48o 



NA TURE 



[September 14, 1893 



nitrogen and a phenyl radical were eliminated, and the puri- 

 fied residue was found to approximate in composition toSlNPh, 

 which would represent the body as phenylsilicocyanide or a 

 polymer of it. Even careful heating of the diimide in ammonia 

 gas has not enabled me to remove all the phenyl from the 

 compound, but rather to retain nitrogen, as the best residue 

 obtained from such treatment consisted of SijNjPh, or the 

 phenylic derivative of one of the substances produced by Schut- 

 zenberger and Colson from the ammonia reaction. It may be 

 that both these substances are compounds of silicocyanogen 

 with an imide group of the kind below indicated — 



SiN. 



>NH 



SiN/ 



SiN., 

 SiN 



")>NPh 



Further investigation must decide whether this is a real re- 

 lationship ; if it be, we should be able to remove the imidic 

 group and obtain silicocyanogen in the free state. One other 

 point only need be noticed, namely, that when the above silicon 

 compounds are heated in oxygen they are slowly converted into 

 SiOj ; but the last traces of nitrogen are removed with great 

 difScalty, unless water-vapour' is present, when ammonia and 

 silica are quickly formed. 



Much remains to be done in this department of comparative 

 chemistry, but we may fairly claim to have established the fact 

 that silicon, like carbon, can be made to form perfectly well- 

 defined compounds in which it is exclusively united with the 

 triad nitrogen of amidic and imidic groups. 



Now, having proved the capacity of silicon for the formation 

 of compounds of this order with a triad element. Nature very 

 distinctively lets us understand that nitrogen is not the particular 

 element which is best adapted to place the triad lole towards 

 silicon in its high- temperature changes, which are ultimately 

 dominated by oxygen. We are not acquainted with any natural 

 compounds which include silicon and nitrogen ; but large 

 numbers of the most important minerals contain the pseudo- 

 triad element aluminum combined with silicon, and few include 

 any other triad. Phosphorus follows silicon in the periodic 

 system of the elements as nitrogen does carbon, but silicates 

 containing more than traces of phosphorus are rare ; on the 

 other hand, silicates are not uncommon containing boron, the 

 lower homologue of aluminum ; for example, axinite, datholite, 

 and tourmaline, 



Moreover, it is well known that silicon dissolves freely in 

 molten aluminum, though much more of the former separates 

 on cooling. Winkler has analysed the gargue of aluminum 

 saturated with silicon, and found that its composition is ap- 

 proximately represented by the formula SiAl, or, perhaps, 

 Si2Al2, if we are to regard this as analogous to CjNj or 

 cyanogen. Here aluminum at least resembles nitrogen in 

 directly forming a compound with silicon at moderately high 

 temperature. It would appear, then, that while silicon can 

 combine with both the triads nitrogen and aluminum, the marked 

 positive characters of the latter, and its extremely low volatility, 

 suit it best for the production of permanent silicon compounds 

 similar to those which nitrogen can afford. 



With these facts in mind we may carry our thoughts back to 

 that period in the earth's history when our planet was at a higher 

 temperature than the dissociation point of oxygen compounds. 

 Under such conditions the least volatile elements were probably 

 liquids, while silicides and carbides of various metals were 

 formed in the fluid globe. We can imagine that the attraction 

 of aluminum for the large excess of silicon would assert itself, 

 and that, as the temperature fell below the point at which 

 oxidation become pcssibb, these silicides and carbides under- 

 went some degree of oxidation, the carbides suffering most 

 owing to the volatility of the oxides of carbon, while the fixity 

 of the products of oxidation of silicides rendered the latter pro- 

 cess a more gradual ore. The oxidation of silicides of metals 

 which had little attraction for silicon would lead to the forma- 

 tion of simple metallic silicates and to the separation of the large 

 quantities of free silica we meet with in the solid crust of the 

 earth, whereas oxidation of silicides of aluminum would not 

 break up the union of the two elements, but rather cause the 

 ultimate formation of the alumino-silicates which are so abundant 

 in most of our rocks. 



Viewed in ihe light of the facts already cited and the infer- 

 ences we have drawn from them as to the nitrogen-like relation- 

 ship of aluminum to silicon, I am disposed to regard the natural 

 alumino-silicates as products of final oxidation of sometime 



NO I 246. VOL. 48] 



active stlico aluminum analogues of carbo-nitrogen compounds 

 rather than ordinary double salts. It is generally taken fo 

 granted that they are double salts, but recent work on ihi 

 chromoxalates by E. A. Werner has shown that this view is no 

 necessarily true of all such substances. 



Without going into undue detail we can even form some con 

 ception of the general course of change from simple aluminun 

 silicide to an alumino-silicate, if we allow the analogies alread; 

 traced to lead us further. 



We recognise the existence of silico-formyl in Friedel an( 

 Ladenburg's silico-formic anhydride ; hence silico-triformamid 

 is a compound whose probable formation we can admit, and, oi 

 the basis of our aluminum-nitrogen analogy, an aluminun 

 representative also. Thus — 



^COH 



N— COH 



^COH 



Triformamide. 



.SiOH 



N— SiOH 



^SiOH 



Silico-Irifor- 

 matnide. 



.SiOH 



Al— SiOH 



^SiOH 



.SiO^R' 



AI— SiOjR' 



\siO,R' 



Silico-alumino- Salt ofanaluminc 

 triformamide. silicic acid. 



Now, oxidation of triformamide would lead to complet 

 resolution into nitrogen gas, carbondioxide gas and water ren 

 dering it an extremely unstable body ; under similar condition 

 silico-triformamide would probably afford nitrogen gas am 

 silicic acid (or silicon dioxide and water) ; while the third com 

 pound,* instead of breaking up, would (owing to the fixity o 

 aluminum as compared with nitrogen) be likely at first to affon 

 a salt of an alumino-silicic acid, in presence of much basi 

 material. 



The frequent recurrence of the ratios Si3Al,Si|,.\l2, &c., in th 

 formulae of natural alumino-silicates, suggests that some at leas 

 of these minerals are derived from oxidation products of th' 

 above triformic type. Without stopping to trace all the pos 

 sible stages in the oxidation of the primary compoun< 

 Al(Si0.2R)3, or variations in basicity of the products, I may cil' 

 the four following examples out of many others which might b 

 given of resulting representative mineral groups : — 



^SiO,R' 



^SiO^R'a 



/SiOiR'j 



SiO^R"' 

 Al— SiOiR" 



Al— SiOjR' : Al— SiOjR'a : Al— SiOjR 

 "^SiOjR' \siO4R'" \siO.,R"' ^SiajR" 



Beryl type (hemi-). Garnet type. Muscovite type. Xenolite type. 



Five years ago Prof. F. W. Clarke, of the United State; 

 Geological Survey, published a most interesting paper on thi 

 structure of the natural silicates. In this he adopts the view 

 that the mineral xenolite, SiaAljOu, is the primary fron whicf 

 all other alumino-silicates may be supposed to arise by varioa: 

 substitutions. Nature, however, seems to teach us that sue! 

 minerals asxenolile, fibrolite, and the related group of "clays' 

 are rather to be regarded as end-products of a series of hydro- 

 lytic changes of less aluminous silicates than primary substances 

 themselves ; hence the sketch which I have ventured to give 

 above of the probable genesis of alumino-silicates seems to pro- 

 vide a less arbitrary basis for Claike's interesting work, withoal 

 materially disturbing the general drift of his subseqa«nt 

 reasoning. 



We may now consider for a moment in what direction evi- 

 dence can be sought for the existence in nature of derivatives 

 of the hypothetical intermediate products of oxidation between 

 a primary silicide and its fully oxidised silicate. 



In the absence of a working hypothesis of the kind which I 

 have already suggested it is not probable that direct evidence 

 would yet be obtainable — this must be work for the future— but 

 when we consider that the existence of compounds of the oid« 

 in question would manifest themselves in ordinary mineni 

 analyses by the analytical products exceeding the original wei(^ 

 of material, we seem to find some evidence on the point iB 

 recorded cases of the kind. A deficiency of a single atom of 

 oxygen in compounds having the high molecular weights of 

 those in question, would be indicated by very small excesses 

 (from 2 to 3 per cent.) whose real meaning might be easily 

 overlooked. Now, such results are not at all unusual W 

 analyses of mineral alumino-silicates. For instance, Amphilmts 

 containing a mere trace of iron have afforded 10275 pirts from 

 100, and almost all analyses of i)/;W-c.wOT«;;V^ are high, givii^s* 

 much as 103 parts. In less degree Vestivianite and members of 



1 In these cases where R'" = Al it is, of course, assumed that the latter » 

 acting only as a basic radical. 



