Principles and practice of agricultural analysis. Volume 1 (of 3), Soils — Reading Companion

ortant ingredient of sea water. Combined with silica sodium is an important element in many silicates. Sodium, although closely related to potassium chemically, cannot in any case be substituted therefor in plant nutrition. In combination with nitrogen it forms soda or Chile saltpeter which is a valuable fertilizer on account of its content of nitric acid.

=20. Iron= is the most abundant of the heavy metals, and occurs in nature both free and combined with other elements. In the free state it is found only to a limited extent in basaltic rocks and meteorites, but in combination with oxygen it is one of the most widely diffused of metals, and forms the coloring matter of a large number of rocks and minerals. In this form, too, it exists as the valuable ores of iron known as magnetite and hematite. In combination with sulfur it forms the mineral pyrite, FeS₂. The yellow and red colors of soils are due chiefly to iron oxids. It is an important plant food, although not taken up in any great quantity by the tissues of plants.

=21. Manganese=, next to iron, is the most abundant of the heavy metals. It occurs in nature only in combination with oxygen, in which form it is associated in minute quantities with iron in igneous rocks or in the forms known mineralogically as pyrolusite, psilomelane and wad. As the peroxid of manganese it occurs in concretionary forms scattered abundantly over the bottom of the deep sea. It is found in the ash of some plants but is not believed to be an essential to plant growth.

=22. Barium= occurs in nature combined with sulfuric acid, forming the mineral barite, or heavy spar, or with carbon dioxid forming the mineral witherite. It is of small importance from an agricultural standpoint.

=23. Relative Abundance of the More Important Chemical Elements.=—It will be of interest to the agricultural analyst to know as nearly as possible the relative abundance of the more important chemical elements. This subject has been carefully studied by Prof. F. W. Clarke in a paper read before the Philosophical Society of Washington.[19] The materials considered in these calculations are the atmosphere, the water, and the solid crust of the earth to the depth of ten miles below the sea level. Of these materials the relative quantities of the three constituents named are as follows:

Per cent. Atmosphere 0.03 Water 7.08 Solid crust of the earth to the depth of ten miles 92.89

According to these calculations the relative abundance of the important elements composing the atmosphere, the water of the ocean and the solid crust of the earth to the depth given is as follows:

Solid crust, Ocean, seven per Mean, including ninety-three per cent. air. cent. Oxygen 47.29 per cent. 85.79 per cent. 49.98 per cent. Silicon 27.21 „ „ „ „ 25.30 „ „ Aluminum 7.81 „ „ „ „ 7.26 „ „ Iron 5.46 „ „ „ „ 5.08 „ „ Calcium 3.77 „ „ 0.05 „ „ 3.51 „ „ Magnesium 2.68 „ „ 0.14 „ „ 2.50 „ „ Sodium 2.36 „ „ 1.14 „ „ 2.28 „ „ Potassium 2.40 „ „ 0.04 „ „ 2.23 „ „ Hydrogen 0.21 „ „ 10.67 „ „ 0.94 „ „ Titanium 0.33 „ „ „ „ 0.30 „ „ Carbon 0.22 „ „ 0.002 „ „ 0.21 „ „ Chlorin 0.01 „ „ 2.07 } „ „ 0.15 „ „ Bromin „ „ 0.008} „ „ „ „ Phosphorus 0.10 „ „ „ „ 0.09 „ „ Manganese 0.08 „ „ „ „ 0.07 „ „ Sulfur 0.03+ „ „ 0.09 „ „ 0.04+ „ „ Barium 0.03 „ „ „ „ 0.03 „ „ Nitrogen „ „ „ „ 0.02 „ „ Chromium 0.01 „ „ „ „ 0.01 „ „ —————— ——————— —————— 100.00 „ „ 100.000 „ „ 100.00 „ „

=24. Fluorin= is not mentioned in this table but it is stated that its probable percentage is 0.02 to 0.03 making it thus slightly more abundant than nitrogen.

One of the chief points of interest in connection with this table is that the nitrogen which is regarded by most persons as one of the most abundant of the elements is almost the least abundant of those mentioned.

THE MINERALS OCCURRING IN ROCKS.

=25. The Soil=, as before stated, being comprised almost exclusively of decayed rocks, its characteristics would naturally be determined by the character of the minerals contained in the rocks.

A rock may be composed of a single mineral or an aggregation of several minerals.

According to the authority of the National Museum[20] it may occur, either in the form of stratified beds, eruptive masses, sheets or dikes, or as veins and other chemical deposits of comparatively little importance as regards size and extent. The mineral composition of rocks is greatly simplified by the wide range of conditions under which the commonest minerals can be formed. Thus quartz, feldspar, mica, the minerals of the hornblende, or pyroxene group, can be formed from a mass cooling from a state of fusion; they may be crystallized from solution, or be formed from volatilized products. They are therefore the commonest of minerals and are rarely excluded from rocks of any class, since there is no process of rock formation which determines their absence.

Most of the common minerals, like the feldspars, micas, hornblendes, pyroxenes, and the alkaline carbonates possess the capacity of adapting themselves to a very considerable range of compositions. In the feldspars, for example, lime, soda, or potash may replace one another almost indefinitely, and it is now commonly assumed that true species do not exist, but all are but isomorphous admixtures passing into one another by all gradations, and the names albite, oligoclase, anorthite, etc., are to be used only as indicating convenient stopping and starting points in the series. Hornblende or pyroxene, further, may be pure silicate of lime and magnesia, or iron and manganese may partially replace these substances. Lime carbonate may be pure, or magnesia may replace the lime in any proportion.

These illustrations are sufficient to show the reason for the great simplicity of rock masses as regards their chief mineral constituents.

Whatever may be the conditions of the origin of a rock mass, the probabilities are that it will be formed essentially of one or more of a half a dozen minerals in some of their varieties.

But however great the adaptability of these few minerals may be they are, nevertheless, subject to very definite laws of chemical equivalence. There are elements which they cannot take into their composition, and there are circumstances which retard their formation while other minerals may be crystallizing. In a mass of rock of more or less accidental composition formed under these widely varying conditions it may, therefore, be expected that other minerals will form, in considerable numbers, but minute quantities. It is customary to speak of those minerals which form the chief ingredients of any rock, and which may be regarded as characteristic of any particular variety, as the essential constituents, while those which occur in but small quantities, and whose presence or absence does not fundamentally affect its character, are called accessory constituents. The accessory mineral which predominates, and which is, as a rule, present in such quantities as to be recognizable by the unaided eye, is the characterizing accessory. Thus a biotite granite is a stone composed of the essential minerals quartz and potash feldspar, but in which the accessory mineral biotite occurs in such quantities as to give a definite character to the rock.

=26. Classification Of Minerals.=—The minerals of rocks may also be conveniently divided into two groups, according as they are products of the first consolidation of the mass or of subsequent changes. This is the system here adopted. We thus have:

(1) The original or primary constituents, those which formed upon its first consolidation. All the essential constituents are original, but on the other hand all the original constituents are not essential. Thus, in granite, quartz and orthoclase are both original and essential, while beryl and zircon or apatite, though original, are not essential.

(2) The secondary constituents are those which result from changes in a rock subsequent to its first consolidation, changes which are due in great part to the chemical action of percolating water. Such are the calcite, chalcedony, quartz, and zeolite deposits which form in the druses and amygdaloidal cavities, of traps and other rocks.

Below is given a list of the more common, original and secondary minerals occurring in rocks. It will be observed that the same mineral may, in certain cases, occur in both original and secondary forms. The tables following were prepared by Dr. George P. Merrill.

2a. Orthoclase. Anhydrous silicate of alumina with varying amounts of lime, potash, or soda and rarely barium. 2b. Microcline. „ 2c. Albite. „ 2d. Oligoclase. „ 2e. Andesite. „ 2f. Labradorite. „ 2g. Bytownite. „ 2h. Anorthite. „

3a. Hornblende. Anhydrous silicates of lime and magnesia with iron and alumina in the dark varieties. 3b. Tremolite. „ 3c. Actinolite. „ 3d. Arfvedsonite. „ 3e. Glaucophane. „ 3f. Smaragdite. „

4. The Monoclinic Pyroxenes:

4a. Malacolite. Anhydrous silicates of magnesia and lime with alumina and iron in the dark varieties. 4b. Diallage. „ 4c. Augite. „ 4d. Acmite. „ 4c. Aegerite. „

5. The Rhombic Pyroxenes:

5a. Enstatite (bronzite). Silicates of magnesia and iron. 5b. Hypersthene. „

6a. Muscovite. Anhydrous silicates of alumina with potash, soda, and iron. 6b. Biotite. „ 6c. Phlogopite. „

13. Garnet, variable common form.

24. Elaeolite and Nepheline.

27. The Sodalite Group:

33. Titanite (sphene).

41. Halite (common salt).

44. The Metallic Sulfids:

3. The Amphibole Group:

4. Muscovite (sericite).

6. Calcite (and aragonite).

ROCKS AND ROCK DECAY.

=27. Types Of Rocks.=—Rocks may be divided in reference to their structure into four types: First, crystalline; second, vitreous; third, colloidal; fourth, fragmental.

Of these classes there may be selected, as types of the first order, granite and crystalline limestone.

The second class is typically represented by obsidian. Rocks of this kind are confined to a volcanic origin.

The third class of rocks is completely amorphous in its structure and is less common than the others. It is found only in rocks of chemical origin. Types of this class are the siliceous sinters, opals, flint nodules, and many serpentines.

Of the fourth class of rocks, sandstone is typical, being comprised wholly of fragments of rocks pre-existing. The particles may be held together either by cohesion or by a cement composed of silica, iron oxids, carbonate of lime or clayey matter.

=28. The Microscopical Structure of Rocks.=—A great deal more light is thrown upon the nature of rock materials by microscopical study than by their study in bulk. The requisites for a microscopical study of rock are that the material should be cut into extremely thin laminae with parallel sides and polished so as to transmit the light freely. The study of the crystalline structure of the material is then conducted by means of a microscope furnished with polarizing and analyzing appliances. The light before passing through the mineral film is polarized by a Nicol prism. After passing through the film it is analyzed by a second Nicol prism. In this way the crystalline structure of the rock as affecting polarized light is distinctly brought out. The thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an inch.

The method of rock study by thin microscopic sections is one of comparatively recent origin. It is scarcely more than a dozen years since the process was fairly adopted by mineralogists. The value of the method is based upon the fact that every crystalline mineral has certain definite optical properties. Therefore, when a crystalline mineral is distorted or misshapen so as to be incapable of identification by the ordinary method, it can be at once identified by its optical examination in the manner just described. In this way not only can one mineral be distinguished from another, but the crystalline system to which it belongs can be accurately pointed out. The value of the method is well summed up by Merrill,[21] who says that it is not merely an aid in determining the mineralogical composition of a rock, but also, which is often much more important, its structure and the various changes which have taken place in it since its first consolidation. Rocks are not the definite and unchangeable mineral compounds they were once considered, but are rather ever varying aggregates of minerals which even in themselves undergo structural and chemical changes almost without number.

Another valuable result of such a study is illustrated by the discovery that the structural features of a rock are not dependent upon its chemical composition or geologic age, but upon the conditions under which it cooled from the molten magma. Portions of the same rock may vary all the way from a wholly crystalline to a pure vitreous form.

Some typical microstructures of crystalline rocks are shown in the accompanying figures 1–6.[22]

Although this method of study has thus far been confined mainly to crystalline rocks, its efficiency is by no means limited to them. The fragmental rocks and their decomposed débris to which the name soil is given are equally worthy of study by this method. Indeed, the full value of a chemical analysis of any rock or soil can not be ascertained unless such an analysis is accompanied by a microscopic examination. It is desirable to know not merely what there is in any soil, but in what form these compounds exist. To this latter question the chemical analysis as ordinarily made will give no clew. In Germany a beginning has been made in this line of work, and American scientists are beginning to realize its importance. An outline of this method of analysis will be given in the proper place.

=29. Specific Gravity.=—Much information in regard to the properties of a rock, or mineral constituent thereof, may be derived from its specific gravity.

The internal structure of a rock may have much to do with its apparent specific gravity. As an instance of this, it may be stated that an obsidian pumice will float upon water, buoyed up by the air contained in its vesicles, while a compact obsidian of the same composition will sink immediately. A careful discrimination must, therefore, be made between apparent and true specific gravity. In general it may be said that crystalline rocks have a higher specific gravity than those of a vitreous nature. The specific gravity is, therefore, largely dependent upon chemical and crystallographic properties; for instance, among siliceous rocks those which contain the largest amount of silica are the lightest, while those with a comparatively small amount, but rich in iron, lime, and magnesia, are heaviest.

=30. Chemical Composition of Rocks.=—Rocks are often classified with respect to the chief mineral constituent which they contain. Rocks which are composed largely of lime are termed calcareous; of silica, siliceous; of iron, ferruginous; and of clay, argillaceous. In respect of eruptive rocks, it is customary to speak of those which show above sixty per cent of silica as acidic, while those containing less than fifty per cent of silica and a correspondingly larger amount of iron, lime, and magnesia, are spoken of as basic. Illustrations of the classification of rocks on the above principles are given below.[23]

STRATIFIED ROCKS. ──────────────────────────┬──────────────┬───────────────────────────── Kind. │ Specific │ Composition. │ Gravity. │ ──────────────────────────┼──────────────┼───────────────────────────── Calcareous: │ │ Compact limestone │ 2.6 to 2.8 │ Carbonate of lime. Crystalline limestone │ „ │ „ │ │ Compact dolomite │ 2.8 to 2.95 │ Carbonate of lime and │ │ magnesia. Crystalline dolomite │ „ │ „ │ │ Siliceous: │ │ Gneiss │ 2.6 to 2.7 │ Same as granite. Siliceous sandstone │ 2.6 │ Mainly silica. Schist │ 2.6 to 2.8 │ 60 to 80 per cent silica. │ │ Argillaceous: │ │ Clay slate (argillite) │ 2.5 │ Mainly silicate of alumina. ──────────────────────────┼──────────────┼───────────────────────────── │ │ ERUPTIVE ROCKS. ──────────────────────────┬──────────────┬───────────────────────────── │ Specific │ Per cent silica. │ Gravity. │ ──────────────────────────┼──────────────┼───────────────────────────── Acidic Group: │ │ Granite │ 2.58 to 2.73 │ 77.65 to 62.90 Liparite │ 2.53 to 2.70 │ 76.06 to 67.61 Obsidian │ 2.26 to 2.41 │ 82.80 to 71.19 Obsidian pumice │ Floats on │ 82.80 to 71.19 │ water. │ │ │ Intermediate Group: │ │ Syenite │ 2.73 to 2.86 │ 72.20 to 54.65 Trachyte │ 2.70 to 2.80 │ 64.00 to 60.00 Hyalotrachyte │ 2.4 to 2.5 │ 64.00 to 60.00 Andesite │ 2.54 to 2.79 │ 66.75 to 54.73 │ │ Basic Group: │ │ Diabase │ 2.66 to 2.88 │ 50.00 to 48.00 Basalt │ 2.90 to 3.10 │ 50.59 to 40.74 Peridotite │ 3.22 to 3.29 │ 42.65 to 33.73 Peridotite (iron rich) │ 3.86 │ 23.00 Peridotite (meteorite) │ 3.51 │ 37.70 ──────────────────────────┴──────────────┴─────────────────────────────

=31. Color Of Rocks.=—The color of rocks is determined chiefly by the oxids of metals which they contain and the degree of oxidation of the mineral in each particular case. There are, however, many colors of rocks which seem to depend not upon any particular mineral ingredient which they contain, but upon some particular crystalline structure or physical condition.

The chief coloring matters in minerals are those which form colored bases such as iron, manganese, chromium, etc. The yellow, brown, and red colors, common to fragmental rocks, are due almost wholly to free oxids of iron. The gray, green, dull brown, and even black colors of crystalline rocks are due to the presence of free iron oxids or to the prevalence of silicate mineral rich in iron, as augite, hornblende, or black mica. Rarely copper and other metallic oxids than those of iron are present in sufficient abundance to impart their characteristic hues. As a rule, a white or light-gray color denotes an absence of an appreciable amount of iron in any of its forms. The bluish and black colors of many rocks, particularly the limestones and slates, are due to the presence of carbonaceous matter.

In still other cases, and particularly the feldspar-bearing rocks, the color may be due in part to the physical condition of the feldspar.

Inasmuch as the color of rocks is due so largely to metallic oxids, it is easy to see that they may undergo changes when exposed to weathering, or the degree of oxidation may change, and either, together with changes in the physical structure of the rock, may cause a distinct change in color. Luster is often considered in connection with color, and is due almost exclusively to physical conditions.

=32. Kinds of Rocks.=—The rocks which form any essential part of the earth’s crust are grouped under four main heads, the distinction being based upon their origin and structure.[24] Each of the main divisions may be subdivided into groups or families, the distinction being based mainly upon chemical composition, structure, and mode of occurrence. The four chief families are:

First, aqueous rocks, formed mainly through the agency of water as chemical precipitates or as sedimentary beds.

Second, aeolian rocks formed from wind-drifted materials.

Third, metamorphic rocks, changed from their original condition through dynamic or chemical agencies, and which may have been partly of aqueous and partly of igneous origin.

Fourth, igneous or eruptive rocks, which have been brought up from below in a molten condition, and which owe their present structural peculiarities to variations in composition and conditions of solidification.

=33. Aqueous Rocks.=—Aqueous rocks may be divided into the following general classes:

First, rocks formed as chemical precipitates.

Second, rocks formed as sedimentary deposits and fragmental in structure. The second class may again be subdivided into rocks formed by mechanical agencies and mainly of inorganic materials; and second, rocks composed mainly of the débris of plant and animal life.

In regard to the first form of aqueous rocks, namely, those formed as chemical precipitates, it may be said that while their quantity is not large they are yet of considerable importance from an agricultural point of view. They embrace those substances which, having once been in a condition of vapor or aqueous solution, have been deposited or precipitated, either by cooling or by the evaporation of the liquor holding them in solution, or by coming in contact with chemical substances capable of precipitating them. The influence of water as a solvent is perhaps not fully appreciated. Its solvent influence will be noted particularly under the head of weathering or decay of rocks. Its importance, however, in producing stratified rocks has been very great. Water, especially when under great pressure and at a high temperature, has the power of dissolving many minerals. This power is often greatly increased by the mineral matter previously in solution in the water or by the gases which it may contain. As an illustration of the latter property, the solvent action of water charged with carbon dioxid on limestone may be cited.

When mineral matters have been dissolved by the water in the ways mentioned and carried with the water beyond the condition where the solution has taken place, new conditions are found favorable to the precipitation of the dissolved matters. The water, which before may have been very hot, may reach a place where it cools, and being a supersaturated solution, the excess of the material is thrown down as the water cools.

On the other hand, if the solution be due to the presence of carbon dioxid and the water reach a place where it is exposed to the air or where the pressure under which the abundance of the gas has been due is diminished, the carbon dioxid will escape and the mineral matters which have been dissolved thereby will be precipitated.

The incrustations which often appear round the mouth of springs and the occurrence of stalagmites and stalactites in caves are illustrations of this action.

In respect of the formation of rocks as precipitates from a state of vapor we have scarcely any illustrations excepting in volcanic regions. Rocky materials with which we are generally acquainted are practically non-volatile at the highest temperature which can be secured on the earth’s surface, but it is possible that in the interior of the earth the temperature may be so high as to maintain many substances in a state of vapor.

They may, in this case, become disassociated so that the compounds or elements exist distinctly in a vaporous condition. Such a vapor transported to regions of diminished temperature would first of all on cooling permit a union of the chemical elements forming new compounds less volatile, which, of course, would be at once precipitated.

The rocks and minerals formed in this way which are of some agricultural importance may be classified as follows:

Oxids, carbonates, silicates, sulfur, sulfids, sulfates, phosphates, chlorids, and hydrocarbon compounds, the most important from an agricultural point of view being the phosphates.