Document JoeZbbEJ8GaoQ0BBzn2oN3LX

SILICATES __ The silicates comprise the largest chemical group among minerals. They show a wide range in composition which Is frequently very complex in character. Recently, however, X-ray investigation has revealed important fundamental facts concerning their atomic structure and thrown much light- upon tiie intricate problem of their composition. It is no*?; established that the fundamental structural unit of all silicates has a tetrahedral shape with four oxygen atoms surrounding eac.i atom of silicon. But these Si04 groups may be linked together in various ways to form Indefinitely, extended series'. In the orthosilicates these 310* groups occur independently with relations to uhe other elements present, as in an ordinary salt, such as a sulphate, a carbonate, etc. In other types of silicates, however, the SlO^ groups are linked together by having 0170 or more atoms of oxygen shared in common by neighboring groups. If two , adjacent silicate groups have one atom of oxygen in common, individual groups with such formulas as Slg.0*?, SiB0g, SI40|S Si 60nq may. occur. Further, the SiO* groups may fora chains in which two oxygen atoms of two adjacent groups are held . in common# Single chains of this type with a composition ' corresponding to SiOB occur in pyroxene. In amphlbole two such chains are linked together, giving a composition represented by Si4 0j|. ^ueh. chains may be- further linked together tc form a silicon-oxygen sheet, thought to be characteristic of the micas, with a composition of Si8O c . Again the linking of the silicon-oxygen chains may take"' place so as to give rise to a three-dimensional network. Such a grouping is characteristic of the different forms of silica (quarts, tridymlte, etc.), and by supposing a partial replac. r-ient of silicon by aluminum an extended acid network may 'be obtained into which basic atoms can be introduced. This latter structure is supposed to be charaeterist of the zeolites, where metallic atoms may be artificially substituted for each"'other and the water content varied without disturbing tne fundamental structure. In these various silicon-oxygen structures, metallic atoms are placed in such ways as to bind the whole structure together. It has been shown that where isoraorphous replacement takes place the number of oxygen atoms i including F and OH) remains constant for each unit cell of the structure. Variations in composition may involve the substitution of Si by Al, of Al, Mg, and Fe by each other, of Ka by Ca, etc. A general no statement by Bragg* Is "that a silicate should be regarded as a structure having a constant number oi oxygen atoms in cot the unit, with a constant number of places for- metal and silicon which can be filled by these elements In varying o o proportions consistent with a balance between valencies." It can be seen that this view of the structure of the silicates largely does away with the customary assumption of the existence of distinctive silicate acid radicals and alters greatly the usual method of the classification of the silicates. &ueh a study of the silicates is, however only 2. just begun, and tils' time has not yet arrived when a fundamental reclassification can be attempted, Therefore, in this edition, the usual chemical classification of the silicates has'been kept, but short statements concerning the new facts of structure and relations between species have been introduced in their appropriate places. ' She silicates are in part strictly anhydrous, in part hydrous, as the zeolites and. the amorphous clays, etc. Furthermore, a large number of the silicates yield more or less water upon ignition, and in many cases it is known that they are, therefore, to be regarded as basic. (oi' acid) silicates. I'he line, however, between the strictly anhydrous and hyuroua silicates cannot foe sharply drawn, since with many species which yield water- upon ignition the part played by the elements forming the water is as yet uncertain, furthermore, in the cases of several groups, the strict arrangement must be deviated from, since the relation of the. species is best exhibited by introducing the related hydrous species, irasedlately after the others. The substance of these paragraphs has been condensed from an article entitled, The Structure of Silicates, by if L. Bragi,1 is. Kr . , j[h, 2 5 7 , 1950 Copied from 'Dana, E, S, and hf h. ford, A textbook of mineralogy. John: Wiley, E. Y. ij.th, ed. 1952 f, -yf K f 0003776