MINEKALOGY. 538 MINEKALOGY. is of primary inii)ortaiioe in tlio investigation of any niiuoral species. The iilenlilieation of the faces of the crystal, which is often attended with considiiable diHiculty, is accomplished by meas- uring the interfaeial angles by means of an in- strument called a goniometer {q.v. ) and compar- ing these with the calculated relations obtained from simple mathematical formulas based on spherical trigonometry. The optical properties of minerals as well as their presence and relations in rocks are determined by means of the petro- graphic microscope. (See Mit'RO.scoPK.) I'"or exhaustive study along the line of physical char- acters, elaborate and accurate apparatus is re- quired, while a well-cc|uipped chemi<'al lalmratory is almost indispensable to the mineralogical in- vestigator.
- Mixeb.ls and Kocks. One of the most impor-
tant phases of mineralogical study, and one which is replete with interest to the geologist, is the relation of minerals to rocks. The division known as the crystalline rocks, in particular, presents a wide and varied series of rock-forming minerals. These may be classed as essential and accessory constituents according as they give char- acter in the rock in which they occur or are present only in insignificant proportions. Quartz, the feldspars, the micas, hornblende, augite, enstatite, hypersthene, chrysolite, garnet, leucite, serpentine, caleite, and dolomite arc essential constituents of many crystalline rocks, while such minerals as gypsum, salt, limonite, hematite, sidcrite, kaolin, magnetite, and apatite often oc- cur in such extensive deposits as to constitute rock masses. Among the accessory rock-forming minerals way be mentioned graphite, corundum, vesuvianite, chiastolite, cyanite, tourmaline, zir- con, titanite, etc. Many geologists have made use of the mineralogical character of rocks as a basis of classification. i)artieularly in the case of the igneous rocks, niul. though open to some objection from the standpoint of geological inquiry, the system as applied to crystalline rocks has much to connnend it, iliNERAL Chemistry. Comparatively few ele- ments exist in nature uncombined : the great majority of minerals occur as salts of relatively few mineral acids. Minerals crystallizing from a mineralizing lluid. whether it be a solution or a fusion, combine the clcTiients existing in that fluid in strict accordance wilh the laws of chem- istry. The resulting minerals may. however, be somewhat modified by the presence of elements foreign to their ty|)ical fornuilas. as in the case of tlie emerald variety of beryl, which owes its brilliant green color to the presence of a small amount of chromium not re|>rcsented in the nor- mal ccniiposition of beryl. .Again, certain ele- ments closely related in chemical character fre- quently re|)lace one another in mineral eomposi- tiiin. the relative proportions varying between limits anil giving rise to a group of closely re- lated ecimpoimds. Such is the eolumbite-tanta- lite group, which presents all the gradations from normal columbite (l'VNb.O„) to normal tantalite (FeTa-0„). Minerals closely related in compo- sition often exhibit a striking similarity in crys- tal form. WTien such isomorphons compounds are present in the same magma they are not separable in the process of crystallizntion. but tend to produce a mineral intermediate In com- position: as when the isomorphous carbonates dolomite and siderite grade into the interme- diate compound ankcrile. An extremely inter- esting group is that of the trielinic feldspars or plagioclases which are regarded as isomorphous mixtures of the molecules of the two isomorphous species albitc and anorthite. Mineralogy atl'ords several examples of mineral species identical in chemical composition, but crystallizing in forms which are essentially different. This condition, which is known as dimorphism, is represented by the two calcium carbonates calcitc and arago- nite and by the two iron disulphides pyrite and marcasite. Titanium 'dioxide, which is trimor- phous, occurs as rutile. octahedritc, and brookite, CussiFic.VTioN OF Minerals. The most logi- cal and convenient scheme of classification of minerals is that which is adopted by Dana in his System of Mineralogy and which is now, with slight moditlciitions, universally used. By this method mineral s])ecics of similar composition are jilaced together in classes which are subdi- vided into divisions. These in turn are split up as far as possible into isomorphous groups. The principal classes are: ( 1 ) Native elements. (2) Sulphides — Sulphides, selenides, telluridcs, arsenides, and antimonides. (3) Sulpho-salts — Sulpharsenidcs, sulphanti- monides. and sutphobismuthitcs. (4) Haloids — Chlorides, bromides, iodides, and fluorides. (5) Oxides. (U) O.xygen Salts — Carbonates, silicates, and titanates; niobates and tantalates: phosphates, etc.; borates and uranates; sulphates, etc.; tung- states, molybdates, (7) Salts of the organic acids, (8) Hydrocarbons. Uses of Minerals. By far the most important of the uses to which minerals are put is that of producing metals from those of them which con- tain metal constituents in suflicicnt quantity to render their mining profitable. The discussion of the distribution and mode of occurrence of metallic ores involves many questions of a purely technical nature and belongs essentially to the province of ore deposits (q.v.). few metals such as gold, platiniun, copper, arsenic, and to some extent silver, antimony, bismuth, and mer- cury are found native, that is. uncombined with other elements. The majority of the metallic ores, however, occur as sulphides, oxiiles. or car- bonates of the various metals, or more rarely as arsenides, telhiridcs. chlorides, or silicates. Asso- ciation of metallic minerals in more or less inti- mate mi.xtures often gives rise to highly complex ores. Many ores which are essentially com- pounds of the base metals contain gold and silver in appreciable amounts and are profitably mined for the latter metals, as is the case with the argentiferous galena of Colorado, Jlontana, and Utah. The non-metallic minerals, although of less importance commereially than the metallic ores, are none the less of great and incre:ising value in the arts. These are grou|ied. with reference to their application, into: (1) Substances used for chemical purposes, embracing the minerals cm- ployed in the manufacture of acids, chemicals, soda, alum, plaster of Paris, etc. (2) Ceramic materials >ised in making pottery, bricks, tilinsr, paving blocks, terra-cotta. porcelain, and glas.s. (.3) Refractory materials, used in the manu- fncture of fire-proofing, linings of furnaces, cruci-
