412 
DR, A. E. H. TUTTON ON THE 
Rubidium Ferrous Selenate, Rb 2 Fe (Se0 4 ) 2 .6H 2 0. 
Morphology. 
This substance has not been previously investigated, either goniometrically or 
optically. It forms beautifully transparent and, considering its composition (as 
containing ferrous iron), remarkably permanent crystals of a very pale bluish green 
colour. 
Ten crystals were measured, selected from the three best crops of the several 
prepared. The ferrous selenate employed in one case had been prepared by the action 
of selenic acid on iron wire. An equivalent of pure rubidium selenate, prepared by 
the action of selenic acid on rubidium carbonate, was dissolved in only slightly more 
than the minimum amount of water required for solution at the ordinary temperature, 
and added to the cold solution of ferrous selenate immediately after the filtration 
from the precipitated selenium, in order to avoid the somewhat rapid decomposition 
(oxidation) which solutions of ferrous selenate suffer. Crystallisation occurred under 
the receiver of an air pump, under reduced pressure. Several crops of remarkably 
transparent and permanent crystals of rubidium ferrous selenate were also obtained 
by the method described in the introduction. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes .— 
a :b : c = 07424 : 1 : 07000. 
Axial Angle.—/3 = 104° 57'. 
Forms Observed. —-a{l00}, b {010}, c {001}, p {110}, p' {120}, q {Oil}, r'{20l}, 
o'{l 11}, and n'{T2l}. 
Habit. —Thick blocks, more or less tabular, parallel to c {001}. A typical crystal 
is shown in fig. 4. 
The crystals exhibited the usual characteristic relative development of the faces of 
the basal plane c {001} and ^ {011} common to all the 
rubidium salts of the series, namely, the c-faces predomi¬ 
nated, but the g-faces were also well developed as fairly 
broad bevellings. The faces of the clinopinakoid b {010} 
were also frequently quite large on this salt, forming the 
two sides of the blocks. The ends of the blocks were 
formed by the faces off the primary prism p {110} and the 
orthopinakoid r'{20l}, which latter were often relatively 
very large, forming the two predominating end planes. 
Good little faces of the primary orthopinakoid a {100} were usually present, 
and indeed large crystals often showed the a-faces quite considerably developed. 
There were also usually several faces of the hemipyramid (/{Ill} present, but their 
reflections were poor. Excellent, although narrow faces of the prism g/{l20} were 
also observed on two of the crystals measured. Minute faces of the hemipyramid 
