MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
425 
Although the crystals were largely of the two principal types just mentioned, and 
shown in the figures, practically all the types of crystals met with in this large series 
of double sulphates and double selenates were observed. That is, crystals were found 
of the three main types (and also of all intermediate varieties) which have been shown 
in previous communications to be characteristic respectively of the potassium, 
rubidium, and caesium salts of the series; namely, for the potassium salts prismatic 
parallel jojllO} with large broad c{00l}, and very small q {Oil}, for the caesium 
salts prismatic parallel q {011} with narrow c {001}, and for the rubidium salts the 
midway type with only moderately prominent c {001} faces and with q {011} faces 
relatively smaller than the immense ones of the caesium salts yet larger than the 
very small ones of the potassium salts. One prominent characteristic was noticeable, 
however, on these crystals of ammonium ferrous selenate, namely, the very pronounced 
development of the orthopinakoid ? / {20l}. This form is characteristically well and 
often obtrusively developed in all the iron salts of this grand series, both those of 
the double sulphates and those of the double selenates, and is remarkably so in the 
common well-known salt ferrous ammonium sulphate, which ferrous ammonium 
selenate closely resembles. 
The reflections from the crystals measured were of general excellence, and relatively 
but little trouble was experienced from striation of the c and £>-faces. Certain crops 
rarely showed any traces of the faces of b {010} and (/{Ill}, while on the crystals of 
other crops these faces were quite well developed. 
Cleavage. —A good cleavage was observed parallel to r'{20l}, the cleavage 
direction common to the whole series. No trace could be discovered of cleavage 
parallel to b {010}, which has been observed to be developed in some of the ammonium 
salts of the series. 
Volume. 
Relative Density .— Four determinations by the immersion method yielded the 
following values :— 
I. 
Density for 17°'9/4° 
. . 2-1915 
For 20°/4° . . 
. 2-1910 
II. 
1 —‘ 
<£> 
o 
o 
0 
. . 2-1904 
„ 2074° . . 
. 2-1902 
III. 
„ l9°l/4° . . 
. . 2-1914 
„ 2074° . . 
. 2-1912 
IY. 
o 
OO 
o 
OO 
pH 
. . 2-1914 
„ 2074° • • 
. 2-1911 
Mean . 
. 2-1909 
Accepted value for 20°/4°, 2T91. 
The value 2T60 was obtained by Tops0e. 
Molecular Volume .— M — 220’39. 
d 2U91 
Molecular Distance Ratios (topic axial ratios ).— 
X : V': « = 6-3212 : 8'5043 : 4'2684. 
