310 The American Geologist. ^^'^^' ■'•^°^- 
III. Pyroxene from olivine diabase of Pigeon Point, Minn. 
Partial Analysis by R. B. Riggs. Bull. 109, U. S. Geol. 
Survey, p. 36. 
The pyroxene whose composition is given under II above 
has been called pigeonite by the writer ; it is unique in possess- 
ing an optic axial angle which is quite variable, but always 
distinctly less than the usual value for pyroxene. That this 
abnormally small optic angle is not due to the high per cent 
of titanium dioxide present in pigeonite seems to be quite well 
established by the work of Becker. In general pyroxenes pos- 
sess an optic angle which is remarkably constant ; angle V very 
rarely varies* more than two degrees from the normal value 
which is nearly 60°. In the tliree known instances* in which 
the optic angle shows a marked variation from the normal 
value it has the values: 65° 3', 66° 44', and 68°. These are all 
larger than the normal value, and the variation even here is 
insignificant when compared with the variation in the pyroxene 
of Pigeon Point, where the optic angle becomes so small in 
some cases as to give almost the effect of an uniaxial mineral. 
x\nother peculiarity of pigeonite is that in it the optic angle 
seems to be extremely variable even in a single thin section. 
These characters of pigeonite can ha'rdly be due to the man- 
ganese content, since Flink* has shown that schefferite from 
Sweden with six to eight per cent of MnO has an optic angle 
of 65°. 
The only two occurrences of pigeonite at present known 
are both in Minnesota: — at Pigeon Point, and at Duluth, that 
is at the two extremities of a single petrographic province. 
These lines have been written in the hope that they may lead 
to the discovery of other occurrences of this anomalous pyr- 
oxene. 
• Cf. HiNTZE. Handb. Mineral, ii, pp. 1025-1029. 
