MINERAL Citemisray. C'omparalively few ele ments exist in 11:11.1111. 11110 P1111) 114. great majority of minerals occur as salts of relatively few mineral acids. )1inerals crystallizing front a mineralizing fluid. whether it a solutiim a fusion. eombine the elements existing in that fluid in strict accordance with the laws of chem istry. The resulting minerals may, however, he modified by the presence of elements foreign to their typical formulas. is in the ease of the emerald variety of beryl, which owes its brilliant green Polir to the• presence of a small amount of chromium not represented in the nor mal composition of beryl. .\ gain, certain ments ,'lonely related in eliendeal character fre quently replace one another in mineral composi tion, the relative proportions varying between limits and giving rise to a group of closely re.
lafed compounds. Such is ti lit,. group, which presents all the grad,ition. from normal eolumhite ( FeNb to mat tantalite ( (t„). closely related in compu siti.n often exhibit a striking similarity in rry-t :II J1.1111 \\•hen isomorphous compounds are present in the same magma they are not separable in the process of erystallization. hut tend to produce a mineral intermediate in eom po‘ition: as when the isomorphous carbonates dolomite and siderite grade into the diate compound ankcriie. extremely inter esting group is that of the triclinic feldspars or plagioclases which are regarded as isomorphous mixtures of the meleenles of the two isomorphous species albite and anorthite. ,lineralogy affords
several examples of mineral species identical in chemical composition, hut crystallizing in forms which are different. This condition, '1\111(.11 is known as dimorphism, is represented by the two calcium carb(mates, ealeite and arago nite and by the two iron disulphides pyrite and marcasite. Titanium dioxide, which is (rumor plums, ()emirs as rutty. octaliedrite, and brookite.
('LAssirtc.vrtox ov „NltsEam.s. The most logi cal and convenient scheme of classification of minerals is that which is adopted by Dana in his System of I/hien/logy and which is now, with slight modifications, universally used. By this method mineral species of similar composition are placed together in classes which are subdi vided into divisions. These in turn are split up as far as possible into isomorphous groups. The prineipal classes are: (1) Native elements.
(•') Sulphides—Sulphides, selonides, tellurides, arsenide.. and antinionides, (3) Sulpho-salts—Sullilmrsenides, sulphanti moi ides. and sulphobisitoithites.
(4) Ilaloids—Chlorides, bromides, iodides, and fluorides.
(5) Oxides.
On oxygen Salts—Carbonates, silicates. and titanates: niobates and tantalatcs; phosphates, etc.; borates and uranates; sulphates. etc.; twig states, molybdates.
(7) Salts of the organic acids.
( I yd rocarbons.