Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 137 



4. I may further emphasize the similarity of riebeckite, 

 in its occurrence and petrographic characters, with the tourma- 

 line from tourmaline granites, aplites, etc. The poikilitic 

 structure of the large crystals, hypidiomorphic and allotrio- 

 morphic forms in one and the same rock, are characters com- 

 mon to both riebeckite and tourmaline. The rocks with 

 tourmaline are very alkalic like those with riebeckite* ; rie- 

 beckite like tourmaline eliminates other black constituents 

 such as biotite. The riebeckite rocks have their special acces- 

 sory mineral, zircon, in the same way that tourmaline rocks 

 carry cassiterite.f 



5. The quartz and the feldspars (which are orthoclase either 

 with patches of soda-microcline or with albite in microperthite 

 intergrowths, all more or less idiomorphic) contain many inclu- 

 sions of riebeckite, zircon, and liquids with bubbles and cubes 

 of common salt. 



6. A great deal of zircon accompanies the riebeckite ; 

 Brogger, Washington, Mrazec, Lacroix and Souza Brandao 

 (1905) emphasize this fact. Lacroix found as much as 7*5 per 

 cent zircon in the rocks of Madagascar. The barkevikite rocks, 

 on the other hand, contain much titanite. It is worthy of men- 

 tion in this connection that Brogger;}; states that in the middle 

 of a dike of quartz-lindoite (of west Aker, Christiania) riebeckite 

 occurs with much zircon, crystallized after riebeckite, whilst at 

 the salband there is katoforite and segirite without zircon ; on 

 the other hand, zircon is very frequent in the pegmatitic dikes 

 at the Christiania region. Zircon and titanite have been formed 

 during the whole time of the consolidation of the magma-like 

 riebeckite. 



7. In miarolitic cavities of these rocks fluorspar, galena, 

 zircon (spinel?) and riebeckite have been found together. 

 Brogger, Lacroix and Washington state that fluorspar is often 

 a constituent of the rocks rich in soda, which contain segirite 

 and riebeckite, and in general I have also found it in many of 

 the rocks of the region studied by me and in those of other 

 places, such as in the Quincy granite, the trachyte of Berkum 

 near Remagen, the microgranite of Ailsa Craig, etc. 



The occurrence of fluorspar in certain granites is very 

 important from a theoretical standpoint ; in them fluorspar 

 occurs as small crystals, often microlites, grown together with 

 or included in the segirite ; riebeckite when inter-grown with 

 segirite is quite free from such fluorspar inclusions, but may 

 contain small pockets of rare carbonates (parisite \). In general 



* The complicated composition of riebeckite is well known. I know of 

 six analyses and no two alike ; the differences can not come from mistakes 

 only. On the other hand, riebeckite contains Fl also. 



f A. Lacroix, Materiaux de Madagascar, loc. cit., i, p. 89. 



% W. C. Brogger, loc. cit., pp. 137, 138. 



