136 Murgoci — Genesis of Rieheckite and Riebeckite Rocks. 



3. The riebeckite rocks represent in the massives the peg- 

 matitic varieties, consolidated under special physical conditions, 

 riebeckite being a mineral which requires pneumatolitic con- 

 ditions for its formation. All penologists, who have studied 

 riebeckite rocks, mention pegmatitic, micropegmatitic or grano- 

 phyric structures as being characteristic of them. Brogger* 

 has remarked that riebeckite (and arfvedsonite) occurs espe- 

 cially in rocks rich in quartz (over 60 per cent, according to 

 the analyses of Butureanuf and E. LudwigJ, up to 78*5 per 

 cent), and on the other hand, while segirite occurs in almost all 

 the rocks of the Christiania region, riebeckite appears only in 

 those which indicate high pressure, and is wanting in those peg- 

 matitic dikes, where gegerite is the most frequent mineral. 

 Flink & Boggild§ have described riebeckite (" type II of arfved- 

 sonite," Riebeckite V) in the pegmatitic schlieren of Greenland 

 (Narsarsuk), which are also very rich in segirite. It must be 

 noted, that the syenite pegmatites of Narsarsuk occur not as 

 veins or dikes, like those from Christiania, but " there are 

 dike-like syenite formations, which differ from the ordinary 

 typical syenite, in having as chief elements constituents, rich 

 in iron, of the pyroxene and amphibole series. There can be 



no question of dikes or deposits, the minerals show 



that the pegmatitic formations have arisen simultaneously with 

 the bulk of the rock." Heddle, Prior, Konig, etc., have also 

 studied perfect crystals from quartz dikes or miarolite cavities 

 only. On the other hand, Lacroix|| has described very charac- 

 teristic pegmatites, containing the only large crystals of rie- 

 beckite known, which come from Colorado (San Petro's Dom), 

 Corsica, Madagascar, etc. The riebeckite crystals occur in the 

 pegmatite-like black tourmaline in common pegmatites ; the 

 riebeckite pegmatites pass over gradually into granitic or 

 microgranitic rocks. 



* W. C. Brogger, Eruptivgesteine der Kristiania Gebietes, i, 1894, pp. 36, 

 39. 184, 186, etc. 



f V. Butureanu, Sur la composition des granite a Riebeckite de Jacob- 

 deal, Dobrogea. Annales scientifiques de l'Universite de Jassy, 1893. An 

 analysis of riebeckite too. 



% A. Pelikan, Petrographische Untersuchungen von Gesteine der Inseln 

 Sokotra, Abdel Ktiri nnd Semba. Denkschriften der Math. JNTaturw. Klasse 

 der k. Akademie Wien, 1902, lxxi. Analyses by Prof. E. Ludwig. 



§ J. Flink, 0. Boggild and Chr. Winter. Untersuchungen fiber Mineralien 

 von Julianehaab. Meddelelser om Groenland, 1899, 24. Eeference in Zeit- 

 schr. f. Kryst. andMin. xxxiv, 1901. — N. v. Ussing (ibidem, 1894, Abstract in 

 Neues Jahrbuch f. Miner., etc., 1901, 45) cites riebeckite also in syenite. 



I A. Lacroix, after the communications to Academie de Sciences, Paris 

 (Colorado, Comptes Rendus cix, 1889, Mt. Saber, C. R. cxxviii, 1899) returns 

 to the Corsican rocks (described first by Le Vevier, C. R. cix, 1899, and 

 Nentien, Mem. carte geol. France, 1897) in Mineralogie de la France, i, 695, 

 and recently discusses the general question in Materiaux pour la Mineralogie 

 de Madagascar. Nouv. Archives du Museum d'Hist. Natur. 1902-1903. See 

 also : Les travaux de M. A. Lacroix, 1903. 



