Wright — Three Contact Minerals. 



553 



From these optical and crystallographic data, it appears that 

 hillebrandite is orthorhombic with possibly c—t, a=& and 

 cleavage after 110 (?). Its most characteristic optic features 

 are: refractive index about 1-61, birefringence weak to medium, 

 2E medium with very strong axial dispersion 2E V > 2£ , which 

 in parallel light gives rise to abnormal blue interference tints 

 which are readily recognized. Optical character, negative. 



The absence of crystallographic faces of any degree of per- 

 fection precluded any attempts at etching which might have 

 been made. 



The specific gravity at 25° was determined by pycnometer 

 with water at 2 - 692, and also in xylene at 2*692. The check 

 determination in xylene was made because the analysis of 

 hillebrandite shows it to have been slightly hydrolized. 



In hydrochloric acid (1:1) hillebrandite separates some 

 silica at once but enters otherwise into solution. Hillebrandite 

 decomposes very slowly with cold water as the test by adding 

 a few drops of phenolphthaline to the mixture indicates. 

 j"J Before the blowpipe thin splinters of hillebrandite fuse 

 down with difficulty to a colorless glassy bead, at the same 

 time giving a strong calcium flame reaction and glowing 

 briskly. 



Chemical Analysis. 



Si0 2 __. 



TiG 2 



Al 



1 



32-59 



-02 



-23 



la 



•5398 

 •0003 

 •0023 



•0009* 



•0001 

 •0010 

 1-0296 

 •0005 

 •0005 

 •5019 



2 

 31-74 



-*- 23 



Fe 2 3 ) 



FeO f "- 



MnO 



MgO... 



CaO 



Na : 



•15 . 



•01 



-04 



57-76 



_ -03 



58-81 



K 9 



-05 





HO 



9-36 



9-45 



F 



co 2 .- - — 



none 



none 





100-24 



2-0769 100-00 



1. Hillebrandite, Terneras Mine, Velardena, Durango, Mexico, 



E. T. Allen analyst. 

 la. Molecular proportions of 1. 



2. Percentage weight composition of formula H 2 0.2 CaO. Si0 2 . 



* Calculated as Fe 2 3 . 

 Am. Jour. Sci. — Fourth Series, Vol. XXVI, No. 156. — December, 1908. 



