391 Pen field and Minor — Chemical Composition and 



giving the smallest value of 2E = 71° 32', must be the richest 

 in water or hydroxy! and poorest in fluorine of any topaz that 

 has thus far been examined. 



The indices of refraction also show a progressive change along 

 with the variations of 2E as may be seen by the following 

 determinations, given for yellow light by different investi- 

 gators. 





2E. 



2V. 



a. 



P. 



7- 



Thomas Range, Utah,* 



126° 24' 



67° 18' 



1-6072 



1-6104 



1-6176 



Nerchinsk, Adunchal Mts.,f 121 55 



65 30£ 



1-61327 



1-61597 



1-62252 



Colorless Crystal, Brazil,:): 



120 40 



65 14 



1-6120 



1-6150 



1-6224 



Schneckenstein, Saxony, f 



114 17 



62 33 



1-61549 



1-61809 



1-62500 



" § 



110 12 



60 55 



1-6156 



1-6180 



1-6250 



Minas Geraes, Brazil,f 



86 21 



49 37 



1-62936 



1-63077 



1-63747 



As hydroxyl replaces fluorine, therefore, the indices of refrac- 

 tion increase and the strength of the double refraction de- 

 creases, as shown by the values y— a in the extremes : 



Thomas Range, Utah y—a — 0-0104 

 Minas Geraes, Brazil y—a= 0-00811 



The crystallographic axes are also affected by the isomorph- 

 ous replacement of fluorine by hydroxyl. The variation 

 however is not very great and only exact determinations can 

 be used for showing it. Mr. C. A. Ingersoll has kindly made 

 for us some careful measurements on a crystal from the Thomas 

 Range, Utah, which, next to the topaz from Zacatecas, contains 

 the least water of any examined by us, and upon which the 

 forms o, 221 andy, 021 were well developed and gave beautiful 

 reflections. Also on one from Brazil upon which very exact 

 measurements could be obtained from they, 021 faces only, the 

 other forms, the striated prism and the pyramid u, 111, not 

 being suitable for measurement. 





Utah. 







Measured. 





Measured. 



Calculated. 



f*f, 021 A 021 = 87° 19'* 



0*0, 221^221 



= 105° 10' 



105° 10' 



0*0, 221^221 — 49 36* 



o*o, 221 ^ 221 



= 127 47 



127 51 



Brazil. 



Measured. 

 /a/, 021^021 = 86°55£'* 



The axial ratios are given in the following table and with 

 them a number of others given by investigators who regard 

 them as very exact. 



* Ailing. This Journal. Ill, xxxiii, p. 146, 1887. 



\ Mulheims. Zeitschr. Kryst., xiv. p. 226, 1888. 



% Des Cloizeaux. Manuel de Mineralogie, p. 475, 1862. 



§ Zimanyi. Zeitschr. Kryst, xxii, p 339, 1893. 



