectance and visual efficiency increased in the case of the dark soils and color purity
increased in the case of the red soils.
D.T.A. and x-ray diffraction data indicate that color changes of
clays from addition of Fe, organic matter, and Mn can be related to a
two-stage process of surface coating. Initially a thin film forms
which is chemically bonded to the clay; secondly, as the film thickens
the coating becomes a surface accumulation independent of bonding to
the clay surface. No change in the clay structure was indicated.