308 S. L. Penfield — Crystallization of Willemite. 



eter, was so attached that only a part of its faces were devel- 

 oped, which are represented in about their natural proportion 

 in fig. 5. The prominent rhombohedron u is of the second 

 order, and if it corresponds in position to u in the New Mexico 

 willemite, 2ll3, ■§— 21, then the terminal faces are those of the 

 lower, instead of the upper end of the crystal,* and if sym- 

 metrically developed would appearas in fig. 6. The rhoinbo- 

 hedron s is 1123, §— 2r and e is 0112, —J. The prisms are of 

 the first order w, of the second order a and of the third order 

 h, 3120, i-\ I, although the symbol of the latter is uncertain, as 

 satisfactoiw measurements could not be made. 



The second specimen contained long prisms of nearly trans- 

 parent, pale-green willemite, imbedded in a pink manganifer- 

 ous calcite. Only one of these, measuring about 6 mm in diam- 

 eter, was well terminated, fig. 7. The rhombohedron x is of 



3P3. i 

 the third order and has the symbol 3121, l 3 l or — - — . The 



prismatic faces are bright but the rhombohedron is dull so 

 that measurements were made only with a contact goniometer, 

 as follows : 



a^x, 2110 ,v 3121 



a~x, 1120 ^ 3121 



x*x, 3121 * 23 



Measured 28|° 



54J° 



102° 



Calculated ..28" 5' 



53 59' 



102 10' 



This interesting and very unusual combination of prisms 

 terminated only by a rhombohedron of the third order is simi- 

 lar to that exhibited by some of the phenacite crystals from 

 Mt. Antero, Colorado. f 



On the cleavage of Willemite. 



Statements are made by various authors that willemite has a 

 distinct basal cleavage, while the manganese variety, troostite, 

 cleaves parallel to a prism of the second order, and this varia- 

 tion, together with the differences in crystalline habit and 

 chemical composition have been cited as grounds for regarding 

 the two minerals as distinct species. 



During the course of this investigation the subject of cleav- 

 age has been carefully considered. On the crystals from Mor- 

 esnet either a basal or a prismatic cleavage may be produced 

 by placing a knife blade in appropriate positions on the crys- 

 tals and pressing until they break. The cleavages are poor, 

 they can scarcely be called distinct and both are about equally 

 perfect, although, owing to the prismatic shape, the basal is 



* Owing to the rhombohedral-tetartobedrisin the upper and lower ends of a 

 crystal can not be interchanged without changing the symbols of the faces, 

 f This Journal, III, xxxvi, p. 320, 1888. 



