Vol. 5] Louderback. — Benitoite. 349 



is also very suggestive of the color given by the sesquioxide of 

 titanium. The reduction test stands opposed to this idea unless 

 the quantity in the material used for the test was very small— 

 or, in other words, unless very small quantities can give dis- 

 tinct colors. 



The color is apparently quite stable. No evidence of fading 

 has been noticed in the cut stones and fragments heated to a 

 bright red, just short of fusion, for five or six minutes showed 

 no change whatever after they were again cooled. 



Pleochroism is very intense in the deep colored varieties 

 and is probably the most important test applicable to cut 

 stones. In the lighter parts the extraordinary ray is a very 

 slightly greenish blue, inclining to indigo as it becomes darker, 

 and is very similar to one of the axial colors shown by some 

 cordierites. In the deeper colored crystals and the thicker lay- 

 ers it is an intense purplish blue. The ordinary ray is white. 

 The color of the mineral in ordinary light is therefore merely 

 the color of the extraordinary ray diluted with the white of 

 the ordinary ray. The extraordinary ray shows strong absorp- 

 tion of sodium light, and renders a determination of the re- 

 fractive index for that light difficult. 



An attempt has been made to represent the pleochroism in 

 plate 33, figures 2 to 4, as shown in a large cut stone. Figure 

 2 shows the natural color, and figures 4 and 3 the colors of the 

 ordinary and extraordinary rays. The stone used is of only 

 moderate depth of color. Dark colored specimens, if fairly thick, 

 give such very strong absorption along the extraordinary ray that 

 the depth of color is hardly reproducible. 



It fuses quietly to a transparent glass at about 3. 



Chemical Characters. 

 In hydrochloric acid it is practically insoluble, and this per- 

 mits us to dissolve crystals out of their natrolite matrix with- 

 out injury to the crystal faces. The natrolite dissolves, leaving 

 the benitoite in a matrix of hydrous silicic acid which is easily 

 removed. Sulphuric acid has also been used for this purpose 

 at the mine. The mineral is attacked by hydrofluoric acid, and 

 dissolves readily in fused sodium carbonate. Blasdale also finds 

 it but slowly attacked by molten potassium pyrosulphate. 



