Mineralogy

Window-glass may be used in an emergency as a substitute for apatite, and flint for quartz.

The hardness test may also be made by endeavoring to scratch the specimens enumerated ‘in the list with the mineral under examination. If, for example, the mineral scratches orthoclase felspar but does not scratch quartz, it has a hardness between 6 and 7. A greater precision is sometimes attempted by giving the hardness as 6 ¼, 6 ½, 6 ¾, according to whether the mineral in question approaches more nearly to felspar or quartz in hardness.

Hardness may also be tested by means of a penknife or even the finger-nail, the former scratching up to about 6 ½, the latter up to 2 ½. Finger-nails, however, vary in hardness.

Several precautions are to be observed in testing hardness. A definite scratch must be produced in the softer mineral and this is best seen by blowing away (or licking away, if the observer cares to) the powder produced by scratching and then examining the place with a lens. A softer mineral drawn across a harder mineral often produces a whitish stripe which may be mistaken for a scratch in the harder mineral; in the same way an attempt to scratch harder minerals with the knife produces a steel mark on them. Granular specimens may give a kind of scratch by the breaking out of the mineral grains. Finally, it is of course necessary that a fresh surface, that is, one not coated with decomposition products or the like, of the mineral is subjected to the hardness test.

During the hardness trial, the color of the powder produced by the scratch is observed, this giving the streak of the mineral.

Tenacity

Minerals possess certain properties dependent upon their tenacity, of which the following are the most important:

1. Sectility - A mineral is said to be sectile when it can be cut with a knife and the resulting slice breaks up under a hammer. Examples: graphite, steatite, gypsum.
2. Malleability - A mineral is malleable if a slice cut from it flattens out under a. hammer. Examples: native gold, silver and copper.
3. Flexibility - is the property of bending. In some minerals it can be observed by experimenting with their plates or lamina, only. A flexible mineral remains bent after the pressure is removed. Examples: talc, selenite, etc.
4. Elasticity - as the term is usually employed in mineralogy differs from flexibility in the fact that the portion bent springs back to its former position. Mica yields flexible elastic plates, whilst the somewhat similar mineral, chlorite, gives plates that are flexible but not elastic.
5. Brittleness - is a character common to many minerals and is shown by their crumbling or flying to powder instead of yielding a slice. Examples: iron pyrites, apatite and fluor-spar.

Fracture

It is very important to note the characters of the fractures displayed on the broken or chipped surfaces of minerals. It is equally important to distinguish between the smooth flat surfaces resulting from what is called the cleavage of a mineral, and the irregular surfaces characterizing true fracture, these latter being totally independent of cleavage. Whilst the fracture is an important diagnostic character and, further, a recent fracture reveals the true color of certain minerals, it is unwise to break or chip good crystals, as crystalline form is a far more valuable and constant a character by which to determine a mineral than its color and, in many cases, than its fracture.

Fracture is said to be:

1. Conchoidal -The mineral breaks with a curved concave or convex fracture. This often shows concentric and gradually diminishing undulations towards the point of percussion, somewhat are resembling the lines of growth on a shell. Conchoidal fracture is well shown by quartz, flint and natural glasses.
2. Even - The fracture-surface is flattish or nearly fiat, as in chert.
3. Uneven - The fracture-surface is rough by reason of minute elevations and depressions. Most minerals have an uneven fracture. (4) Hackly.-The surface is studded with sharp and jagged elevations, as in cast-iron when broken.
4. Earthy.-As in the fracture of chalk, meerschaum, etc.

Cleavage

The tendency to split along certain definite planes-the cleavage-planes-possessed by many minerals is closely related to crystalline form and the internal structure of the crystal. In each cleavable mineral, the directions of the cleavage-planes are parallel to a certain face or to certain faces of a form in which the mineral may more closely packed together or the mutual electrical charges are greater than in directions at right angles to the cleavage-plane. This plane, therefore, is a plane of least cohesion and hence splitting or cleavage easily occurs along it. It is important, as already stated, to distinguish between fracture and cleavage, as the former is irregular and not connected with the crystalline structure of the mineral. Substances with no crystalline structure, that is, amorphous substances, show no cleavage. Certain rocks, such as slate, which split readily into thin sheets are said to be cleaved, but this property of slaty cleavage, as it is best called, is the result of recrystallization produced by pressure and has no connexion with the cleavage which exists in minerals.

Minerals may cleave in one, two, three or more directions, but one cleavage is generally to be obtained with greater ease than the others. Cleavage is described by stating the crystallographic direction followed by the cleavage-planes and the degree of perfection shown by such planes. With regard to the latter, cleavage is described as perfect or eminent, good, distinct, poor, indistinct, difficult, etc. As examples of minerals with perfect cleavage, we may give fluor-spar, galena, calcite, and mica. Fluor-spar commonly crystallizes in cubes; if such a cube is taken and tapped with a hammer it will be found to cleave along planes truncating the corners of the cube, and if this cleaving is done in a regular wayan octahedron is produced. Fluor-spar is said, therefore, to have a perfect octahedral cleavage, and to give octahedra as its cleavage fragments. Galena, which also crystallizes in cubes, cleaves parallel to the faces of the cube, so that its cleavage is cubic and its cleavage-fragments are cubes. Calcite, no matter what shapes its crystals are, produces rhombohedral cleavage-fragments on being crushed. Mica possesses one perfect cleavage parallel to which exceedingly thin sheets of the mineral may be split off.
Cleavage is a very important property in the recognition of minerals, both in the hand-specimen and, as is shown later, under the microscope.

Gliding-planes and secondary twinning are related to cleavage, and are produced in a mineral by pressure. For example, during the preparation of a thin slice of calcite for examination under the microscope, the pressure of grinding the mineral may cause it to show an excellent cleavage and some secondary twinning. Twinning is discussed in later pages. The secondary twin-planes and the gliding planes are often planes along which the mineral separates fairly readily-such planes are called partings.

Surface Tension Effects

The difference in adhesive power of various liquids to different minerals has formed the basis for numerous processes of ore separation and concentration. The surface tension between various metallic sulphides and oil is greater than that between the gangue minerals quartz, calcite, etc., and the same medium. In the original Elmore Process a paste of sulphide and gangue was mixed with oil and water and agitated; the oil separated into a layer above the water and carried the sulphides with it. Somewhat the same principle underlies the method of extracting diamonds from their matrix, blueground, by causing them, to adhere to grease upon shaking tables. The various Flotation Processes depend on surface tension. In these, bubbles of gas or air attach themselves to say, fine-powdered zinc blende, agitated in oil-mixtures, and float this mineral to the surface, leaving other sulphides and gangue material at the bottom of the liquid. By varying the conditions of flotation clean separations of various ore-minerals can be produced and in this way the working of mixed ores has been made economically possible.