290 



Pirsson and Wells — Occurrence of ' LeadhilUte. 



In color the leadhillite varies from colorless in the smaller 

 crystals which are often beautifully clear and limpid, to an 

 exquisite clear sea-green in the larger translucent individuals. 



From their freshness, high luster, delicate tints, size and 

 good crystallization these specimens are by far the most strik- 

 ing ones of this mineral that have come under our observation. 



Crystallography. — From a crystallographic standpoint the 

 mineral from this new locality is very simply developed, the 

 majority of the crystals appearing like simple hexagonal 

 prisms terminated by the base and consisting of the forms c, o, 

 (001) ; m, I, (110) and a, i-l (100). Often the positive hemi- 

 dome <?, 2-1, (201) is present and the crystals have the habit 

 shown in fig. 1. More rarely the negative hemidome ic, -2-i 

 (201), and the ortho-pyramids q, 1-1, (il4) and u, |-i(118) occur 

 and these are shown in fig. 2, which represents the most 



Fig. l. 



Fi°r. 2. 



complex form observed. These have been identified by 

 the following angles measured on the reflecting goniometer. 

 For the calculated angles the elements derived from the meas- 

 urements of Laspeyres" have been used in which 



a : b : 6 : : 1*74764 : : 1 : 2-21545, ang. £=89° 47' 38". 



c a« (001^100) 



Calc. 





Meas. 







89° 47' 



38" 



89° 38', 



89° 37' = 



= ang. p. 



120 27 





120 22 







89 54 





89 39 







68 39 





68 54 



69 04 





68 18 





6S 27 







54 16 





54 14 







34 09 





34 37 







C a.w»(001/v110) 

 c ^e (001 /x 201) 

 c A M (001^201) 

 c A.q (001 /s 414) 

 c ^ (001^418) 



The above measurements represent the best obtained ; as a 

 general rule the reflections of the signal are poor in spite of 

 the brilliancy of the luster, owing to the fact that the crystals 

 are usually much striated by lines parallel to the trace of the 

 basal plane and are moreover often slightly curved or twisted. 

 The measurement of c A « (001 ^ 100) = ang. /9 was one of the 

 best and obtained on a small crystal which gave good reflec- 

 tions and was free from distortion ; it is somewhat less than 



* Zeit. f. Kryst., i. p. 199, 1877. 



