Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 139 



strup* by melting arfvedsonite has obtained gegirite ; Doelter 

 bj melting gastaldite in sodium fluoride has also obtained 

 gegirite or akmite, but without fluoride there resulted an 

 amorphous mass. These two experiments and many others 

 have shown that gegirite and pyroxene can originate in molten 

 masses under ordinary conditions. This is, however, not the 

 case with amphiboles, and recently Vogtf has demonstrated 

 again that amphiboles require high pressure for their forma- 

 tion. Considering the facts more closely, for a medium to be 

 capable of giving rise to the riebeckite or gegirite * molecule 

 (which may be expressed by SigO^Fe"', Fe" Na a J), there 

 appear to be two chief factors necessary, namely pressure and 

 miner alizers%. The obvious fact that these two minerals can 

 originate at the same time in a magna, shows that there cannot 

 be much difference between their coefficients of solubility, i. e., 

 their capacity for forming saturated solutions in the molten 

 mass ; on the other hand, the melting point of these minerals 

 (aeg. = 940°, rieb. = 945° C. according to Doelter) is not differ- 

 ent under ordinary circumstances and cannot vary much if 

 the circumstances vary in the same way for both minerals. 

 The structure of the riebeckite-aegirite rocks and the mode of 

 occurrence of these minerals support the statement of Hopfner 

 verified by Yogt (1. c), that pressure has not much influence 

 on the order of separation of the minerals in a magma and on 

 the composition of eutectic mixtures. I may add, with 

 respect to the ideas of Loewinson-Lessing,|| that pressure alone 

 is not sufficient to force a dimorphous substance to crystallize 

 in one form rather than in another, although one may have a 

 smaller true molecular volume than the other. According to 

 this general dynamic rule, gegirite, with the smaller molecular 

 volume, should be the characteristic mineral of the abyssal 

 rocks rich in soda. Observation and experiment contradict 

 this : gegirite can form under ordinary pressure and occurs 

 much more in hypabyssal and volcanic rocks than in abyssal 

 ones ; riebeckite has not been obtained at ordinary pressure, 

 but it occurs in trachyte with fluorspar and in rhyolites, which 

 clearly show evidences of a pneumatolytic process. On the 



* The best argument for the primary existence of the gegirite is its occur- 

 rence, in the same rock, with little thin needles of riebeckite, which could 

 not resist even the slowest and slightest action of transformation. 



f T. H. Vogt, Die Silikatschmeltzlosungen. Mem. of the Acad, of Chris- 

 tiania, 1904. 



% This formula given by Konig, and confirmed by Butureanu on the 

 Dobrogean riebeckite, agrees very closely with the analyses of gegirite by 

 Doelter. 



§ This question in particular I intend to take up again, after some experi- 

 ments, with more detail. 



I F. Loewinson-Lessing, Studien iiber Eruptivgesteine. Memoires du Con- 

 gres Geol. de St. Petersburg, 1897, p. 325 f. 



