As a result of all this fertility of combination, the total number of silicious minerals in igneous rocks is large. It is the function of the mineralogist to treat of these minerals as such. The geologist deals with them as constituents of the earth and as factors in its history. Only a few of them are so abundant as to require special individual notice in a general study of the earth. It may be remarked also that only a few of them can be identified by simple inspection as they occur in the rocks, partly because of the delicacy of the distinctions between many of them, and partly because of their minuteness and intricate intermixture. The resources of the polarizing microscope are necessary for safe determination in most cases. The student need not feel embarrassment or discouragement if he is often unable to recognize the constituents of the intimately crystalline rocks. Their determination has grown to be a profession by itself.
The leading minerals of igneous rocks.—Fortunately for the simplicity of geological study, a few minerals make up the great mass of the igneous rocks. These few are quartz, the feldspathic minerals, the ferromagnesian minerals, and the iron oxides. Quartz (silica, SiO2) is the free acid already mentioned. The feldspathic and ferromagnesian minerals are the leading silicates of the earth’s crust, and vastly surpass all others in abundance. The feldspathic group embraces minerals formed by silica in union with alumina, together with either potash, soda, or lime, or two or more of these together. The ferromagnesian group embraces minerals formed by the union of silica with iron, magnesia, and lime, together with more or less of the other basic oxides. These statements are only true in a very general sense. Admixtures, replacements, and impurities are so frequent as to break down all sharp, simple definitions. The feldspathic minerals are normally light in color, ranging from white to red or gray. The ferromagnesian minerals are normally dark (commonly greenish) from the presence of iron, the great coloring element of rocks. But these color distinctions do not hold good in detail and cannot be much trusted as a means of identification.
The feldspathic minerals ([p. 462]) embrace the potash feldspars, orthoclase and microcline; the soda feldspar, albite; the lime feldspar, anorthite; and the mixed feldspars intermediate between albite and anorthite, viz., the soda-lime feldspar, oligoclase, the lime-soda feldspar, andesine, in which lime and soda are nearly equal, and the lime-soda feldspar, labradorite, in which the lime predominates; together with leucite, a potash silicate higher in alkali than orthoclase, and nephelite, a soda silicate higher in soda than albite. Leucite and nephelite are usually classified as feldspathoids, not as feldspars. It is to be understood that alumina is normally present in all these. Additional details respecting these minerals may be found in the reference list, [p. 460].
Among the ferromagnesian minerals the most important are the pyroxenes, the amphiboles, and the biotite type of mica. Olivine is of subordinate importance. The pyroxenes ([p. 465]) and amphiboles ([p. 460]) have nearly the same chemical composition, but differ in crystallization and physical properties. Hornblende (an amphibole) has been melted, and on cooling under proper conditions found to take on the form of augite (a pyroxene). Pyroxene is sometimes altered into uralite, one of the amphiboles. The pyroxenes and amphiboles are the most abundant of the dark minerals in crystalline rocks. The leading members of the pyroxene group are augite, diallage, hypersthene, enstatite, and soda pyroxene. The chief members of the amphibole group are hornblende and the soda amphiboles. All are essentially silicates of magnesia and iron oxide, with or without the addition of lime, soda, and alumina. Details respecting these may be found in the reference list.
The two leading micas are the iron-magnesia mica, biotite, and the potash mica, muscovite, the familiar “isinglass” of the stove-door. Chemically, muscovite should go with the potash feldspars, but it is distinguished from them by its crystalline habit and physical properties. The biotite should go chemically with the pyroxenes and amphiboles, which it closely resembles except in its crystalline properties. Details respecting the micas may be found in the reference list, [p. 464].
Two iron oxides, magnetite (Fe3O4) and hematite (Fe2O3) are widely disseminated in igneous rocks. They constitute the free bases already mentioned.
Summary of salient facts.—The salient facts are, therefore, (1) that out of the seventy-odd chemical elements in the earth, eight form the chief part of it; (2) that one of these elements uniting with the rest forms nine leading oxides; (3) that one of these oxides acts as an acid and the rest as bases; (4) that by their combination they form a series of silicates of which a few are easily chief; (5) that these silicates crystallize into a multitude of minerals of which again a few are chief; and (6) that these minerals are aggregated in various ways to form rocks. Possessed of these leading ideas, we are prepared to turn to the consideration of some of the conditions under which these combinations take place in the formation of rocks from molten magmas.
THE NATURE OF MOLTEN MAGMAS.
We easily fall into the habit of thinking of molten rock as we think of a molten metal, merely as a substance which has passed from the solid to the liquid condition because of high temperature. With the return of low temperature a molten metal returns to the solid state usually in the same molecular condition which it possessed before. The point of fusion and the point of solidification are the same and are rigidly fixed. If this were true of the constituents of a rock, a definite order for the solidification of the several minerals might be anticipated. As a matter of fact, the order is not the same under all conditions, and, what is especially significant, the order is far from being that in which the constituents would fuse or would solidify separately. For instance, in a granite composed chiefly of quartz, feldspar, and mica, the quartz is often the last to take form, although it is more infusible than the feldspar or the mica. This and other phenomena show that a molten magma is not to be viewed simply as a fused substance, but rather as a solution of one silicate in another, or as a solution of several silicates in one another mutually. The high temperature is to be regarded merely as a condition prerequisite to solution, or as the condition of fusion of some one constituent which then dissolves the others. If crystals of snow, sugar, and salt be mixed at a low temperature and compacted, the mass may be regarded as an artificial rock. On raising the temperature, all will pass into solution while the temperature is still somewhat below the melting-point of the snow, the most fusible, and while it is much below that of either the sugar or the salt. This particular case is instructive because the ice is not simply fused by temperature; the affinity of the salt plays a part. If the temperature were again lowered, the sugar and salt would not crystallize out at their fusing-points, but would remain in solution down to and even below the normal freezing-point of water; in other words, they would remain in solution until the water crystallized out and forced them to take the solid state. This holds good when the amounts of the sugar and salt are small relative to the water. If, on the contrary, their quantity is large relatively, crystallization will take place at higher temperatures and before the water crystallizes to ice. From this it appears that the salt and sugar might crystallize either before the water or after it, according to the degree of concentration. The behavior of mixtures of minerals in passing into and out of the molten condition appears to be quite analogous to this, and hence a great variety of results attend the process, dependent upon the number, the nature, and the relative quantities of the ingredients. The approved conception of the genesis of a rock from a molten magma (when ample time is given) is that one compound after another crystallizes out as the temperature falls and its point of saturation for each is reached, until the whole has been solidified. The modes of combination of the elements in the molten magma are not necessarily the same as those in the derivative crystals; indeed, the combinations doubtless change as the process proceeds; certain constituents being taken out, the remaining ones probably rearrange themselves.
Time required in crystallization.—The liquid magma of igneous rocks is essentially a fluid glass or slag. It is analogous to common glass, which is a silicate of potash, soda, or other base, except that usually common glass is relatively free from iron and other coloring substances, while these abound in the natural magmas and render them dark and more or less opaque; but the fundamental nature is the same, except that the natural lavas are usually mixtures of several silicates, while the artificial glasses consist of only one, or at most a few. Furnace slag is essentially an artificial lava.