W. H. Hobbs—Mineralogical Notes. 127 
To determine the optical properties of the mineral a large 
twinned crystal was sawed so as to make three thin sections ; 
the first parallel to the basal cleavage of a considerable part of 
the crystal (oP), the second normal to this section and parallel 
to the noticeable straight twinning line and its parallel edge 
(«© Po ), and the third normal to the first two sections (« Pa ), 
From an examination of these sections it was determined that 
the mineral is either monoclinic or triclinic with a close approach 
to monosymmetric character. The basal cleavage is very per- 
fect and a second distinct cleavage runs parallel to either a 
steep pyramid or a prism, for the two directions are inclined 
about 120° to one another and about 90° to the base. A third 
irregular cleavage follows the clinopinacoid. The index of 
refraction is high and the double refraction very low. The 
plane of the optic axes bisects (nearly) the obtuse angle of the 
cleavage and the positive bisectrix (probably obtuse) is nearly 
normal to the base. The absorption is: 
a olive green to greenish gray. 
b deep robin’s egg blue. 
c light yellow green. 
with b> a>c 
_ The erystal is polysynthetically twinned so that the “ ortho- 
pinacoidal ” section afforded lamelle corresponding to three 
positions—one of which has parallel extinction, and the other 
two have extinction angles of 14-24° on opposite sides of the 
basal cleavage lines (see Fig. 5.) This corresponds to the 
Tschermak law where the individuals are inclined 120° to one 
another. These three sorts of lamelle are also observed on the 
“ clinopinacoidal ” section. 
