418 
DR. A. E. H. TUTTON ON THE 
Caesium Ferrous Selenate, Cs 2 Fe (Se0 4 ) 2 .6H 2 0. 
Morphology. 
The salt was prepared in the manner described in the introduction, the ferrous 
selenate being prepared by- the action of selenic acid on ferrous sulphide, and the 
caesium selenate by the action of selenic acid on pure caesium carbonate. 
Ten crystals were measured, selected from four different crops, all of which were 
composed of excellent transparent single crystals, sometimes grown to a considerable 
size, and of a pale (almost colourless) bluish green colour containing more yellow than 
the crystals of rubidium ferrous selenate. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes. — a :h :c = 07308 : 1 : 0'4979. 
Axial Angle. —/3 = 106° 2'. 
Forms Observed.—h {010}., c {001}, jo {110}, q {Oil}, r' {201}, and o'{Ill}. 
Habit. —-Prismatic parallel to the inclined axis a. 
A typical crystal is represented in fig. 6. 
The crystals were usually very definitely of this 
habit, which is distinguished by narrow faces of 
c {001} and broad faces of q {Oil}, and which is very 
characteristic of the caesium salts of the series. In a 
few crops the c-faces were somewhat broader, and 
almost conferred a tabular habit on the crystals. As 
in several of the double sulphates containing iron, 
notably ammonium ferrous sulphate, the faces of the 
orthopinakoicl r'{20l} were usually more prominent 
than those of the primary prism p {110}, and in many cases practically formed the 
ends of the prisms, the p-faces being very small. The faces of the clinopinakoid 
b {010} and of the hemipyramid o'{Tll} were only occasionally present, and then 
were very subsidiary, although some good reflections were obtained from them. 
Cleavage. —There is an excellent cleavage parallel to r’ {201}. 
Volume. 
Relative Density. —Four determinations by the immersion method yielded the 
following values :— 
I. 
Density for 15°‘9/4° . 
. . 3-0497 
For 20°/4° . 
. . 3-0484 
II. 
„ 15°7/4° . . 
. . 3-0485 
„ 20°/4° - . 
. . 3-0472 
III. 
„ 16 0, 2/4° . 
. . 3-0495 
0 
o 
o 
. . 3-0483 
IV. 
„ 16“l/4° . 
0 
. . 3-0504 
„ 20°/4° . 
. . 3-0492 
Mean . 
. . 3-0483 
Accepted value for 20°/4°, 3'048. 
