ON A REMARKABLE CUBE OF PYRITE, CARRYING CRYS- 

 TALLIZED GOLD AND GALENA OF UNUSUAL HABIT 



By JOSEPH E. POGUE 

 Assistant Curator, Division of Mineralogy, U. S. National Museum 



With One Plate 



The intergrowth or interpenetration of two or more minerals, 

 especially if these be well crystallized, often shows a certain mutual 

 crystallographic control in the arrangement of the individuals, sug- 

 gestive of interacting molecular forces. Occasionally a crystal upon 

 nearly completing its growth exerts what may be termed "surface 

 affinit}'," in that it seems to attract molecules of composition differ- 

 ent from its own and causes these to crystallize in positions bearing 

 definite crystallographic relations to the host crystal, as evidenced, 

 for example, by the regular arrangement of marcasite on calcite, 

 chalcopyrite on galena, quartz on fluorite, and so on. Of special 

 interest, not only because exhibiting the features mentioned above, 

 but also on account of the unusual development of the individuals 

 and the great beauty of the specimen, is a large cube of pyrite, 

 studded with crystals of native gold and partly covered by plates of 

 galena, acquired some years ago by the U. S. National Museum. 



This cube measures about 2 inches (51 mm.) along its edge, and 

 is prominently striated, as is often the case with pyrite. It contains 

 something more than 130 crystals of gold attached to its surface, 

 has about one-fourth of its area covered with galena, and upon one 

 face shows an imperfect crystal of chalcopyrite. The specimen came 

 into the possession of the National Museum in 1906 and was ob- 

 tained from the Snettisham District, near Juneau, Southeast Alaska. 

 It is now on exhibition in the Mineral Department under number 

 86045. Three similar specimens were exhibited at the Seattle Expo- 

 sition during 1909, one of which is stated by the owner, Mr. L. V. 

 Winter, of Juneau, Alaska, to be 4 inches square and to show 170 

 crystals of gold upon its surface. So far as can be learned the four 

 specimens are the sole representatives of a very unique association. 



Crystallography of the pyrite. — The pyrite has four of its faces 

 well developed ; each of the two remaining ones is marred by an 

 irregular pit, about one-half inch deep and the same in diameter, 

 and the edge joining these two faces is imperfect. The crystal is 

 striated parallel to the pyritohedron e (210), due to oscillatory com- 



477 



