Related Physical Properties of Topaz. 393 



Physical properties and their relations to the chemical com- 

 position. — The specific gravities were very carefully deter- 

 mined on a chemical balance, pains being taken to boil the 

 crystals for some time in water to expel any air bubbles. The 

 results vary within the limits 3-574 and 3-533, a difference of 

 only 0*041, and as a rule they decrease as the molecularly 

 lighter hydroxyl replaces fluorine. 



Also basal plates were prepared and the divergence of the 

 optical axes 2E measured on a large axial angle apparatus. 

 The values for 2E have been found to vary in topaz from dif- 

 ferent localities and according to the observations of Des 

 Cloizeaux they extend from 129° 30' on crystals from Du- 

 rango, Mexico* to 71° 32' on those from Mugla, in Natolien, 

 Asia Minor,f both measurements being for red. These varia- 

 tions have generally been supposed to be connected with some 

 change in chemical composition, but a satisfactory explanation 

 has never been given. In the following table the measure- 

 ments that we have made are arranged according to decreasing 

 values of 2E for yellow, and with these the determinations of 

 the specific gravity, fluorine and water are given : 









Specific 









2E yellow. 



gravity. 



Fluorine. 



Water. 



Zacatecas, Mexico, 



126° 



28' 



3-574 







•18 



Thomas Range, Utah, 



125 



53 



3-565 



20-37 



0-19 



Nathrop, Colorado, 



125 



51 



3-567 



20-42 



0-29 



Pike's Peak, Colorado, 



122 



42 



3-567 







0-48 



Tenagari, Japan, 



120 



59 



3-565 



19-50 



0-57 



Adun Chalon, Siberia, 



118 



46 



3-562 



1924 



0-58 



San Luis, Mexico, 



118 



17 



3-575 



19-53 



0-80 



Schneckenstein, Saxony, 



114 



28 



3-555 



18-50 



0-93 



Stoneham, Maine, 



J13 



50 



3-560 



18-56 



0-98 



Minas Geraes, Brazil, 



84 



28 



3-532 



15-48 



2-45 



cc a n 







3-523 





2-50 



On the last mentioned crystal the value of 2E was not 

 measured owing to the strong optical anomalies which the sec- 

 tion presented ; it was observed, however, that the angle was 

 small. It is evident from the results given in the table that 

 the value of 2E decreases as the percentage of water increases 

 or as fluorine is replaced "by hydroxyl, and this relation is so 

 constant that the percentage of water can be told from the 

 value of 2E. It is evident, therefore, that the topaz from Du- 

 rango, mentioned by Des Cloizeaux as giving the largest value 

 of 2E = 129° 30', must be the nearest approach to the fluorine 

 compound, while that from Asia Minor, also cited by him as 



*Bull. Soc. Min. de France, ix, p. 135, 1886. 

 fNouv. rech., Inst. France, xviii, p. 612. 



