Spodumene and its Alterations. 33? 



fibrous network, an interwoven congeries of needles, scales, and 

 blades, some of which have parallel sides, and are crossed ob- 

 liquely by minute planes (x 175), which suggest a cleavage 

 structure. There are also here and there a few more distinctly 

 crystallized blades of Muscovite, with parallel sides, which seem 

 to lie across the general direction, but which never present good 

 terminations. Among these blades are minute oblong bodies, 

 often in groups, which under a sufficiently high power are also 

 found to be perfect and doubly-terminated crystallized scales. 

 Their length varies from 0.0019 to 0.0114 mm. Many are scat- 

 tered through the material, having evidently crystallized between 

 the laminae, and others are gathered in groups of considerable 

 numbers. There are also numerous minute ochreous granules 

 dispersed throughout. With strong illumination, interference 

 fringes, with bright colors, are produced by the fine fibrous struc- 

 ture of the Aglaite. Between the crossed nicols of the polari- 

 scope, the mineral possesses a strong double refraction, with 

 colors varying from brownish yellow, through purple, into bright 

 blue. 



The mineral from Chesterfield also presents a fine fibrous 

 structure, somewhat curved, and without any interference fringes. 

 Muscovite is occasionally distinguishable, either in rectangular 

 blades or minute crystalline groups, sometimes 0.4 mm. in 

 length : its presence, as well as that of occasional remnants of 

 unaltered Spodumene in tiny fibres, seems to throw light on the 

 excess of protoxides found in the analyses. Ochreous granules 

 are not uncommon, and the existence of ferric oxide or hydrate 

 in this accessory condition indicates at least that a portion of it 

 should not have consideration in the formula. 



The fibres of Cymatolite seem to spring from the terminal 

 edges of those of the Spodumene, and form a continuation of 

 their structure : so that it is evident that the process of altera- 

 tion has most rapidly and favorably attacked the Spodumene 

 from the ends of its fibres. Between the crossed nicols, the 

 fibres present the usual strong double refraction and vivid colors, 

 while the grains of unaltered Spodumene are feebly illuminated 

 with colors varying on rotation from light brown to bluish gray. 

 In a thin section made from a sliced cross-section of an entire 

 pseudomorphous crystal, about an inch in diameter, of which the 



