14:0 Mtirgoci — Genesis of Riebeckite and Riebeckite Rocks. 



other hand, the schistose metamorphosed riebeckite rocks of 

 Gloggnitz and Alter Padroso, etc., show no transformation in 

 this respect, segirite and riebeckite having the same character 

 and both being primary, often grown together or zonal, as in 

 the impressed eruptive rocks. We are forced accordingly to 

 invoke, besides the composition of the magma and the pressure, 

 also that important agent, which has left its traces in the com- 

 ponents of these rocks, the mineralizers* 



If now, p represents the conditions of pressure, which may 

 be unity, 'Or one atmosphere, and m the mineralizersf, which 

 may be much reduced, of a magma in which the whole sub- 

 stance Si 6 1B Fe /,/ , Fe" JS"a 2 may crystallize as segirite, then P and 

 M may be the conditions of minimal pressure and mineralizers 

 in which the same substance may crystallize as riebeckite. 

 One can imagine that between the points p, m and P, M there 

 may be a large number of stages (P x , M y ), where there can 

 arise a variable percentage A of segirite to R of riebeckite, 

 more or less respectively, accordingly as a particular stage is 

 nearer to the point p, m or P, M.J The variation of the 

 medium /jl (composition of magna + mineralizers) influences 

 in large proportion the phenomena, and the representative 

 curve of the phenomena f (m, p) is displaced in plane, and for 

 a definite value of //. we meet with a critical point for the for- 

 mation of riebeckite; in a magma below this limit, — rie- 

 beckite can no longer form under any pressure by the given 

 mineralizers. And according to these different conditions of 

 a magma, pressure and mineralizers, there can originate in one 

 and the same igneous mass and at the same time of consolida- 

 tion rocks with riebeckite only, rocks with riebeckite and 

 segirite in all proportions, and those with segirite alone. 



In this way we can explain the relations which have been 

 observed between riebeckite, segirite and fluorspar. The min- 



* F. Loewinson-Lessing in his interesting discussion (1. cit. p. 359) admits 

 the necessity for pressure and an active gas (he means water) for the forma- 

 tion of amphiboles. The experiments which he has made by melting pyrox- 

 enes and amphiboles in an atmosphere of water vapour have not been suc- 

 cessful in producing hornblende. It would be very interesting to know what 

 would be the result of an experiment in a fluorine atmosphere under a high 

 pressure ! I may note that the only synthetic hornblende (with 2% Na 2 0) 

 was obtained by Chrustschoff (1891) in sealed tubes in the presence of water 

 at high temperature. 



f That is to say, the capability of the mineralizers for forming minerals. 



X This question is a very complicated one and we do not know how many 

 substances and how many phases there are at a given moment. We may, 

 however, imagine the simplified case of n substances (magma, iron-sodium 

 silicate and fluorine mineralizers) and n + 1 phases (magmatic solution, rie- 

 beckite, segirite and gas). The most analogous example is to be found in 

 the crystallization of calcite and aragonite (or conchite) from a dilute solu- 

 tion at varying temperatures. See : Beitrage zur mineralogischen Kenntnis 

 der Kalkausscheidung im Tierreich von Agnes Kelly. Jenaischen Zeitschrift 

 fur Naturwissenschaft, 1900. 



