Spodumene and its Alterations. 349 



The Muscovite from this locality is thus determined as a half- 

 silicate, 



R 2 Al Sr O e = j Ar g 1 ^ Q11 



The Albite appears to be the sesqui-silicate — 

 Na 2 Al Si O 16 - i Na ' 4 Si3 ° 8 



It is probable that there has commonly occurred a splitting 

 up of two molecules of the bi-silicate, Spodumene, by a simple 

 transference of two atoms of silica from one to the other, into 

 one molecule of Muscovite and one of Albite, the lithium being 

 replaced by potassium or sodium out of the solution during this 

 change. This may be represented — 



2 Mol. Spodumene 1 Mol. Muscovite 1 Mol. Albite. 

 Hj At 2 Si 8 O 24 = E 2 Al Si 2 O 8 + Na 2 A± Si 6 1S 



It is further possible that the formation of the great masses of 

 Muscovite has been in large part due merely to the separation 

 of two molecules of silica from a molecule of Spodumene, the 

 silica thus isolated being deposited as quartz within many of the 

 pseudomorphs. 



Microscopic Structure. — In a thin section of Spodumene con- 

 taining Muscovite, the latter appears under a low power as clear 

 rounded glassy lakes ; while in some cases the Spodumene is 

 prolonged into slender threads, projecting into the mica parallel 

 to the bladed lamina? of the latter. 



V. 



Albite aftek Spodumexe. 



This form, generally intermixed with a little Muscovite and 

 less quartz, is a mere variety of III, much rarer than IV, and 

 always with rather indistinct outlines and terminal faces ob- 

 literated. 



VI. 



QlJAKTZ AFTER SPODUMENE. 



The pseudomorphs in quartz are very rare, sometimes six to 

 nine inches in length, with the original faces and striations of 



