475.4 
XXXV. 
ON THE PSEUDO-OPACITY OF ANATASE. 
By J. JOLY, Sc.D., F.B.S., 
Honorary Secretary, Royal Dublin Society. 
[Read Frrruary 20; Received for Publication Frsruary 22; Published 
JUNE 11, 1901.] 
THE mineral species octahedrite exhibits remarkable variations in 
erystalline habit, colour, and transparency. The variety anatase, 
found in many of the Swiss metamorphic rocks (as in the Made- 
ranerthal and the valley of Binn) occurs in black and generally 
highly lustrous octahedra, exhibiting often not the smallest trace 
of transparency, and resembling, save for the obvious dimetric 
nature of the octahedron and a higher lustre, the mineral mag- 
netite. On the other hand, the variety wiserine (which is more 
especially abundant in the Binn locality) possesses a rich yellow- 
brown colour, high adamantine lustre, and is transparent. Again, 
found often side-by-side with the anatase, the orthorhombic species 
Brookite occurs in plates having the same transparency as the 
wiserine, but generally with more or less symmetrically arranged 
inclusions of dark and nearly opaque matter within. 
It is remarkable that the first-named species, anatase, owes its 
opacity almost entirely to its crystalline form and its high refrac- 
tive index. The opacity is, in fact, only apparent, the substance 
of the mineral being most generally transparent or highly trans- 
lucent, and of a pale, very delicate blue colour. In some in- 
dividuals, the transparency is diminished by irregularities and 
inclusions, and in some few specimens, the crystals are truly 
opaque, or nearly so. These last are wanting in the splendid 
lustre of the transparent specimens. Others, again—but these seem 
rare—are visibly transparent; but this seems due largely to the 
scattering of light by cracks and inclusions within the crystals, or 
to absence of the smooth and lustrous surface. In longest dimen- 
sion anatase crystals from the Swiss localities seldom exceed 3 to 4 
millimetres. 
