August 6, 1909] 



SCIENCE 



169 



Nephelite, 



Eucryptite, 



Kaliophilite, 



Andaliisite, 



Topaz, 



Al3(SiO.)3Na3. 

 AljlSiOJjLij. 



AlaCSiOaK,. 



Al3(SiO,)3(A10),. 

 Al3(SiO,)3(AlF.)3 



All of these formulae are equivalent to the 

 simplest empirical formulaj tripled ; a mul- 

 tiplication which is suggested by the fact 

 that all of the minerals named alter into 

 muscovite, Al3(Si04)3KH,. This species 

 in turn is correlated with the other normal 

 micas, as follows: 



Muscovite, AJjlSiOJjKH,. 



Biotite, Al,(SiO.)3MgJ<H. 



Phlogopite, AliSiOJjMg^KH,. 



Between these micas there are intermediate 

 mixtures, and sometimes admixtures of 

 molecules derived in precisely the same 

 way from trisilicic acid, H^SisOs. That is, 

 the radicles SiOi and SiaOg replace each 

 other isomorphously, a relation which is 

 also indicated in the feldspar group. Al- 

 bite is a trisilicate, anorthite is an ortho- 

 silicate; the other soda lime feldspars are 

 mixtures of these two. 



This formulation of the micas has had 

 considerable acceptance, and it brings the 

 allied compounds under one general set of 

 expressions. These again connect with the 

 vermiculites and chlorites, and with a num- 

 ber of other species such as the garnets, 

 prehnite, some zeolites, etc. In short, a 

 system of formula has developed from 

 work done in the survey laboratory which 

 expresses in s3Tnbolic form known relations, 

 and is therefore legitimate so far as it 

 goes." It is probably not final, but its 

 usefulness is apparent and has been tested 

 for many years. When a better system 

 offers, one which correlates a larger number 

 of facts, it will be time to abandon this and 

 to accept the new. 



"See U. S. Geol. Survey Bulletin 125 for a 

 complete statement of the silicate theory. Some 

 of the conclusions reached in that memoir need 

 to be revised in the light of more recent knowledge. 



Another group of reseai'ches, now under 

 way in the survey laboratorj^, relates to the 

 theory of ore deposition. It had been ob- 

 served by geologists connected with the 

 survey that ore deposits often exhibit the 

 phenomenon of secondary enrichment ; that 

 is, the heavier metals are more or less dis- 

 solved from the upper part of a lode, to be 

 i-eprecipitated at lower levels. The chem- 

 ical processes governing this leaching and 

 redeposition are among the subjects under 

 investigation in the laboratory. This prob- 

 lem, obviousl}', interlaces with other related 

 problems, and some interesting results have 

 already been obtained. Dr. H. N. Stokes 

 has studied the relations between pyrite 

 and marcasite,^^ and in another investiga- 

 tion^^ he has determined some of the condi- 

 tions governing the solution, transportation 

 and deposition of silver, copper and gold. 

 The details of these experiments do not 

 admit of anj-- brief summary here. Dr. E. 

 C. Sullivan^' has shown that many secon- 

 dary precipitations of the heavy metals are 

 of the nature of double decompositions; a 

 copper solution, for example, reacting with 

 silicates such as the feldspars to give up its 

 copper and to receive some other base in 

 return. He has also found^* that the wall 

 rock of a vein may act like a semi-per- 

 meable membrane and so effect the separa- 

 tion of certain bases from their salts. A 

 solution of ferric sulphate, for instance, 

 hydrolyzes, and then contains ferric hy- 

 droxide in the colloidal condition. When 

 this operation takes place in a Pasteur filter 

 tube the colloidal substance is retained, 

 while the liberated acid passes through. In 

 short, dialysis seems to be one of the proc- 

 esses by which mineral solutions are sepa- 

 rated into fractions of different composi- 



" Bull. 186, U. S. Geol. Survey; and Economic 

 Geology, 2, 15. 



"Economic Geology, 1, 044. 



"Bull. 312, U. S. Geol. Survey. 



" Economic Geology, 3, 750. 



