Common Minerals and Rocks

by William O. Crosby

(3) Siliceous Group.—These rocks are composed of pure silica in the forms of quartz and opal. When first deposited, whether organically, like tripolite, or chemically, like siliceous tufa, the siliceous rocks are soft and light, and the silica is in the form of opal. Subsequently it changes to quartz, and the rocks assume the much harder and denser forms of chert and novaculite, respectively.

Tripolite or Diatomaceous Earth.—This interesting rock is soft, light, and looks like clay; but it is lighter, and the argillaceous odor is faint or wanting. It does not effervesce with acid. Hence, it is neither clay nor chalk. Notwithstanding its softness, it is really composed of a hard substance, viz., silica, in the form known as opal. By rubbing off a little of the dust, and examining it under the microscope, we easily prove that the silica is mainly or entirely of organic origin; for the dust is seen to be simply a mass of more or less fragmentary organic remains, occurring in great variety, and of wonderful beauty and minuteness. There are few rocks so unpromising on the exterior, and yet so beautiful within. We have already learned that these organic bodies are principally Diatom cases, Radiolaria shells, and Sponge spicules. We can form some idea of their minuteness from Ehrenberg’s estimate that a single cubic inch of pure tripolite contained no less than 41,000,000,000 organisms.

The lightness of tripolite (sp. gr., 1-1.5) is due to the facts that opal is a light mineral (sp. gr., 1.9-2.2), and that many of the shells are hollow. Tripolite is a good example of a soft rock composed of a hard mineral; and it owes its value as a polishing material to the fact that it consists of a hard mineral in an exceedingly fine state of division. Tripolite, when pure, is snow-white; but it is rarely pure, being commonly either argillaceous or calcareous. This rock is now forming in thousands of places, in both fresh water and the ocean.

Flint and Chert.—During the course of geological time, beds of tripolite are gradually consolidated, chiefly by percolating waters, which are constantly dissolving and re-depositing the silica; and, finally, in the place of a soft, earthy rock, we get a hard, flinty one, which we call flint if it occurs in the newer, or chert if it occurs in the older, geological formations. Besides forming beds of nearly pure silica, which we call tripolite, the microscopic siliceous organisms are diffused more or less abundantly through other rocks, especially chalk and limestone. In such cases the consolidation of the silica implies its segregation also; i.e., the silica dissolved by percolating water is deposited only about certain points in the rock, building up rounded concretions or nodules. Thus, a siliceous limestone becomes, by the segregation of the silica, a pure limestone containing nodules of chert, which are usually arranged in lines parallel with the stratification. Specimen 16 is a fragment of a flint-nodule from the chalk-formation of England.

Siliceous Tufa.—Hot water, and especially hot alkaline water, circulating through the earth’s crust, is always charged with silica dissolved out of the rocks; and when such water issues on the surface in a hot spring or geyser, it is cooled by contact with the air, its solvent power is diminished thereby, and a large part of the silica is deposited around the outlet as a snowy-white porous material called siliceous tufa. Silica deposited in this way is, like organic silica, always in the form of opal. Siliceous tufa is distinguished from clay, slate, chalk, and limestone by the same tests as tripolite, and from tripolite itself by the absence of microscopic organisms.

Novaculite.—Through the action of percolating water and pressure, siliceous tufa, like tripolite, becomes harder and denser and is thus changed to novaculite, which holds the same relation to chemically deposited silica that chert and flint do to organically deposited silica. The white novaculite obtained at the Hot Springs of Arkansas, and commonly known as Arkansas stone, is a typical example of this rock. The rock which, on account of the use to which it is put, is known as buhr-stone, is also an excellent example of chemically deposited silica. It is usually somewhat porous and fossiliferous.

(4) Calcareous Group.—These are the lime-rocks, including the carbonate of lime, in limestone and dolomite, the sulphate of lime, in gypsum, and the phosphate of lime, in phosphate rock. These rocks are even more closely connected in origin than in composition; and it is for this reason that rock-salt, which of course contains no lime, is also included in this group. Limestones are formed abundantly in the open sea, through the accumulation of shells and corals; but when portions of the sea become detached from the main body and gradually dry up, like the Dead Sea and Great Salt Lake, dolomite, gypsum, and rock-salt are deposited in succession as chemical precipitates. Phosphate rock may be regarded as a variety of limestone, resulting from the accumulation of the skeletons and excrement of the higher animals.

Limestone.—This is the lithologic or rock form of carbonate of lime or calcite, and one of the most important, interesting, and useful of all rocks. Although so simple in composition,—calcite being the only essential constituent,—limestone embraces many distinct varieties, and is really equivalent to a whole family of rocks. A highly fossiliferous limestone, such as specimen 38, is, perhaps, the best variety with which to begin the study of the species. The softness of the fossil shells of which the rock is so largely composed, and the fact that they effervesce readily with dilute acid, proves that they are still carbonate of lime; and by applying the acid more carefully, it can be seen that the compact matrix of the rock also effervesces, consisting of shells more finely broken or comminuted and mixed with more or less clay and other impurity, almost the entire rock being of organic origin.

On the coast of Florida, and in many other places, we find beautiful examples of shell-limestone now in process of formation. These are at first very open and porous, because the interstices between the nearly entire shells are not yet filled up with smaller fragments and sand. But when that is done, we shall have a rock similar to the old fossiliferous limestone. Specimen 37.

The shells and fragments, and the grains of calcareous sand, are, as a rule, quickly cemented together by the deposition of carbonate of lime between them; so that limestone is nowhere observed occurring abundantly in an unconsolidated form.