1214 
of benitoite. This very beautiful mineral, which is used as a gem, 
and whose chemical composition is: BaTiSi,O,, has some importance 
from the mineralogical standpoint, because it has been considered 
by mineralogists as the only representative of the trigonal-bipyramidal, 
or, — with greater probability, — of the ditrigonal-bipyramidal class. 
The first opinion was expressed by Rogers *), the last by PaLacne’), 
who as a proof of the correctness of his view emphasized the presence 
of a form {2241} in many crystals. Later on however JEZEK") made 
it probable that the ternary axis was of a polar nature, and that 
benitoite-erystals were twins with respect to the basal face (0001; ; 
by this author benitoite is considered to be a ditrigonal-pyramidal 
mineral. 
Evidently the question about the real symmetry of this remarkable, 
enormously diehroitie mineral, is not yet settled; for that reason 
this object was chosen for the study of the diffraction phenomena 
of RÖNTGEN-rays. . 
We had at our disposition very beautiful pink erystals from San 
Benito County in California, where benitoite is accompanied by 
natrolithe and neptunite. They were flattened parallel to {0001}, and 
showed a combination of the forms: {0001}, {1011}, {0111}, {1010} 
and {0110}. From a beautiful, homogeneous crystal three plates were 
prepared exactly parallel to {OO01{, to 1010} and to {1210}. The 
optical investigations of the plate perpendicular to the c-axis, very 
soon proved that the crystals show only pseudo-trigonal symmetry, 
and that they are in reality not uniaxial, but biaxial, with a very 
small axial angle, and with positive character of their tirst bisectrix 
which coincides with the direction of the c-axis. In no position was 
the plate completely dark when between crossed nicols. On rotating 
the microscope-table the interference-image often showed a deform- 
ation of the central part and distinct lemniscate-shaped inner rings, 
as well as the transformation of the dark cross into two branches 
of a black hyperbola; the plane of the optical axes is evidently 
perpendicular to {L010}, with the c-axis as the direction of the first 
bisectrix, which has a positive character. The birefringence of the 
mineral is strong; in basal sections local disturbances of the image 
are also observed, suggesting at once the memetic character of the 
1) Rogers, Science, 28, 676, (1908). 
2) Patacue, Amer. Journ. of Science, 27, 398, (1909). 
8) Jezek, Bull. intern. de Acad. des Sciences de Bohème, Prague (1909). Vide 
also on benitoite: LouperBack. Publ. of Calif. Univ. 5, 9, 149, (1907); Kraus, 
Science 27, 696, 710, (1908). 
