410 



A A TURE 



[August 24, 1905 



insoluble ; the experiments which are wanted are experi- 

 ments upon the solubility of certain minerals in magmas of 

 known composition under known conditions ; in these and 

 similar instances the order of separation is that of the 

 solubility, but such physical factors as the velocity of 

 crystallisation (which varies very considerably with the 

 temperature), and the viscosity, may completely invert the 

 order. 



Direct experiments made by Barus and Iddings upon the 

 electric conductivity of silicate magmas afford evidence that 

 such magmas contain dissociated as well as undissociated 

 molecules, so that they cannot be regarded as merely fused 

 mixtures of certain minerals. If two or more rock-forming 

 minerals be fused together it may happen that thev form 

 new compounds and crystallise out as different minerals, 

 or if one or the other remains unchanged it may crystallise 

 out in a different proportion. .-Ml this shows that double 

 decomposition goes on in the liquid. We cannot therefore 

 expect, without knowing the degree of dissociation, to 

 make much use of the lowering of the freezing point in 

 order to calculate the other factors in the process of rock- 

 formation. 



Doelter concludes that upon the whole the normal order 

 of crystallisation in rocks is in the main that laid down 

 by Rosenbusch long ago, namely, an order of increasing 

 acidity, but that it is determined by the mutual affinities 

 of the molecules in the magma, and by the relative power 

 of crystallisation of the components into which they unite 

 themselves, and that the physical factors which I have 

 already enumerated play a very important part in the 

 process. No one has endeavoured more systematically than 

 Doelter to determine for the rock-forming minerals the 

 melting points and the solubilities, without which it is 

 impossible to make much progress in our reconstruction 

 of the history of rocks. He has recently shown us how 

 the microscope may be used in the study of fused silicates 

 at high temperatures, and has so opened up a new field 

 of research. 



Vogfs Afyplications of the La-^'s of Solutions. 



The work of Vogt has extended over many years, and is 

 now summarised in two remarkable memoirs recently pub- 

 lished by him, in which are expressed his mature opinions 

 upon silicate magmas ; the reasoning is based upon his own 

 experiments, upon those of Doelter, and upon the classic 

 researches of Ebelmen. It is now generally conceded that 

 the particular minerals produced in a silicate magma depend 

 much more upon the chemical composition of the magma 

 than upon tempprature and pressure ; Lagorio and Moroze- 

 wicz were led to this conclusion by their own experiments 

 upon fused silicates. Experiments upon slags at ordinary 

 temperatures and pressures may, therefore, be invoked 

 to elucidate the formation of rocks. 

 _ In 1902 Vogt slated his conviction that the laws of solu- 

 tions may be applied to igneous rocks, and his two recent 

 memoirs are, in fact, an attempt to explain the experiments 

 upon slags and fused silicates as examples of the operation 

 of these laws. 



All important, according to him, is the composition of 

 the eutectic mixture ; he finds that if the analyses of silicate 

 magmas be arranged according to their oxygen ratio or 

 acidity, the various minerals of which they consist make 

 their appearance within fairly well-defined limits. For ex- 

 ample, in the case of the Ca-Mg-Fe-Mn slags, which contain 

 little alumina, olivine and the melilite minerals only make 

 their appearance in the more basic slags, and the meta- 

 silicates in the more acid, the limit between the two corre- 

 sponding to an acidity of about i-6. 



The limit of individualisation between the various 

 minerals is supposed to correspond to their eutectic mixture. 

 Such slags may, therefore, be regarded as a mutual solution 

 of two or more of the minerals olivine, enstatite, hyper- 

 sthene. augite, the gehlenite-melilile group, akermanite, 

 wollastonite, and the hexagonal metasilicate, which is so 

 characteristic of the more acid slags. The particular 

 minerals which make their appearance are practically de- 

 termined by the acidity of the magma and by the relative 

 proportion of the bases present, particularly by the ratio of 

 the calcium to the magnesium-iron-manganese group ; in 

 other words, Vogt asserts that a silicate magma is a mutual 

 solution of the various crystalline compounds that actually 

 NO. 1869, VOL. 72] 



make their appearance as it solidifies, and that the order 

 of crystallisation depends upon their proportion in the 

 magma as compared with their proportion in the eutectic. 

 The old conception of a solvent and a solute ceases to 

 have much meaning ; the matter which is of supreme 

 importance is the nature of the eutectic mixture when the 

 constituents are given ; thus micropegmatite and micro- 

 felsite represent the eutectic of felspar and quartz, and 

 correspond to a mixture of about 74 parts of felspar to 

 26 of quartz, as indeed has been stated by Teall. 



Now, if we are justified In regarding rock-magmas and 

 fused silicates as mutual solutions of certain definite com- 

 pounds, and if these compounds are actual minerals or 

 other silicates which crystallise out of the magma when it 

 cools, we are also justified in making use of the properties 

 of these minerals when we apply to the magma the known 

 physico-chemical laws which govern solutions. 



The number and nature of the minerals which can be in 

 equilibrium with each other and the solution are to be 

 determined by experiments upon their solubility interpreted 

 by the phase-rule of Willard Gibbs, and especially by the 

 laws which Roozeboom and other physical chemists have 

 deduced for components which form double salts or iso- 

 morphous mixtures. Knowing the components we ought, 

 therefore, to be able to determine their latent heat of fusion, 

 their specific heat, the lowering of the freezing point of 

 their mixtures, and from these data to calculate the true 

 formula? of the rock-forming minerals. It will readily be 

 understood that in a mixture of quartz and orthoclase, the 

 lowering of the freezing point below that of either of the 

 constituents, as calculated by van 't Hoff's formula, from 

 their melting points, latent heats, and molecular weights, 

 will be very different according as the formula of quartz 

 is taken to' be SiO, or Si,0,. 



Vogt boldly attacks the whole problem as one that can 

 be solved on these lines ; we have good reason to believe 

 that the slags and rock-magmas are solutions ; we know 

 their constituents ; we can therefore proceed to experiment 

 with these constituents and to predict the behaviour of 

 their mixture according to the principles of physical 

 chemistry. The order of crystallisation is mainly deter- 

 mined by the relative composition of the magma and the 

 eutectic, and the composition of the eutectic may be cal- 

 culated from the intersection of the freezing-curves. 



One interesting result is the conclusion that in the 

 silicate magmas which have been the subject of experiment 

 the minerals produced are all of very simple constitution : 

 that, for example, olivine, diopside, akermanite, melilite, 

 and anorthite have the simplest possible formulje corre- 

 sponding to their analyses and are not polymerised. 

 Mineralogists will welcome this conclusion if it be true, 

 for it has occasionally been the fashion on theoretical 

 grounds to attribute a high degree of polymerisation to 

 many minerals, and nothing is easier than to account for 

 many difficulties if one may multiply the formula of a 

 mineral by any number that is required. It should be 

 added, however, that Doelter, calculating from his own 

 experiments, is led to think that some of the minerals 

 must have formulas which are multiples of their empirical 

 formula. 



Vogt even goes a step further in his application of the 

 principles of modern chemistry. The order of crystallisa- , 



lion appears to be by no means always that of the solu- ■ 



bility, but indicates that a mineral is sometimes not so | 



soluble as might be supposed. Now another principle in 

 the modern physics of solutions is that by adding to a 

 solution of one substance a new electrolyte containing an 

 ion common to both the solubility of the first is di- 

 minished, and Vogt does not hesitate to apply this principle. 



Thus spinel and felspar in mutual solution, when felspar 

 is in large excess, should on cooling yield felspar first. 

 But in many basic rocks spinel is the first to crystallise ; 

 this is, according to Vogt, due to the presence of ferro- 

 magnesian silicates containing the Mg-ion which is also 

 present in spinel : if these be partially dissociated the solu- 

 bility of the aluminate will be lowered. 



An obvious criticism on this argument is that if the 

 dissociation is so slight that it may be ignored for one 

 purpose, it is hardly fair to invoke its powerful action 

 for another, and it is possible that Vogt in his enthusiasm 

 for a theory attempts to explain too much by its aid. 



