« EelmineJätka »
(Figs. 13 and 14), and scalenohedrons (Fig. 15), are included under this system. Graphite, Red Silver ores, Cinnabar, Specular Iron Ore, Corundum, Quartz, Beryl, Apatite or phosphate of lime, Cal
careous Spar, Dolomite, and Carbonate of Iron, are some of the principal minerals which belong to it.
The Trimetric or Rhombic system.-This system includes rightrhombic prisms, rectangular prisms, rhombic octahedrons, &c., and their combinations. Fig. 16 is a rhombic prism; figs. 17 and 18 are
combinations beionging to this system. White iron-pyrites, mispickel or arsenical pyrites, native sulphur, topaz, staurolite, arragonite, heavy spar, celestine, and Epsom salt, are some of the principal minerals which belong to this group.
The Monoclinic or Oblique Rhombic system.-Rhombic prisms and pyramids, and rectangular prisms and pyramids, with oblique or sloping base, belong to this system. Figs. 19 and 20 are monoclinic
The principal minerals comprise: Augite, Hornblende, Epidote, Orthoclase, or potash feldspar, Stilbite, and Gypsum.
The Triclinic, or Doubly Oblique system. The forms of this system are oblique (or they incline) in two directions. The crystals in general are more or less flat and unsymmetrical in appearance. No two planes meet at right angles; and there are never more than two similar planes present in any crystal belonging to the group. Axinite, Albite or soda feldspar, Labradorite or lime feldspar, and sulphate of copper, are the principal triclinic minerals.
Such is a brief exposition of the six crystal systems. For present purposes it will only be necessary for the student to impress upon his memory the following forms, so as to be able to recognize them when met with. The cube (Fig. 1), the regular octahedron (3), the rhombic dodecahedron (2), the pentagonal dodecahedron (5), the cubo-octahedron (6), the regular six-sided prism (10), a combination of a six-sided prism and pyramid (12) a rhombohedron (13 and 14), a scalenohedron (15), a rhombic prism (16).
The irregular forms presented by minerals are of very subordinate importance; so that a few of the more common need only be mentioned. Most of the terms used in reference to these, explain themselves.
Irregular mineral forms:- Globular or nodular, ex. quartz, iron pyrites; reniform or kidney-shaped, ex. quartz, &c.; botryoidal mammillated: a form made up of a series of rounded elevations and depressions, or otherwise exhibiting a surface of this character, ex. red and brown iron ore, calcedony, &c.; stalactitic, ex. cale spar, &c.; coralliform, resembling certain branching corals, ex. arragonite; dendritic or arborescent, a branching form, often made up of small aggregated crystals, ex. native silver, native copper, &c.; filiform or wire-like, ex. native silver; acicular, in needle-like crystallizations, ex. many varieties of augite, hornblende, epidote, &c. When a
mineral has a perfectly indefinite shape it is said to be "massive" or "amorphous."
Structure:-In the majority of minerals, a certain kind of structure, or, in other words, the shape as well as the mode of aggregation of the smaller masses of which they are composed, is always observable. Structure in minerals may be either lamellar, laminar or foliated, prismatic, fibrous, granular, or compact. When the mineral, as in most varieties of calc-spar, heavy-spar, feldspar, and gypsum, for example, is made up of broad tabular masses producing a more or less stratified appearance, the structure is said to be lamellar. When the tabular masses (whether straight, wavy, or curved,) become extremely thin or leafy, as in mica more especially, the structure is said to be laminar, or foliated, or sometimes micaceous. The scaly
structure is a variety of this, in which the laminæ are of small size. When the component masses are much longer than broad or deep, as in many specimens of tourmaline, beryl, calc-spar, &c., the structure is said to be prismatic or columnar. When the prismatic concretions become very narrow, the fibrous structure originates. Fibrous minerals may have either: a straight or parallel-fibrous structure, as in many specimens of gypsum, calc-spar, &c.; a confusedly-fibrous structure, as in many specimens of augite and hornblende; or a radiated-fibrous structure, as in the radiated varieties of iron pyrites, in natrolite, wavellite, &c.,-the fibres radiating from one or more central points. Minerals made up of small grains or granular masses are said to have a granular structure; ex. granular or saccharoidal limestone, granular gypsum, &c. Finally, when the component particles are not apparent, the mineral is said to have a compact structure, as in the native malleable metals, obsidian, and most varieties of quartz. Hard and vitreous minerals of a compact structure (ex. obsidian), generally show when broken, a conchoidal fracture, or a series of circular markings resembling the lines of growth on the external surface of a bivalve shell.
Almost all minerals, especially those of a lamellar structure, separate more readily in certain directions than in others. This peculiarity is called cleavage. The fragments resulting from "cleavage" have often a perfectly regular or definite forin. Thus the purer specimens of calc-spar, no matter what their external form, break very readily into rhombohedrons, which measure 105°5' over their obtuse edges. Galena, the common ore of lead, yields rectangular
or cubical cleavage forms; whilst the cubes of fluor-spar break off most readily at the corners or angles, and yield regular octahedrons, fig. 3.
Hardness.-The hardness of a mineral is its relative power of resisting abrasion, not that of resisting blows: many of the hardest minerals being exceedingly brittle. Practically, the character is of great importance. By its aid gypsum may be distinguished in a moment from calc-spar or limestone, calc-spar from feldspar, and copper pyrites from iron pyrites, not to mention other examples.* The degree of hardness in minerals is conventionally assumed to vary from 1 to 10 (1 being the lowest) as in the following scale drawn up by a German mineralogist, Möhs, and now generally adopted:
Scale of Hardness-Möhs' Scale.
1. Foliated TALC.
2. ROCK SALT, a transparent cleavable variety.
7. ROCK CRYSTAL.
10. THE DIAMOND.
In order to ascertain the hardness of a mineral by means of this scale, we attempt to scratch the substance under examination, by the different specimens belonging to the scale; beginning with the hardest, in order not to expose the specimens to unnecessary wear. Or, we take a fine file, and compare the hardness of the mineral with that of the individual members of the scale, by drawing the file briskly across them. The comparative hardness is estimated by the resistance offered to the file; by the noise produced by the file in passing across the specimens; and by the amount of powder so
Gypsum may be scratched by the finger nail. Calc-spar and copper pyrites may be scratched easily by a knife; whilst feldspar and iron pyrites are hard enough to scratch window-glass. Not long ago, as mentioned by Sir William Logan, a farmer in the Ottawa district was put to much expense and annoyance by mistaking feldspar for crystalline limestone, and attempting to burn it into lime.
obtained. The degree of hardness of the mineral is then said to be equal to that of the member of the scale with which it agrees the nearest. Thus, if the mineral agrees in hardness with Fluor-spar we say, in its description, H (or hardness) =4. If, on the other hand, it be somewhat softer than fluor-spar, but harder than calcareous spar, we say, H=3.5. Finally, if, as frequently happens, the hardness of a mineral vary slightly in different specimens, the limits of the hardness are always stated. Thus, if in some specimens, a mineral agree in hardness with calc-spar, and in others with fluorspar, we say, H = 3 to 4; or, more commonly, H = 3-4. If the hardness be very rigorously tested, it will frequently be found to differ slightly on different faces of a crystallized specimen, or on the broad faces and the edges of the lamina of foliated specimens,-but this, so far as regards the simple determination of minerals, is prac tically of little moment.
As the minerals of which the scale of Möhs consists, may not be in all places obtainable, or always at hand when required, the author of this paper contrived some years ago another scale, agreeing closely enough for practical purposes with that of Mohs, and exacting for its application only such objects as are always to be met with. The following is the scale in question; its use explains itself:
Chapman's Convenient Scale of Hardness, to correspond with that of Möhs.
1. Yields easily to the nail.
4. Not scratched by a copper coin. Does not scratch glass
5. Scratches glass very feebly. Yields easily to the knife.
edge of a file.
Yields with difficulty to the
8, 9, 10. Harder than flint or rock-crystal.
Convenient terms of comparison for degrees of hardness above No. 7 cannot be easily obtained; but that is of little consequence, as there are but few minerals of common occurrence which exhibit a higher degree; and these are readily distinguished by other char