306 F. E. Wright — Optical Properties of Roscoelite. 



is of little avail. The micaceous flakes are relatively large (up 

 to 2 mm in length) and in the aggregate are irregular in shape 

 and size and show the characteristic wavy surfaces of the micas. 

 They are elastic and easily bent. Their color is olive-green ; 

 the luster, splendent, almost submetallic and bronze-like. 

 Cleavage after 001 is perfect; after 010, good. The hardness 

 is not satisfactorily determinable on the minute flakes, but it is 

 apparently 2*5 to 3. 



Under the microscope the flakes are olive-green to green- 

 brown in color and noticeably pleochroic, although much 

 less intensely so than lepidomelane or biotite. In thin flakes 

 the pleochroism is 7 = green-brown, /3 = olive-green and 

 a = olive-green. Absorption 7 > ft > a. The absorption is 

 fairly strong, thick sections appearing practically opaque. 

 Crystallographically, the flakes are usually bounded only by the 

 cleavage faces 001 and less perfectly by 010. The prism faces 

 are rarely developed and ordinarily such terminations are 

 wanting altogether. On the whole, the cleavage after 010 

 appears more highly developed in roscoelite than in other mem- 

 bers of the mica group. 



The basal flakes do not extinguish uniformly, but are fre- 

 quently divided into areas running parallel with the direction 

 of elongation of the flakes and extinguishing at small angles 

 (up to 2^-° were measured) with the common line of junction. 

 The appearance of these areas is frequently that of twinning 

 lamellae, and it is perhaps possible that such is the case, but 

 since the optical orientation is very nearly the same in any two 

 adjacent lamellae — the optic axial plane of the one making an 

 angle of 5° or less with that of the second and the acute bisec- 

 trices in both lamellae being practically parallel — the appear- 

 ance of such lamellge witli noticeable extinction angles on basal 

 sections would place the mineral in the triclinic system. This 

 is, however, hardly probable in view of the universal monoclinic 

 nature of the micas. To test this deduction attempts were 

 made to produce etch figures on cleavage flakes. Both hydro- 

 fluoric acid and sodium hydroxide were used as etch media, 

 and in both instances with only poor results. The best figures 

 were obtained by immersing the flakes for 30 seconds in hot 

 commercial hydrofluoric acid. The etch figures were com- 

 paratively rare, but, when observed, were either five-sided or 

 six-sided, and in every instance appeared to be symmetrical to 

 the cleavage lines after 010, the plane of symmetry. The 

 results, however, were not very satisfactory, and too much 

 stress cannot be placed on them. The flakes occur frequently 

 in radial rosettes, and it is possible that the observed difference 

 in extinction positions of two adjacent lamellae is due to actual 

 difference in orientation resulting from radial growth. 



