OF THE DOUBLE SELENATES OF THE SEPHES R,M(SeO,),,GlLO. 
2G1 
Rubidium Magnesium Selenate, RboMg(Se0.j.)o,GH30. 
A determination of magnesium in 0’4133 gramme of the crystals employed 
afforded as result 0'0821 gramme magnesium pyrophosphate, which corresponds to 
4'29 per cent, of magnesium. The theoretical percentage is 4’08. 
Goniometry. 
Twelve highly perfect small crystals were used in the goniometrical work. They 
were selected from the four most suitable crops. 
Habit; thick tabular to prismatic. 
Axial angle : /3 = 74'^ 46'. 
Ratio of axes : a : h : c = 0'7424 : 1 ; OAOll. 
Forms observed : h — [010 j coRco;c = {001{-oP;p = (t^Oj- ooP;(2= [OlljPoo; 
o'= {In;- + P; r' = [201} + 2R oo . 
The accompanying table sets forth the results of the measurements. 
The crystals of this salt are not distinguished by richness in the number of faces, 
and are frecpiently of a remarkably simple character. No other faces were present in 
a large proportion of the crystals examined besides those of the basal plane c, ortho¬ 
dome r', and clinodome q. The prevailing type was very similar to that of potassium 
magnesium selenate, except that the q faces were relatively more largely develo] 3 ed 
with respect to the faces of the basal plane, in accordance with the rule which has 
been established in the cases of the douljle sul 2 )hates and selenates already studied. 
No faces of the orthopinacoid a were obsei'ved, but those of the clinopinacoid b were 
frequently present, although small. The faces of the liemi-pyramid o' were but 
rarely observed, and were only measurable in one instance, the reflections in this case 
being good. 
