Fig. 453.—Crystals of the fifth system.
The sixth, or Triclinic system, or the doubly oblique. In this system we have three axes differing in length, and all forms which can be arranged about these unequal and oblique axes. Sulphate of copper will be found in this group. The system has been called anorthic, or triclinic, because the axes are unequal and inclined, as in the oblique prism based upon an obliqued angled parallelogram. Axinite crystal, as annexed, will show one form in this system.
Fig. 454.—Sixth system.
As may be gathered from the foregoing, it is not easy to determine a crystalline form with certainty,—a great part of the crystal may be invisible. A crystalline mass is a mineral, which consists of an arrangement of crystals heaped together. If it does not possess these the mineral is amorphous, or shapeless. We will now endeavour to describe some of the physical characteristics of minerals.
Fig. 455.—Wollaston’s Goniometer, an instrument for measuring the angles of crystals.
The Goniometer (see fig. 455) is the instrument used for measuring the angles of crystals. Wollaston’s reflecting instrument is most generally used. It consists of a divided circle, graduated to degrees, and subdivided with the vernier. The manner of working is easy, though apparently complicated. The vernier is brought to zero, when an object is reflected in one face of the crystal. The crystal is turned till the same object is viewed from another face. The angle of reflection is then measured, and can be read off on the circle.
We have already referred to the physical characteristics of the minerals, and one of these attributes is cohesion. When we find a substance is difficult to break, we say it is “hard.” This means that the cohesion of the different particles is very great. Minerals vary in hardness; some are extremely difficult to act upon by force, and a file appears useless. At the other side we find some which can be pricked or scratched with a pin; and these degrees of hardness being put as extremes, we can in a manner relatively estimate the hardness of all other minerals. We can test this by scratching one against another; whichever scratches the other is the harder of the two, and thus by taking up and discarding alternately, we can at length arrive at a comparative estimate of the hardness of all. Such a scale was arrived at by Mohs, and arranged in the following order. The softest mineral comes first:—
- Talc.
- Gypsum (rock-salt).
- Calcareous spar.
- Fluor-spar.
- Apatite-spar.
- Felspar.
- Quartz.
- Topaz.
- Corundum.
- Diamond.