This week we examine the mineralogical composition of rocks and the
common kinds which form many of our soils. In a local sense, soil is primarily the result
of disintegration and decomposition of the minerals contained within rocks. The kind of
soil and its nutrient status found in a given locality is probably more dependent upon the
material from which it weathered than upon any other factor. In a real sense then, the
geologic material found in a locality is the 'parent material' of the soil. Remember this
term as you will be using it throughout the rest of the course. Now, you can begin to
appreciate the importance of geology to the study of soil. The purpose of this lab is to
provide an understanding of weathering processes. If you have had some geology, the
exercise will be a good review.
The term 'mineral' may be used in three different contexts. The first is that of a a
crystalline substance having definite physical structure and composition. There are many
known minerals, such as quartz, which may be classified, and they are the building blocks
of rocks. Rocks, in turn, may be classified on the basis of their mineralogical
composition - that is, the minerals they contain. The second context is the
differentiation of soils containing predominantly inorganic mineral material ('mineral'
soils) from those soils containing predominantly organic material (organic soils). The
third context is differentiation of the inorganic material of the soil from the other
three components: organic matter, air, and water. The mineral component acts as the
skeleton or fabric of the soil.
Perhaps this is carrying the idea to an extreme, but slide 1
is an example of a seedling precariously rooted in a small handful of fragmented material
that has resulted from the disintegration and decomposition of the boulder. The joint or
crack in the boulder is a plane of weakness opened by expansion and contraction by
temperature changes or perhaps freezing. Rainwater, washing down over the boulder, may
carry some nutrients to the limited soil. We can only speculate how long the seedling will
survive in this small niche, because the roots will soon outgrow the soil volume.
Fortunately, most of the soil of the earth's mantle is thicker than this.
The physical and chemical properties that are used to identify the various minerals
present on the earth are listed for you on page 1 of the laboratory exercise. More
importantly, these properties determine the weathering rates of the mineral and the rock
containing them, as well as the byproducts formed therefrom
. Illustrated in are
the various crystal forms based upon the alignment of the several axes and the relative
dimensions of the basic crystal unit. These features reflect the structure of the
molecules within the mineral. There are six systems of crystal form, of which only three
will be mentioned here. The 'cubic' form has three axes at right angles to each other and
they are equal in length. Example minerals are halite (salt), galena, and pyrite (Fool's
Gold). A second system is "hexagonal" having three axes of equal length but with
two axes at 120 degrees to each other in one plane and the third axis at right angles to
the plane of the other two. Calcite and some forms of quartz are examples of this system.
A third system is a "triclinic" system having three unequal length axes and none
form a right angle to any other axis. Plagioclase feldspar is the best example of this
system.
Hardness is a physical property that is strongly influenced by crystal form and
composition. A scale of hardness of 1 to 10 has been developed for application to
minerals. Talc is the softest and can be scratched by most any other mineral. Most
minerals have a hardness of 3 to 7. A penny, hardness of 3, is scratched by a knife blade
with a hardness of 5.5. The rapidity of mineral weathering is directly related to
hardness- the harder the mineral, the slower it weathers. Diamonds don't weather! Hence, a
diamond is "forever," and talc is used in many cosmetic powders because it is so
soft.
Specific gravity is simply the relative weight of a mineral compared to an equal volume of
water. This property is highly variable and not useful except to say that quartz is
considered a "light" mineral. The degree of weathering of "heavy"
minerals such as rutile or tourmaline is used to detect the stage of soil weathering.
Cleavage is the way minerals split along planes of weakness. Cleavage is related to the
structure of the mineral and is usually parallel to other mineral faces. Three examples of
cleavage are: cubic- halite; rhombohedral- calcite; basal- mica. All of these minerals are
on display in today's laboratory. Fracture is the breakage of a mineral in a way other
than along cleavage planes. Thus, most minerals show fracture rather than cleavage.
Several common fracture types are illustrated here. Several minerals exhibiting these
types of fracture are on display in today's laboratory.
The property of lusture is dependent upon absorption, reflection, or refraction of light
by the mineral surface. It aids in the identification of minerals contained in some common
soil-forming rocks. Some common types are illustrated here.
Rubbing or scratching a mineral fragment across the surface of unglazed porcelain produces
characteristic color frequently unlike the color of the mineral. This is also a distinct
identification attribute when coupled with other properties.
At this point you should read the exercise material concerning the primary and secondary
minerals, the arrangement of quartz in the five groups of rock-forming minerals, and their
degree of resistance to weathering. This information all has significance to the
understanding of the rate of soil formation and the rapidity with which a soil goes
through its life cycle. The life cycle was illustrated for you in Figure 1.4 of the first
laboratory exercise. Here, you should especially study Figure 2.2 on page 3. In
Investigation 1 you will examine some of the common minerals found in soil-forming rocks.
These are hand specimens for ease of examination. The size of mineral fragments contained
in many rocks may be quite small, requiring a hand lens for examination and
identification. These large specimens are found concentrated in veins or other natural
deposits within unweathered rock formations.
Beginning in the second paragraph of page 4, you will find a discussion of the common
soil-forming rocks. One kind is igneous, which is shown here. This rock is derived
directly from molten magma and cooled either below the earth's surface or extruded and
exposed to the atmosphere and cooled. Igneous rocks may contain minerals producing many
plant nutrients. Unfortunately, igneous rock weathers slowly compared to other types.
Granite, basalt, diabase, and obsidian are examples.
Metamorphic rock is the kind that has undergone levels of heat and pressure above that at
the earth's surface or its atmosphere. Metamorphic rock is usually foliated as the result
of pressure and parallel cleavage planes are abundant. This rock may weather relatively
rapidly or extremely slow. Schist, gneiss, and slate are examples.
Sedimentary rock is formed through the cementation of sediments under slight pressure. The
cementing agent may be a variety of substances such as silica, iron, calcium, etc. The
sediments mat have been deposits of eroded material from any of the three types of rock.
Their character is quite variable. Sandstone, shale, limestone, and conglomerate are
examples of sedimentary rock. This type of rock is the most common found at the earth's
surface, simply because the earth's crust has been through many repeated cycles of uplift
followed by erosion and leveling during geologic history.
