S. L. Penfield — Mineralogical Notes. 



115 



m 



m 



eral from Laurion, Greece, of the composition PbO . 2PbCl 2 , 

 which he named after the writer. This was one of the last 

 papers published by Prof. Genth and the last new mineral 

 species described by him. 



All of the specimens in Prof. Genth's 

 possession were sent to the writer for crys- 

 tal lographic study. As described by him, 

 the crystallization is hexagonal. No indica- 

 tions of hemihedrism were observed. The 

 usual combination is that of prism m with 

 basal plane c. A few of the crystals showed 

 a pyramid of the second order p, 1122, 1-2, 

 in combination with prism and base, fig. 2. 

 On one small crystal, which gave fair reflec- 

 tions from three of the pyramidal faces, the 

 following measurements of c^p were made, 

 41° 53', 41° 52' and 41° 55'. Taking 41° 53' 

 as fundamental the axial ratio becomes a : c = 

 1:0-8967. 



Some specimens showed the prismatic faces in combination 

 with steep pyramids of the first order. These were in oscilla- 

 tory combination, giving rise to horizontal striations and often 

 causing the crystals to taper to a point. The reflections from 

 these pyramids were numerous and so uncertain that no defi- 

 nite symbols could be assigned to them. 



The cleavage is distinct parallel to the base and sections 

 show in polarized light a normal interference figure. The 

 double refraction is positive and about as strong as that of 

 calcite. 



3. On the cleavage and parting Planes of Oligoclase and Albite. 



In the November number of this Journal, 1888, page 324, 

 the writer described a very clear glassy feldspar from the 

 Hawk mica mine, near Bakersville, Mitchell Co., N. C, which 

 seemed to show abnormal optical properties. It is found as 

 cleavage masses without crystal faces at a considerable depth 

 in the mine. An analysis by Mr. E. S. Sperry showed that 

 it contained the albite and anorthite molecules in the propor- 

 tion of 3*6 : 1 and was to be classified therefore as oligoclase. 

 The material that was used for the investigation was a single 

 specimen from the collection of Mr. Norman Spang of Etna, 

 Pa. This was without twinning striations and presented a 

 perfect cleavage parallel to the basal, and an imperfect one, 

 supposed to be parallel to the brachy-pinacoid (010). Plates 

 parallel to this second cleavage showed in convergent polarized 

 light an optical axis almost in the center of the field, while 



