MINERALOGY AND PETROLOGY. 945 



Cordlerite. Muscovite. Sericite.] 



Kekequabic lake (Nos. 551G, 1061, 1399), in the granite of White Iron lake (No. 953) 

 and in the granite quarried at East St. Cloud (No. 835). In all these cases it is so 

 environed, either microscopically or taxonomically, that it appears as the result of 

 contact of basic molten rocks on some acid rocks, and in the aporhyolyte it orig- 

 inated by crystallization from a molten acid magma, itself the result of fusion of 

 acid elastics. 



Cordierite, silicate of alumina and magnesia, with a small amount of iron, is one 

 of those colorless secondary minerals which resemble the acid feldspars in affecting 

 the zones of metamorphism and igneous contact. There is no doubt that it has 

 escaped observation in many instances, as it might be taken for feldspar or even for 

 quartz, unless specially examined. It not only occurs in the Animikie when modi- 

 fied by the gabbro to a fine mica schist (Nos. 1708, 2055, 370H), but in the biotitic 

 muscovadyte where it is associated with other magnesian minerals and with quartz 

 and labradovite, as well as much magnetite. In the former case it is plainly divided 

 into vertical sectors (No. 370H) which is not in keeping with the general rule lately 

 stated by Teall,* which requires this feature in cordierite of volcanic rocks rather than 

 in metamorphic. It is evidently frequently of later origin than the most of the 

 minerals of the rocks in which it occurs, as it embraces them in a micropoikilitic 

 manner (No. 1708). In the muscovadyte, i. e., the modified basic elastics of the 

 Keewatin (Nos. 1039, 1042, 1090, 1092), its habit is like that of the other minerals, 

 viz., roundish granular, or granulitic, but it sometimes is wholly surrounded by 

 quartz, by biotite (No. 1042), or by some feldspar, ^n all cases, therefore, in 

 Minnesota, so far as known, cordierite has resulted from metamorphism. 



Muscovite. This term has been applied to a light-colored mica seen in large, 

 sometimes porphyritic, scaly masses (Nos. 923, 2263, 387H, 390H). In this mica 

 silica, alumina and potassa are in greater amounts than in biotite, and magnesia is 

 wanting. It hence affects the alkaline and acid rocks, whether metamorphic or 

 plainly fragmental. In the metamorphic schists (No. 431) it is of late origin and 

 embraces the quartz poikilitically (No. 2061). It is uniformly the product of 

 alteration of alkaline feldspar. 



Sericite differs from muscovite only in being in minute scales. It is the basis 

 of the sericite schists, and is very abundant. It is the most conspicuous element in 

 the rejected products of recrystallization of the old feldspars in the metamorphism 

 of clastic debris (Nos. 1278, 2194, 2229) and remains at the centre of the old grains. 

 It has probably been called kaolinic in numerous instances in the microscopical 

 descriptions, although not having the vermicular grouping of the scales of kaolin, 

 and perhaps holding too much of the earthy or alkaline bases for kaolinite. 



* J, J. J. H. TEALU The Natural History of cordierite and its associates. /'/wm/in.'/* of (jrnlnijixts' Asmrinfion, vol. -xvi, Part 

 II, pp. 61-74, 1899. 



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