﻿358 A W. Clarke— Lithia Micas. 



f[. The Iron-Lithia Micas of Cape Ann. 



In the granite quarries of Eockport, Massachusetts, near the 

 extremity of Cape Ann, there are occasional feldspathic veins 

 which contain the rarer minerals danalite, cyrtolite, fergusonite, 

 and amazon-stone, together with certain remarkable micas. 

 One of the latter, cryophyllite, was described by Cooke in 1867 ;* 

 who also analyzed an associated " lepidomelane," to which 

 Dana afterward gave the name of annite. The vein from which 

 Cooke obtained his material was long ago blasted away or cov- 

 ered up ; but other veins of like nature are still accessible, and 

 from one of them the micas examined in this laboratory were 

 obtained. They were collected by the original discoverer of 

 the locality, Mr. W. J. Knowlton of Boston, and were, to all 

 appearance, identical with the micas described by Cooke. 

 Among the specimens the two types were clearly recognizable ; 

 the one a dark-greenish-black lithia mica, cryophyllite; and 

 the other a black, brilliant lepidomelane. In some specimens 

 the cryophyllite formed a border upon the broad plates of the 

 other mica, precisely as in the association of lepidolite and 

 muscovite at Auburn. The resemblance in this particular was 

 curiously striking ; only the Eockport minerals are less con- 

 spicuous than those from Maine, since they lack the contrast 

 of color which gives the latter their beauty. 



The cryophyllite varies considerably in outward character, 

 and three well marked types of it were analyzed by Mr. Eiggs. 

 They may be described as follows: A. Broadly -foliated, brilliant, 

 blackish-green. B. Paler, dull-green, less lustrous, seemingly 

 altered. C. An aggregation of minute six-sided prisms, dark 

 green, almost granular in appearance, resembling some chlorite. 

 Mean results are given. 



A. B 0. Cooke's Analysis. 



Si0 2 51-96 51-46 52*17 51*49 



Al 2 O a 16-89 16-22 16'39 16*77 



Ee 2 O s 2-63 2-21 4-11 1*97 



FeO 6-32 7*63 5*99 7*98 



MnO -24 -06 -32 Mn 2 3 -34 



CaO -12 trace trace 



MgO -03 -17 trace *76 



Li 2 Q 4-87 4-81 4-99 4*06 



Na 2 -87 -89 *63 trace 



K 2 10-70 10-65 10'48 13*15 



H 2 1-31 1-12 1*46 



F 6-78 7-44 7.02 SiF d 3-42 



102-72 102-66 103-56 99-94 



Less oxygen, 2-86 3-11 2*95 



99-86 99-55 100'61 



* This Journal, II, xliii, 217. 



