and its Spherulitic Crystallization. 107 



The calculated angles are tliose given by Grosser^' and tlie 

 agreement is very good, except in v (101), wiiich was not so 

 well developed as the other faces. The three cleavages paral- 

 lel to 100, 001, and lOl are excellent, as represented in the 

 iignre, tiieir excellence being in the order given. The plane 

 of tlie optic axes lies in the clinopinacoid and the bisectrix a 

 makes an angle of 33° in the acnte angle /3 with the vertical 

 axis ; Des Cloiseaux gives 35i° 12'. The optical character is 

 negative, o being the acute bisectrix. Since, according to 

 Des Cloiseanx, the optic angle in air 2 E = about 70°, it follows 

 that one optic axis emerges almost perpendicular to c, 001, the 

 other at an angle of about 16° to a (100). Since both of these 

 are good cleavages, it follows that when one examines the 

 powder made bj crushing the material, in convergent light 

 under the microscope with crossed nicols, almost invariably 

 each fragment exhibits the locus of an optic axis either just in, 

 or just off, the lield of view. Since the a (100) cleavage is 

 the best, it is mostly the latter case that obtains and in the 

 blades the long direction is the one of least elasticity. 



While these crystallographic and optical properties prove 

 the nature of the mineral, its identity was confirmed by the 

 fact that, when powdered, it dissolves readily in hydrochloric 

 acid and yields gelatinous silica. 



It will be noted in the drawing of the crystal shown in fig. 

 4 that the a (100) faces are extended in a thin plate forming 

 re-entrant angles ; commonly these thin plates extend far 

 beyond the main crystal and from each of the four corners, 

 tapering off indefinitely ; the cross section of the whole then 

 shows an H with the vertical legs greatly extended. There 

 may be other cross connections producing ladder-like affairs. 

 It is also to be recalled that these extend as sheets perpendicu- 

 lar to the plane of the drawing, or along the b axis. More- 

 over the sheets are often curved and numbers of these extended 

 plates are grouped into sheaves or rosette-like groups, and thus 

 a variety of patterns are produced as these are cut by the sec- 

 tion at various angles. The individual filaments as they 

 appear in the section are commonly curved ; if examined with 

 a very high power it is seen that the curve really consists of a 

 series of short minute straight pieces of crystal with wedge- 

 shaped cracks between, continuity obtaining only along one 

 edge. By this curving, and by repeated branching, arborescent, 

 or plumose, forms are produced, and in places the glass 

 between crossed nicols appears filled with them and seems like 

 masses of magnificent ostrich plumes thickly scattered, the 

 beauty of whose effect is greatly heightened by the use of the 

 sensitive tint, which turns them brilliant blue and yellow. 

 *Zeitsclir. f. Kryst., xix, p. 608. 



