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The following are the characters in question :
1. Aspect or Lustre.
1. Aspect or Lustre.—We have here to consider, first : the kind ; and, secondly, the degree or intensity of the lustre, as possessed by the mineral under examination. The kind of lustre may be either metallic, as that of a piece of copper, silver, &c.; or sub-metallic, as that of most kinds of anthracite coal; or non-metallic, as that of stones in general. Of the non-metallic lustre there are several varieties, as, more especially: the vitreous or glassy lustre-example: rockcrystal ; the resinous lustre-ex.: native sulphur ; the pearly lustreex.: talc; the silky lustre (usually accompanying a fibrous structure)-ex. ; fibrous gypsum ; the stony aspect ; the earthy aspect, &c. These terms sufficiently explain themselves. Occasionally, two kinds of non-metallic lustre are simultaneously present, as in obsidian, which exhibits a “resino-vitreous” aspect; and the lustre in some zeolites is pearly within, and vitreous externally. In mica, and some few other minerals, there is frequently a pseudo-metallic lustre. This may be distinguished from the metallic lustre properly so-called, by being accompanied by a degree of translucency, or by the powder of the mineral being white or light-colored : minerals of a true metallic aspect being always opaque, and their powder being always black or dark-colored. So far as regards the metallic and the nonmetallic lustres, there are very few minerals which exhibit (in their different varieties) more than one kind. Thus, galena, the common ore of lead, copper pyrites, &c., always present a metallic lustre ; whilst, on the other hand, quartz, feldspar, calc-spar, gypsum, &c., are never found otherwise than with a non-metallic aspect. Hence, by means of this easily-recognized character, we may divide all minerals into two broad groups; and thus, if we pick up a specimen
and wish to ascertain its name, we need only look for it amongst the minerals of that group with which it agrees in lustre.
The first step towards the determination of the substance will in this way be effected.
The degree of lustre may be either splendent, shining, glistening, glimmering, or dull ; but the character is one of comparatively little importance.
2. Colour.–When combined with a metallic aspect, colour becomes a valuable character in the determination of minerals, because it then remains constant as regards a given substance. Thus galena, the common ore of lead, is always lead-grey; copper pyrites, always brass-yellow; native gold, always gold-yellow; and so forth. When accompanied, however, by a vitreous or other non-metallic lustre, colour is, practically, a character of no value; as in that case, the mineral may present, in its different varieties, every variety of colour. Thus, we have colourless quartz, amethystine or violet quartz, red quartz, yellow quartz, &c., just as in the vegetable kingdom, we have red, white, and yellow roses; and dahlias, &c., of almost every shade. When combined with a metallic aspect, the colour is said to be metallic; and of metallic colours we may enumerate the following: s
ex. Native silver.
ex. Pure tin; cobalt ore. Grey
s Lead-grey.. ex. Galena.
Steel-grey ex. Specular iron ore. Black ....... Iron-black (usually with sub-metallic lustre) ex. Mag
netic iron ore.
Gold-yellow ex. Native gold.
.... ex. Copper-pyrites.
Bronze-yellow (a brownish-yellow) ex. Magnetic pyrites. Red ..... ... Copper-red .... ex. native copper. These metallic colours are often more or less obscured by a black, brownish, purple, or iridescent surface-tarnish. Hence, in noting the colour of a mineral, a newly-fractured surface should be observed. The non-metallic colours comprise, white, grey, black, blue, green, red, yellow, and brown, with their various shades and intermixtures; as orange-yellow, straw-yellow, reddish-brown, greenish-black, &c. In minerals of a non-metallic aspect, the colour is sometimes uniform; and at other times, two or more colours are present together, in
spots, bands, &c., as in the varieties of quartz, called agate, bloodstone, jasper, and so forth. In Labradorite, or Labrador feldspar, a beautiful play or change of colour is observed in certain directions. The finer varieties of Opal also exhibit a beautiful and well-known iridescence.
3. Streak.—Under this technical term is comprised the colour of the poroder produced by drawing or “streaking " the mineral under obserration, across a file or piece of unglazed porcelain. The character is a valuable one on account of its uniformity; as, no matter how varied the colour of a mineral may be in different specimens, the streak will remain of one and the same colour throughout. Thus, blue, green, yellow, red, violet, and other specimens of Auor spar, quartz, &c., exhibit equally a white or “uncoloured” streak. The streak is sometimes “unchanged,” or of the same tint as the external colour of the mineral; but far more frequently it presents a different colour. Thus, Cinnabar, the ore of mercury, has a red colour and red streak; realgar, or sulphide of arsenic, has a red colour and orange-yellow streak ; copper pyrites, a brass-yellow colour, and greenish-black streak; and so forth. In certain malleable and sectile minerals, whilst the colour remains unchanged in the streak, the lustre is increased. The streak is then said to be “shining.” Finally, it should be remarked, that in trying the streak of very hard minerals, we must crush a small fragment to powder, in place of using the file ; because otherwise, a greyish-black streak, arising from the abrasion of the file, might very possibly be obtained, and so conduce to error. It may be observed, however, that all minerals of a non-metallic aspect, and sufficient hardness to resist the file, have a white streak.
4. Form.—The forms presented by minerals, may be either regular or irregular. Regular forms are called crystals, whether the minerals which present them be transparent or opaque. The term “crystal”. was first applied to transparent vitreous specimens of quartz or rockcrystal ; but, as it was subsequently found that opaque specimens of quartz presented exactly the same forms, and that opaque as well as transparent forms of other minerals existed, the term gradually lost its original signification, and came to be applied to all regular forms of minerals, whether transparent, translucent, or opaque. Minerals of a metallic lustre are always opaque ; and many of these, as iron pyrites and galena, occur frequently in very regular and symmetrical crystals. Crystals originate in almost all cases in wbich matter pas se from a gaseous, or liquid, into a solid state ; but if the process take place too quickly, or the matter solidify without free space for expansion, crystalline masses, in place of regular crystals, will result. If a small fragment of arsenical pyrites, or native arsenic, be heated at one end of an open glass tube (five or six inches long and one-fourth of an inch in diameter), the arsenic, in volatilizing, will combine with oxygen, and form arsenious acid, which will be deposited at the other end of the tube, in the form of minute octahedrons (Fig. 3, below). In like manner, if a few particles of common salt be dissolved in a small quantity of water, and a drop of the solution be evaporated gently (or suffered to evaporate spontaneously) on a piece of glass, numerous little cubes and hopper-shaped cubical aggregations will result. Boiling water, again, saturated with common alum, will deposit octahedral crystals on cooling : the cooled water not being able to retain in solution the full amount of alum dissolved by the hot water. In like manner, sugar, sulphur, and other bodies crystallise by slow cooling from the molten state.
The study of crystal-forms constitutes the science of Crystallography. To enter into the details of this science would extend our present discussion beyond its proposed limits, and carry us altogether beyond the object in view—the simple determination of the names of cominonly-occurring minerals—and hence we shall confine ourselves to the general statement, that crystals admit of being arranged in six groups, or “systems;" the forms of each individual group passing into one another by simple transitions, but having no relations to the forms of the other groups.* The names of these respective groups,
• The reader desirous to take up the study of Crystallography in a more extended manner, may attend the author's special courses of lectures which include that subject. In these, the use of crystallographic instruments is shewn, and the lectures are illustrated by nu. merous wood and porcelain models, drawings, and natural crystals. The following is ex. tracted from the syllabus of the advanced course on Mineralogy:
CRYSTALLOGRAPHY, PART I.-Crystals, how defined. Formation of Crystals. Elements of Crystals: planes, edges, angles; diagonals, axes. Forms and combinations. Replacing planes. General nomenclature of Forms and simple Crystals. Law of constant Angles. Measurement of Angles. Laws of Symmetry: Holohedral, Hemihedral, and Tetartohedral Forms. Classification of Crystals, Dimorphism. (somorphism. Compound Crystals. Distortions. Pseudomorphs.
PART II.-The six systems of Crystallization. The Monometric system. The Dimetric system. The Hexagonal system. The Trimetric system. The Monoclinic system. The Triclinic system. Method of ascertaining the system of a given Crystal.
PART III.--Optical and other physical relations of Crystallography.
with figures of a few of their more common forms and combinations, are given in the annexed tabular view.
The Monometric or Regular System.—This group includes the cube (Fig. 1), the rhombic dodecahedron (Fig. 2), the regular octahedron (Fig. 3), trapezohedrons or leucitoids (Fig. 4), pentagonal dodecahedrons (Fig. 5), &c. Fig. 6 is a combination of the cube and
octahedron ; Fig 7, a combination of the cube and pentagonal dodecahedron. Native gold, silver, copper, iron pyrites, galena, magnetic iron ore, garnet, fluor spar, rock salt, and numerous other minerals, crystallize in this system.
The Dimetric or Square-Prismatic system. This includes, principally, square-based prisms and pyramids (or octahedrons), and their combinations. Figures 8 and nine are examples of Dimetric crystals.
Amongst minerals, Copper Pyrites, Tin-stone, Zircon, and Idocrase, may be cited as belonging to the group.
The Hexagonal system.-Regular six-sided prisms (Fig. 10) and pyramids (Fig. 11), combinations of these (Fig 12), rhombohedrons