Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/396

Rh 378 MINERALOGY Characters depending on Cohesion. These characters are of five kinds: (1) hardness, .(2) tenacity, (3) elasticity, (4) cleavage, (5) fracture. All may be considered as related to the power of resisting attempts to separate one part from another. Hardness. 1. Hardness. A harder body is distinguished from a softer, either by attempting to scratch the one with the other, or by trying each with a file. Each of these methods is used by the mineralogist in determining the hardness of the species, though the latter is in most cases to be preferred. Both methods should be employed when practicable. Certain varieties of some minerals give a low hardness under the file, owing either to impurities or imperfect aggregation of the particles, while they sci atch another mineral upon which a file would have no effect, showing that the particles of the first are hard, though loosely aggregated. Chiastolite, spinel, and sapphire are common examples of this. When the mineral is too hard to be impressed by a file, the peculiarity of the grating sound will suffice for the prac tised ear. Mohs introduced a scale of hardness, consisting of ten minerals, which gradually increase in hardness from 1 to 10. The intervals between 2 and 3 and 5 and 6 are larger than the others. Breithaupt has therefore introduced another degree of hardness between each of the above, and thus his scale consists of twelve minerals. The scale is as follows : 1. Talc, common laminated light green variety. 2. Gypsum, a crystallized variety. 2 5. Mica (muscovite). 3. Calcite, transparent variety. 4. Fluor-spar, crystalline variety. 5. Apatite, transparent variety. 5*5. Scapolite, crystalline variety. 6.&quot; Felspar (orthoclase), white cleavable variety. 7. Quartz, transparent. 8. Topaz, transparent. 9. Sapphire, cleavable varieties. 10. Diamond. If the file abrades the mineral under trial with the same ease as No. 4, and produces an equal depth of abrasion with the same force, its hardness is said to be 4 ; if with more facility than 4 but less than 5, the hardness may be 4 or 4, written in decimals 4 &quot;25, 4 5. Several successive trials should be made to obtain certain results. The use of the file is acquired with very little experience ; usually a single trial is sufficient. Care must be taken to apply the file to edges of equal obtuseness. That part also of the specimen should be selected which has not been altered by exposure, and has the highest degree of transparency and compactness of structure. The pressure for determination should be rather heavy, and the file should be passed three or four times over the specimen. Where the scale of hardness is wanting, or a first rough deter mination is sought, the following experiments may serve : Every mineral that is scratched by the finger-nail has H. =2 5 or less. Minerals that scratch copper haveH. =3 or more. Polished white iron has H. = 4 5. Window-glass has H. =5 to 5 &quot;5. Steel point or file has H. = 6 to 7 ; hence every mineral that will cut or scratch with a good penknife has H. less than 6. Flint has H. =7, and only about a dozen minerals, including the precious stones or gems, are harder. Many specimens present different degrees of hardness on dis similar faces ; as an example of which we mention cyanite and mica. This is confined to the inequilateral primary forms, and like the similar difference of colour, lustre, &c., finds a ready explana tion in the theory of their formation; unlike faces are the result of the action of a polar force acting along unlike axes. This difference in faces parallel to unlike axes may be perceived in nearly all cases, when the methods of trial are sufficiently delicate. Huygens observed long ago that the cleavage face of a crystal of calc-spar differed in hardness from the other faces ; and even in a monometric crystal it has been found that the faces of the cube and octahedron are not exactly alike in this respect. Tenacity. 2. Tenacity. Solid minerals are said to be brittle, sectile, malleable, flexible, or elastic : 1. Brittle, when parts of a mineral separate in powder or grains on attempting to cut it ; as baryte, calc-spar. 2. Sectile, when pieces may be cut off with a knife without fall ing to powder, but still the mineral pulverizes under a hammer; as brucite, gypsum. 3. Malleable, when slices may be cut off, and these slices flatten out under a hammer ; as native gold, native copper. 4. Flexible, when the mineral will bend and remain bent after the bending force is removed ; as gypsum, graphite, talc. 5. Elastic, when after being bent it will spring back to its original position ; as mica, A liquid is said to be viscous when, on pouring it, the drops lengthen and appear ropy ; as petroleum. 3. Elasticity. Investigations on this property have not Elasticity, to any extent been entered upon. The unequal elasticity of unlike faces of crystals has been shown by Savart in his acoustic investigations, and he was able to distinguish the rhombohedral from the other faces in the pyramid of quartz crystals ; he also showed that the figures formed upon vibrating plates of crystals were directly connected with their optic axes. Milne, by measuring the amount of recoil of a sphere of calcite when struck at different points by another of rock-crystal, found that the elasticity, as thus measured, was greatest along the line of the optic axis, and least in directions at right angles to it. He also found that points which lay intermediate between the main and the transverse axes were most indented by the blows. This goes to show that, although there may be fewest molecules arranged along the lines of the transverse axes, yet cohesion operates with greater intensity along these than in intermediate directions. When the tenacity of a mineral is overcome by an over whelming amount of traction, or its elasticity by a sudden shock, its parts are separated, either in flat and continuous surfaces, or in surfaces which are irregular in the extreme. The first of these modes is termed cleavage, the second fracture. In those substances in which cleavage exists it is found that the planes or directions along which it takes place lie in certain strictly definite positions to one another and to the axes of the crystal. They show not the smallest tendency to a transition or gradual passage into the other directions of greater coherence. 4. Cleavage. The number of these parallel cleavage- Cleavage, planes is altogether indefinite, so that the only limit that can be assigned to the divisibility of some minerals, as gypsum and mica, arises from the coarseness of our instru ments. These minima of coherence, or cleavage-planes, are always parallel to some face of the crystal ; and similar equal minima occur parallel to every other face of the same form. Hence they are always equal in number to the faces of the form, and the figures produced by cleavage agree in every point with true crystals, except that they are artificial. They are thus most simply and conveniently described by the same terms and signs as the faces of crystals. Some minerals cleave in several directions parallel to the faces of different forms, but the cleavage is generally more easily obtained and more perfect in one direction than in the others. This com plex cleavage is well seen in calc-spar and fluor-spar, and very remarkably in zinc blende, where it takes place in no less than six directions. As in each of these the division may be indefinitely continued, it is clear that no lamellar structure in any proper sense can be assigned to the mineral. All that can be affirmed is that contiguous atoms have less coherence along a direction normal to these planes than in other directions. When cleavage takes place in three directions, it of course produces a perfect crystal form, from which the system of crystallization and angular dimensions of the species may be determined ; it is thus often of very great im portance. The common cleavage in the different systems is as follows, those of most frequent occurrence being in italics : (1) In the cubic system, Octahedral, 0, along the faces of the octahedron ; Hexahedral, ooOoo, along those of the cube ; and Dodecahedral, ooO. (2) In the tetragonal system, Pyramidal, P, or 2Poo ; Prismatic, ooP, or ooPoo ; or Basal, OP. (3) In the hexagonal system with holohedral forms, Pyramidal, P, or P2 ; Prismatic, ooP, or ooPoo ; or asal,QP; with rhombohedral forms, Rhomlohcdral, R ; Prismatic, ooR; or Basal, OR. (4) In the right prismatic system, Pyramidal, P; Prismatic, ooP ; Macrodomatic or Brachydomatic, Poo or Poo ; Basal, OP ; Macrodiagonal, ooPoo ; or Brachy diagonal, ooPoc. (5) In the oblique prismatic system, Hemipyramidal,P, or - P ; Prismatic, ooP ; Clinodomatic, P c oo ; Ilemidomatic, Too or - Pco ; Basal, OP ; Orthodiagonal, ooP oc ; or Clinodiagoiuil, ooP oo. (6) In the