326 Spodumene and its Alterations. 



4 Si 112. 



2 Al 54.6 



Percentages. 

 Silica 63.75 



Alumina 27.25 



| Mg \ 4.5 \ equivalent to \ Magnesia (or Lime) 1.99 

 Na 2.9 I I Soda 1.05 



Li 10. 5 J [Lithia 5.96 



12 192. 100. 



376.5 



In general, it appears that the alumina is sometimes replaced 

 by ferric oxide ; the magnesia by lime, ferrous oxide, or oxides 

 of other dyads ; the soda by potassa ; and the lithia by variable 

 amounts of all the preceding monad and dyad elements. 



To recapitulate, it appears to me probable, that the duller 

 color and the inferior lustre, translucency, hardness, and spe- 

 cific gravity of the specimens of the mineral from Norwich and 

 other localities, which have hitherto supplied material for analy- 

 sis, indicate that in most of these cases the mineral has been 

 somewhat affected by weathering, facilitated both by the easy 

 cleavage of the mineral and by the abundant rifts of the cpiartz 

 matrix. The loss of a portion of its soluble protoxides — par- 

 ticularly the alkalies — has in most instances increased the con- 

 tent of both silica and alumina in the residue. This slight 

 decomposition has naturally affected the physical characteristics 

 by the differences already described. At both Goshen and 

 Chesterfield the green transparent variety is uncommon, and at 

 Norwich exceedingly rare ; while the existence of this decom- 

 position is confirmed by the corresponding effects upon the 

 minerals associated with Spodumene, especially Garnet, Zircon, 

 Triphylite, etc., and by the deposit upon the superficies of the 

 Spodumene crystals, and within all interstices, of black Pyrolu- 

 site in delicate tracery, reddish-brown iron-ochre, scales of Autu- 

 nite and uran-ochre, and a pink substance, the latter at Norwich 

 acting as a characteristic coloring-film upon the Spodumene. 



Microscopical characteristics. — On the microscopical exami- 

 nation of a thin section of the unaltered green Spodumene from 



* These figures agree closely with those of Brush, (Am. J. Sci., II, x, 370, 1850). 



