394 



8. L. Penfield — Cookeite from Maine. 



tinct six-sided prisms are rarely found ; the crystals are usually 

 radial as if made from a series of wedge-shaped hexagonal 

 plates, fig. 1, grouped with their thin edges together, fig. 2. 

 The form is something like that of prochlorite, figured on 

 page 653 of the sixth edition of Dana's Mineralogy, only 

 more bent. The exterior of the crystals is rather rough, so 

 that they look almost hemispherical or globular. The cleavage 

 plates are naturally wedge-shaped like fig. 1, which interferes 

 somewhat with making exact optical determinations. When 

 examined in polarized light they appear divided up very sym- 

 metrically into sectors fig. 3. The inner portion a is uniaxial 

 and shows a weak, positive double refraction. The outer por- 



tion or rim is composed of six segments, the opposite ones 

 extinguishing simultaneously in directions indicated in the 

 figure, showing that this outer portion may be regarded as a 

 trilling, composed of penetrating individuals. In each seg- 

 ment the acute bisectrix is about at right angles to the section 

 and the plane of the optical axes is parallel to the edge of the 

 hexagon, corresponding to the clinopinacoid. The bisectrix is 

 also slightly inclined in the same plane. The divergence of 

 the axes is large, considerably greater than that of muscovite, 

 although it could not be measured exactly owing to the small 

 size of the sections. The double refraction is positive and not 

 very strong, being about like that of the chlorites. The uni- 

 axial central portion a, fig. 3, may be considered as composed 

 of biaxial plates, superimposed on one another in twin position 

 according to Tschermak's law of twinning for the chlorites.* 

 The relative proportion of center and rim varies. In some 

 sections the center is almost wanting, the rim, however, is 

 always prominent and shows with the microscope a faint 

 striated structure, the striae running at right angles to the edge 

 of the hexagon, as represented in the figure. Except for the 

 uniaxial center the above method of twinning is exactly like that 

 of the clinochlore from Texas, Lancaster Co , Pa., described by 

 Professor J. P. Cooke,f after whom this mineral was named. 



* Sitzungsber. d. kais. Akad. d. Wissenschaften in Wien (1), xcix, p. 174, 1890. 

 fThis Journal, II, xliv, p. 201. 



