Page:The American Cyclopædia (1879) Volume XI.djvu/602

MINERALOGY the doubled atomic weights of the new chem- istry, indicates the relative quantivalences of the combined elements. The change of ideas in regard to the modes of combination has necessarily produced a new mode of writing the symbols of minerals. Under the dualistic theory, when every ternary oxygen compound was supposed to consist of one oxide acting the part of a base and another oxide acting the part of an acid, the formula was constructed by writing the two compounds one after the other, as RO,SiO a , or R,O 3 ,3SiO a , in which R stands for any basic element. The new system endeavors to make its formulas rational, that is, to construct them in such a way as to em- body the present views in regard to chemical combination. The elements are divided into three classes : 1, the basic, which are positive to those following; 2, the acidic, which are negative to the foregoing, but positive to the third class ; 3, the acidific, which are negative to both the first and second. In a ternary compound which consists of one element from each of these classes, the acidific element is supposed to act as a bond between the other two, and for that reason it is placed in the middle, the basic being written first, the acid- ific next, and the acidic last; as Mg 3 1| O 4 || Si. It has been explained that one atom of each element has the power of uniting to itself a certain number of atoms (from one to six) of hydrogen or other univalent element ; and just as the ancients provided the elements with hooks by which they caught hold of each other, so modern chemists express the quanti- valence of an element by saying that it has a certain number of bonds of attraction. Mag- nesium has a quantivalence of 2, and as there are two molecules of it in the above symbol, the total number of its bonds in the formula just given is four. Silicon has a quantivalence of 4, so that the oxygen has four bonds to sat- isfy on each side, or eight in all. As its quan- tivalence is 2, the four molecules present in the compound have a total uniting power of eight. But the whole of the oxygen does not always play the part of a uniting element only. The number of molecules of uniting oxygen (or other acidific element) is equal to the number of bonds of attraction in the basic or acidic element, according as the former or the latter has the smaller number. In the sym- bol given above, the number of oxygen bonds is just sufficient to satisfy those of the other elements, but in the symbol Mg0 3 Si there are two bonds on one side of the oxygen and four on the other. In this case part of the oxygen is considered to be combined with that element which has the greater number of bonds to be satisfied. When this is the acidic ele- ment, as in the present case, the symbol is written with the acidic element at the left side ; as SiO || O 2 || Mg. If the basic element had possessed the greater quantivalence, that element would have been written at the left; as in the symbol for chondrodite, MgsOaj (0,F)i, || Si 3 . Thus the symbols are made to express the mineralogist's views of the consti- tution of minerals. Replacement. Minerals in their chemical composition are elementary, binary, ternary, quaternary, &c., according as the number of molecules of which they are composed is one, two, three, or four, &c. This, however, does not indicate the possible number of elements present, since each molecule may contain several elements. Enstatite, which is composed of magnesium, oxygen, and silicon, is a ternary ; and diaclasite, which has magne- sium, iron, calcium, oxgygen, and silicon in its composition, is also only a ternary, since the first three elements form only one basic mole- cule. In this case each element in the basic molecule is a dyad (that is, it has a quantiva- lence of 2), and it may not seem strange that, with equal powers of combination, they should be able to replace each other. But other min- erals are found which contain elements of the most diverse degrees of quantivalence, and therefore in the most diverse states of combi- nation. Zircon sometimes contains a protoxide, a sesquioxide, and a deutoxide. The law under which these diverse combinations are brought harmoniously together is that " the replacing power of the elements is in proportion to their combining power." Thus one molecule of an element, which has four bonds of attraction, like tin, is able to replace two molecules of an ele- ment which has only two bonds of attraction, like calcium. In stannite, which is a sulphide of copper, tin, and iron, the proportions of these elements are 2:1:1. Copper, which is biva- lent, requires two atoms to occupy toward sulphur the same relation which one atom of tin and iron has. The proportion in which any element in any state of combination must replace other elements in a different state may be ascertained from the following table, in which the line A contains the several oxides that are known, the line B contains the same oxides reduced to a common oxygen stand- ard (O=l), and the line C represents the proportions in which the bases are inter- changeable. The different states are repre- sented by Greek letters in order to avoid con- fusing fractions. Thus the beta state is ses- quioxide and the gamma state is the deutoxide. R is used to represent any basic element, and it is to be remembered that, though only oxides are represented here, the rule holds good for all negative elements. A. RO, R 3 O, R0 a , R 2 0>, RO 9 , R 2 O T , RO 4 B. RO, RfO, R^O, RfO, R0, RfO, RJO C. aR, /3R, yR, <JR, eR, CR, ?R Any element in the tritoxide state (eR) there- fore requires but one basic atom to replace three basic atoms of an element in the pro- toxide or alpha state. The method of writing the symbol of a mineral which has suffered such substitution may be seen from the sym- bols of magnetite and f ranklinite. The former contains iron in the alpha and in the beta state