576 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



those in the specimen here described, which itself shows marked 

 asterism." The conclusion therefore seems warranted that asterism 

 is generally due to the inclusion of regularly arranged rutile laths. 



3. PSEUDOMORPHS OF MARCASITE AFTER PYRRHOTITE FROM 



OSNABRiJCK, PRUSSIA. 



In November, 1909, the specimens here described were given to 

 the U. S. National Museum (Catalogue No. 86666) by Mr. Werth- 

 mann, of Osnabriick, who called them "pseudomorphs of marcasite 

 after an unknown mineral," and requested their more accurate deter- 

 mination. A careful study revealed the original mineral to be pyrrho- 

 tite, and this conclusion was made known to Mr. Werthmann. In 

 the meantime Messrs. F. Schondorf and R. Schroeder had reached 

 the same conclusion regarding similar specimens, having published 

 their results under the title Ueber Markasit von Hannover und Osna- 

 briick in the Zweiter Jahresbericht des Niedersachsischen geologischen 

 Vereins, Hannover, for 1909, pages 132 to 139, a copy of which was 

 kindly sent to the present writer by Mr. Werthmann. Inasmuch, 

 however, as this publication is one not readily accessible to English 

 reader^, and owing to the theoretical interest that pertains to the 

 rare change of one iron sulphide into another, it is ventured to give 

 the present account, even at the risk of some degree of repetition. 



Plate 64 illustrates very completely the appearance and form of the 

 marcasite. It may there be seen that this mineral occurs in single 

 hexagonal pyramids, occasionally terminated by basal planes, and in 

 larger, irregular masses and groups of crystals. The pyramidal faces 

 are dull, usually rough, and frequently striated horizontally from 

 repeated oscillatory combinations of the upper and lower pyramids. 

 The basal planes are smooth, though almost without luster. In size 

 the specimens range from 4 to 21 mm. in greatest length. 



The mineral is sharply differentiated from pyrrhotite and estab- 

 lished as a form of iron disulphide by its color, hardness, density of 

 4.80-4.86, chemical composition,'' lack of magnetism, and ability to 

 yield much sulphur in the closed tube. Its identification as marcasite 

 instead of pyrite, however, is not absolutely without question. Its 

 density and the fact that much free sulphur is left when its powder 

 is dissolved in strong nitric acid are indicative of marcasite. Several 

 crystals were sliced and a polished surface studied under the micro- 



a When a email flame was viewed through a fairly thin sheet, a well-marked six- 

 rayed star was visible, with a secondary similar but less prominent star intermediate 

 in position. 



b Fe= 47.22%. M. Dittrich, analyst; quoted by Schondorf and Schroeder; 



S= 52.61% Ueber Markasit von Hannover und Osnabriick. Zweiter 



Gangue= . 30 Jahresb. Niedersachsischen geol. Vereins, Hannover, 



1909, pp. 132-139. 



100. 13 



