Related Physical Properties of Topaz. 395 



a : ~b : c 



Utah, Ingersoll - 0-528110 : 1 : 0-477115 



Urals, Koksharov*..-. 0-528542 : l : 0*476976 



Schneckenstein, Laspeyres,f 0-531548 : 1 : 0475973 



Brazil, Ingersoll ._ : 1 : 0*473862 



Optical anomalies. — Of the crystals examined by us the 

 only ones that showed optical anomalies were those from 

 Brazil. A basal section of the crystal that was used for the 

 complete analysis showed an interior rhomb, having the out- 

 line of the unit prism, surrounded by four symmetrical trape- 

 ziums, and two opposite V- sna P e d segments, with their angles 

 turned toward and touching the acute angles of the inner 

 rhomb. The disposition of the parts was practically like that 

 described by Mallard;}: and Mack.§ The extinction directions 

 of the outer segments corresponded almost exactly to that of 

 the inner rhomb. On another crystal from Brazil, for which 

 only the specific gravity and water determinations are given, 

 the optical anomalies were much more marked, and the ex- 

 tinction in the different segments undulatory, so that the di- 

 vergence of the optical axes 2E could not be measured. When 

 the sections were examined by transmitted light it was evident 

 that they were not homogeneous, since the well defined out- 

 lines between the inner rhomb and outer segments indicated a 

 variation in the refractive indices. The structure indicates 

 very clearly the existence of an inner core or older crystal, 

 surrounded by a later growth of topaz of different composition. 

 This idea agrees with the observations of Des Cloizeaux, who 

 found that the central and outer segments of a zonal crystal 

 gave different values for 2E. Since it has been shown that the 

 physical properties vary with the composition all the changes 

 to which such a compound crystal is subjected must give rise 

 to mechanical strains and cause the disturbance in the optical 

 orientation of the different zones. 



Comparison between topaz and herderite. — The changes 

 that have been brought about by the partial substitution of 

 fluorine by hydroxyl have previously been studied by one of 

 us, page 329, and it will be interesting in closing to make a 

 comparison of the results that have been obtained. In herd- 

 erite we know the pure hydroxyl compound, hydro-herderite 

 Ca(BeOH)P0 4 and the hydrofluor-herderite Ca[Be(OH . F)] 

 P0 4 with OH : F = 3 : 2. With topaz the nearly pure fluorine 

 extreme is known (AlF) 3 Si0 4 and the hydrofluor topaz from 

 Brazil [A1(F . OH)] 2 Si0 4 , with F : OH = 3 : 1. 



* Materialien zur Minn. Russ., ii, p. 198, 1854. 

 fZeitschr. Kryst, i, p. 351, 1877. 

 % Ann Mines, x, p. 155, 1876. 

 §Wied. Ann., xxviii, p. 153, 1886. 



