MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 627 



of garnet — ^may thus also be formed under high temperature at the 

 pressure of one atmosphere. 



Garnet occurs as a primary formation in some effusives and 

 often in various deep-seated rocks. The mineral further is formed 

 as known inter alia by contact-metamorphism and by intensive 

 dynamo-metamorphism . 



Amphibole, as is known, does not crystallize from anhydrous 

 silicate melts at one atmosphere but it is generally supposed to be 

 conditioned by high pressure. This depends rather certainly on 

 the fact that the magmas only at high pressure may carry the 

 sufficient quantity of water (or hydroxyl) which seems to be a 

 constitutional component of amphibole. 



Biotite has, as is known, been produced synthetically by melting 

 with fluorides, and the magnesia-mica phlogopite has also been 

 estabUshed as crystallized out of common anhydrous slags contain- 

 ing a little fluorine.^ Phlogopite may thus be formed by crystal- 

 lization of silicate melts at a pressure of one atmosphere. But 

 common mica occurs in the igneous rocks preferentially in deep- 

 seated rocks; it occurs also, however, in effusives. On the other 

 hand, muscovite never occurs as a primary mineral in effusives, but 

 occurs in certain granites and especially in granite-pegmatite dikes. 



As we shall show later, in granites of exactly the same composi- 

 tion — excluding the original content of H2O, etc. — hypersthene, 

 biotite or muscovite may crystallize. Which of these three minerals 

 is formed, may depend on the content of H2O, etc., in the magma 

 (when least, hypersthene is formed; when most, muscovite). 



== See my cited treatises from 1884-85 and 1888-90. In slags from the Kafveltorp 

 copper works (containing 41-46 per cent Si02, 8-1 1 AI2O3, 7-15 FeO including ca. 0.5 

 FcaOs, 13-20 CaO, 10-18 MgO, a little ZnO, CuaS, etc., also 3-5 K^O+Na^O and a little 

 F) there has crystallized 10-15 per cent mica, in great leaves up to 5-6 mm. in diameter, 

 with the following characteristics: pseudohexagonale thin leaves, with just as good 

 cleavage as natural mica; elastic flexible; pressure figure as in natural mica; prism 

 angle ca. 121°; optical biaxial and negative; acute bisectrix almost perpendicular on 

 001, differing only by 1-1.5°; 2 V only a very few degrees; colorless; very vivid inter- 

 ference colors; very resistent to acids; chemical composition: 42.20 SiOa, 11.30 AI2O3, 

 S.92 FeO (with a little FcaOj), 22.93 MgO, 2.29 CaO, 1.40 ZnO, 0.33 Cu^S (mechanical), 

 total 86.37 per cent, rest ca. 13 per cent K2O, NaaO and F. The mmeral is thus a MgO- 

 mica rather poor in FeO and FcjOi, viz., a phlogopite, whose content of H2O (or HO) 

 is replaced by another component, F. 



