52 J. W. Mallet— Crystalline form of Sipylite. 
Art. [X.—On the Crystalline form of Sipylite ; by 
J. W. MALLer. 
accurately together, so that the form can be easily made out. 
This little specimen is a tetragonal octahedron, 1°5 centime- 
ter long, weighing 1°627 grm. No faces are visible save those 
of the octahedron (1) and faint indications at one or two places 
of an extemely narrow plane replacing its terminal edges. The 
surfaces are too dull to allow a reflecting goniometer to be used, 
but an application goniometer gives the angles 
1 A 1 (over summit)=53° 0' 
(Hence O ~ 1=116° 30’) 
1 A 1 (adjacent pyramidal)=100° 45’ 
1A 1 (basal)=127° 0’ 
These measurements show a close relation to fergusonite, 
for whic 
O A 1=115° 46’ 
1 A 1 (pyramidal)=100° 54’ 
1 A 1 (basal)=128° 28 
the latter, and this,t as well as Rammelsberg’s analysis of 
fergusonite, supports the view expressed in my former paper 
that sipylite is an ortho-niobate—R”, M’, O,—containing basic 
ydrogen. 
The sipylite crystal shows distinct cleavage parallel to 1. It 
is fully identified with the mineral originally examined by its 
ewes physical characters. e sp. gr. =4°883 at 16° C.; 
ormerly found, 4887 at 12°-5, and 4:892 at 17°°5. 
Univ. of Virginia, May 21, 1881. 
phosphate. But no great weight can be attached to any opinion as to yttrium 
compounds until the confusion at present existing in relation to the metals which 
have together passed under this name has been cleared up. 
