NATURAL SCIENCES OP PHILADELPHIA. Ill 



forms of these reflections. Under a high power, I ohserved perfect equal- 

 sided hexagons, with nearly all possible modifications of that form, by more 

 or less unequal replacement of some of the prismatic sides ; thus, some assum- 

 ing a triangular form, some that of rhombs and rhomboids ; some of the lat- 

 ter being almost linear. These plates are sometimes imperfectly formed, the 

 boundary lines being occasionally irregular and broken, exhibiting one, two, 

 three or four sides, and sometimes no part of the sides present a right line. 

 They usually lie parallel with the principal cleavage of the Feldspar, and, 

 when the rays of light strike their bright surfaces, the reflections are exceed- 

 ingly brilliant. Under the microscope, with a bright light, it will be observed 

 that some reflections are blue, others green, purple, red and yellow. Some 

 of the specimens of Sunstone show parallel lines on the edges of the cleavages 

 parallel to the prismatic sides of the Feldspar, which are evidently occasioned 

 by the regular deposit of the layers. These are quite different from the fine 

 parallel minute striae which lie on the principal faces of the cleavage, and 

 which can only be observed with a high power. Neither of these are con- 

 stant. These spangles or plates are so thin, that I have been unable to 

 detect any perceptible thickness on their prismatic sides. These very inter- 

 esting plates in Sunstone have been known for a long time, but I have not 

 been able to find any analysis of them.* Kenngott states that they are Go- 

 thite, hydrated-per-oxid of Iron (Fe 0.,HO). Sheerer says that " the Aventu- 

 vine character is owing to minute particles of Specular Iron.] I doubt this, 

 as the resplendant crystals are usually semi-transparent, reflecting various 

 colors, as mentioned above. There are in most varieties another set of de- 

 posits, which are much rarer, and present opake, black masses, usually taking 

 the same hexagonal form and its modifications, but often without any regular 

 form. These may be of the same metallic substance iu a different state of 

 oxidation, not transmitting the rays of light. 



Fine specimens of Moonstone are found in Albite, in Delaware County, west 

 of Media, but this species of Feldspar does not give out its beautiful bine 

 color by reflection from any foreign body, but by the absorption of all the rays 

 of light but blue, and this owing to some arrangement of its atoms not yet 

 understood. 



In the examination of various Feldspars with, high power of the microscope, 

 I found in nearly every one which was not entirely white, that more or less 

 foreign matter in a crystallized state, was included in their composition. In 

 the green compact variety which I have described above, and proposed to call 

 Lennilite, there was nothing detected, nor was there in the ordinary green 

 Feldspar of Mineral Hill, near Media, except that in the latter locality there 

 have been specimens found of a glassy structure, and with clear double 

 cleavages, in which reddish spots were interspersed, which spots were always 

 colored by the presence of these crystalline plates, having beautiful bright 

 reflections, and of the usual hexagonal form. 



I ought to mention here, in connection with these beautiful brilliant plates 

 in Sunstone, that Prof. Rood, of Columbia College, New York, some time 

 since, made a " Micro-Stereograph " of a thin plate of Sunstone from Arendal, 

 Norway. In this he succeeded admirably in displaying these numerous modi- 

 fications of the crystals, which were enlarged and photographed on paper ; 

 thus bringing those interesting forms with great perfection to the recognition 

 of the unaided eye. 



I proceed now to the results of my microscopical examinations of various 

 Feldspars, in which I found more or less of these minute crystalline forms. 



* Aventurine Quartz is also called Sunstone, and is considered of some value as a stone of luxury, 

 liut it has not reflections as brilliant as those of Feldspar ; nor are they, so far as I have been able 

 to observe, crystallized plates, but their irregular deposits are of the same brown and red color, 

 and they may be Gothite. 



t Dana's Mineralogy. 



1866.] 



