548 Palache and Warren — Chemical Composition and 



always to a greater or less extent intergrown with aegirite. 

 While the latter is frequently included in the body of the 

 former, its most common position is about the outside, parti- 

 cularly the ends of the crystal, and although the rieheckite con- 

 tains unorientated grains of aegirite, the usual mode of inter- 

 growth is with the prismatic axes of the two minerals parallel. 

 As the sides and particularly the ends of the crystals are 

 approached, narrow strips of aegirite are interlaminated with 

 the rieheckite, the amount usually increasing until the latter 

 is entirely replaced. The rieheckite usually contains consider- 

 able amounts of black dust, often arranged in wavy lines, as 

 well as larger grains of black oxides, mostly ilmenite. Occasional 

 grains of feldspar, fluorite and zircon are also included, some- 

 times singly and again forming patches of varying size. 



Optical. — The deep color and strong absorption of the min- 

 eral makes the determination of its optical properties, with 

 any precision, very difficult. This difficulty is increased by 

 the fact that it has been found impossible after many trials to 

 obtain altogether satisfactory sections of the mineral across 

 the cleavage owing to its extreme brittleness. By the study of 

 finely crushed material and thin sections the following char- 

 acters have been made out :- 



Eay near c~a. For] Kay || to b = c. For j Eay near a = b. For 

 sections O'OS 1111 " or under, - 03 mm thickness, very ' thickness under O'OS"" 11 

 deep blue to bluish, | dark smoky green to [ yellow. For 03 and 

 smoky green. For over almost black. i over brownish yellow 



Q.Q3mm nearly or quite I with a greenish shade, 



black. 



Absorption o<c much greater than b. For many sections 

 intermediate in position between the front and side pinacoids, 

 a peculiar dull, grayish blue (some might call this a drab 

 or even a violet tone of color) is seen. This is particularly 

 true of thin cleavage fragments. In many sections parallel to 

 the clinopinacoid it has been observed that the distribution of 

 color is not uniform, the blue being seen in streaks parallel to 

 the cleavage, or lying along lines crossing the cleavage, sug- 

 gesting in appearance minute cracks along which there has 

 been some slight chemical change. In such cases the remainder 

 of the section has a dull bluish green color. In the rieheckite, 

 from the pegmatites at least, such variation in color does not 

 appear to be connected with any significant change in the 

 chemical composition. Tests with the sensitive tint on very 

 thin cleavage fragments show always a negative elongation. 

 The extinction in 010 sections does not exceed four or five 

 degrees, measured on the prismatic cleavage. Its accurate 

 determination is rendered difficult by the strong natural color 

 and strong dispersion of the mineral. A single section perpen- 



