Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/397

MINERALOGY 379 anorthic system, Hemiprismatie, ooP , or ooP ; Hemidomatic either along the macro-dome or the braehydome; Basal, OP ; Macrodiagonal, oo I 5 oo ; or Brachydiagonal, oof oo. In some minerals, as mica and gypsum, the cleavage is readily procured ; these may be held in the hand and divided by a knife. Others only cleave with more or less difficulty ; these must be placed on a firm support resting on lead, folded paper, or cloth, and a sharp blow struck on a chisel applied in a proper direction. This may often be ascertained by examining the specimen in a strong light. Sometimes it is necessary to subject them to extreme com pression in a vice. Some of the hardest substances have not only a perfect but a facile cleavage, as euclase, topaz, and diamond ; many of the softest species have none. The planes produced also vary much in their degree of perfection, being highly perfect in some, as mica and calc-spar, and imperfect in others, as garnet and quartz. In a very few crystalline minerals cleavage-planes can hardly be said to exist. Cleavage must be carefully distinguished from the planes of union in twin crystals, and the division-planes of laminar minerals. racture. 5. Fracture. This is the irregular manner in which substances may be broken. Even minerals possessed of cleavage may be fractured in other directions ; but in amorphous bodies fracture alone occurs. The following varieties of fracture occur, and are highly characteristic: 1. Conchoidal, almost typical of amorphous bodies, but occas- sionally seen in crystals, rounded cavities, more or less deep. The name is taken from the resemblance to the successive lines of interrupted growth in a bivalve shell. Seen in flint, obsidian, asphalt. In calcite the direction of this fracture is intermediate to the planes of the mineral s cleavage. 2. Even, when the surface of fracture is smooth and free from inequalities. 3. Rough, when the surface of fracture is rugged, with numerous small elevations and depressions. 4. Splintery, when covered with small wedge-shaped splinters. 5. Hackly, when the elevations are sharp, slightly bent, or jagged, as broken iron. 6. Earthy, when it shows only fine dust. Taste, Odour, Touch. te. Taste belongs only to soluble minerals. The different kinds adopted for reference are as follows : 1. Astringent, the taste of blue vitriol. 2. Sweetish astringent, taste of alum. 3. Saline, taste of common salt. 4. Alkaline, taste of soda. 5. Cooling, taste of saltpetre. 6. Bitter, taste of epsom salts. 7. Sour, taste of sulphuric acid. 8. Pungent, taste of sal-ammoniac. 9. Metallic, taste of zinc sulphate. our. Odour. Excepting a few gaseous and soluble species, minerals in the dry unchanged state do not give off odour. By friction, moistening with the breath, and the elimina tion of some volatile ingredient by heat or acids, odours are sometimes obtained which are thus designated : 1. Alliaceous, the odour of garlic. Friction of arsenical iron elicits this odour ; it may also be obtained from any of the arsenical ores or salts by means of heat. 2. Horse-radish odour, the odour of decaying horse-radish. This odour is strongly perceived when the ores of selenium are heated. 3. Siilphtirous. Friction will elicit this odour from pyrites, and heat from many sulphurets. 4. Bituminous, the odour of bitumen. 5. Fetid, the odour of sulphuretted hydrogen or rotten eggs. It is elicited by friction from some varieties of quartz and limestone. 6. Argillaceous, the odour of moistened clay. It is obtained from serpentine and some allied minerals after moistening thorn with the breath ; others, as pyrargillite, afford it when heated. 7. Empyreumatic or ozonic. Quartz, when two portions strike one another. uch. Touch. Some minerals are distinguished by a greasy feeling, as talc ; others feel smooth, as celedonite ; others meagre, like clay ; others cold. This last character distin guishes true gems from their imitations in glass. Some, in virtue of their hygroscopic nature, adhere to the tongue. CHEMICAL PROPERTIES OF MINERALS. Influence of Chemical Composition on the External Relation of Characters of Minerals. That the characters of a com- composi- pound must to a certain extent depend on those of its tl n f . , , ... physical component elements seems, as a general proposition, to p ro p er ti e s. admit of no doubt. Hence it might be supposed possible from a knowledge of the composition of a mineral to draw conclusions in reference to its form and its other properties ; but practically this has not yet been effected. The distinction between the mineralizing and mineralizable or the forming and formed elements lies at the foundation of all such inquiries. Certain elements in a compound apparently exert more than an equal share of influence in determining its physical pro perties. Thus the more important non-metallic elements, as oxygen, sulphur, chlorine, fluorine, are remarkable for the influence they exert on the character of the compound. The sulphurets, for example, have more similarity among themselves than the various compounds of one and the same metal with the non-metallic bodies. Still more generally it would appear that the electro-negative element in the compound is the most influential, or exerts the greatest degree of active forming power. After the non-metallic elements the brittle, easily fusible metals rank next in power ; then the ductile ignoble metals ; then the noble metals ; then the brittle, difficultly fusible ; and, last of all, the metals of the earths and alkalies. Generally each chemical substance crystallizes only in one form or series of forms. Some substances, however, show dimorphism, or crystallize in two forms, and thus may compose two or more minerals. Thus sulphur, which in nature usually crystallizes in the right prismatic system, when melted forms oblique prismatic crystals. Carbon in one form is the diamond, in another graphite; carbonate of lime appears as calc-spar and as aragonite ; the bisulphuret of iron as pyrite and as marcasite. An example of trimorphism occurs in titanic acid, forming the three distinct species anatase, rutile, and brookite. It is remarkable that of dimorphic minerals one form is almost always right prismatic; thus: Rhombic Form. Cyanitc, anortliic Sillimanite, Andalusite. Calc-spar, hexagonal Aragonite. Susannite, do Leadhillite. Anat ase }vJ* Brookite. Pyrolusite, ri ght prismatic Polianite. Cuprite, cubic Chalcotrichite (?) Senarmontite, cubic Valentinite. Pyrite, do Marcasite. Rammelsbergite, do Ctiloanthite. Argentite, do Acanthite. Freieslebenite, obliqu^prismatit- Diaphorite. Sulphur, do. Sulphur. Even the temperature at which a substance crystallizes influences its forms, and so far its composition, as seen in aragonite, Glauber salt, natron, and borax. Isomorphism. Still more important is the doctrine of Isomor- isomorphism, designating the fact that two or more simple or phism. compound substances crystallize in one and the same form, or often in forms which, though not identical, yet approximate very closely. This similarity of form is generally combined with a similarity in other physical and in chemical properties. Among minerals that crystallize in the tesseral system, isomorphism is of course common and perfect, there being no diversity in the dimensions of the primary form ; but for this very reason it is generally of less interest. It is of more importance among crystals of the other systems, the various series of which are separated from each other by differences in the proportions of the primary form. In these perfect identity is seldom observed, but only very great similarity. The more important isomorphic substances are either simple sub stances, as (1) fluorine and chlorine; (2) sulphur and selenium; (3) arsenic, antimony ; (4) cobalt, iron, nickel ; (5) copper, silver, mercury, gold (?); or combinations with oxygen, as (6) lime, magnesia, and the protoxides of iron, manganese, zinc; (7) sesqui- oxides, as of iron, manganese, chromium, and alumina ; (8) phosphoric acid, vanadic acid, arsenic acid; (9) sulphuric, selenic, chromic acids; or combinations with sulphur, as (10) sulphuret of iron and of zinc; (11) sulphuret of antimony and of arsenic; (12) sul phuret of lead, of copper, and of silver. These substances are named vicarious from the singular property that in chemical compounds they can mutually replace each other in definite proportions, and very often without producing any important change in the form or other physical properties. But there are numerous instances among

the silicates where the mutual replacement of the isomorphic