CHAPTER II.
HOW TO TEST FOR MINERALS.
When the mineralogist wishes to know the names of the specimen he holds in his hand, he, in the case of a mineral difficult to determine, considers all the following properties:
- Crystalline form and structure,
- Cleavage,
- Fracture,
- Tenacity,
- Hardness,
- Specific Gravity as compared with that of water,
- Luster,
- Color and Streak,
- Transparency or otherwise,
- Taste,
- Odor,
- Chemical Composition tested by analysis,
- Pyrognostic characters as determined by the use of the blowpipe,
- Mode of occurrence and associated minerals.
- Crystalline Form and Structure. Unfortunately the science of
crystallography is extremely complicated and long study is necessary
to master it; once acquired, however, it is of paramount usefulness to
the student. According to Dana there are six systems, to one of which
every crystal may be referred. They are:
- (1) Isometric; (2) Tetragonal; (3) Hexagonal or Rhombohedral; (4) Orthorhombic; (5) Monoclinic; (6) Triclinic.
In the isometric system there are three equal axes at right angles to each other.
In the tetragonal system there are three axes at right angles to each other. Two of these are equal, while the third, or vertical angle, is longer or shorter.
There are two divisions of the hexagonal system; the hexagonal system properly so-called, and its rhombohedral division. All forms are referred to four axes, three equal axes inclined to each other at angles 60 degrees in a common horizontal plane, and a fourth vertical axis at right angles, and longer or shorter. The rhombohedral division comprises crystals having but three planes of symmetry, intersecting at angles of 120 degrees in the vertical axis. They are regarded as half forms of the corresponding hexagonal crystals.
In the orthorhombic system there are three unequal axes at right angles to each other.
In the monoclinic system there are three unequal axes, of which one, the lateral axis, is inclined to the vertical, while the angles between the others are right angles.
In the triclinic system there are three unequal axes and these intersections are all oblique. The student who wishes to pursue this subject further should consult Dana's System of Mineralogy.
Physical Mineralogy. Cleavage is the line of easiest separation in a mineral. It may be perfect, imperfect, interrupted, etc.