Cleavage is perfect, parallel of the clinodiagonal section, or the plane iì; also distinct parallel to the line cl, as apparent often in the cross fractures of the crystals, and by occasional striæ. This plane of cleavage may be taken as the basal plane O.
The planes I and l are brilliant, especially the former. The oblique plane situated on the back side in the figure, and which may be called r, is quite rough, owing to the oscillatory combination between two hemi-octahedral planes. In many of the crystals, only the right one of the two planes I is present, and also only the left one of the two planes l. The prisms frequently terminate above in an irregular edge, made by the meeting of the one, or two, planes I, and the rough plane r, and this edge is sometimes cut off, more or less deeply, by a single oblique plane, which is one of the planes l.
According to measurements with the reflective goniometer;
| I:I | = | 142° 26’ |
| I:iì | = | 108° 47’ |
| l:iì | = | 101° 40’ |
| I:l | = | 156° 20’ (approximately.) |
The inclination of the I on l could not be accurately measured, on account of the minuteness of the planes in the crystals, in which both planes occur, and the want of perfection in the reflection. The angle obtained for l:iì would give, for l:l, 156° 40’.
By measurement with a goniometer attached to a compound microscope, the plane angle between the lines of cross cleavage, or cl, and the edge I:I (which equals the inclination of O on the orthodiagonal section, or a plane ii) was found to be 117-117½°; and that between edge I:I and edge l:l (which equals ii on li, both unobserved planes) 95°-95½°: whence O:li would equal approximately 147° 30’. The inclination of the rough plane r on the edge l:l is about 110°, but varies much.
The results of calculation, taking as data the above mentioned angles I:I and l:ii, along with the inclination of O to ii = 117° 15’, and that of the edge l:l (or lì) to ii = 95° 15’ are as follows:
C (= O : iì) = 117° 15’ and 62° 45’
a (vert. axis) : O (clinodiag.) : c = 0.5396 : 1 : 2.614
l : l = 156° 46’