trigonal
pyramid and the corresponding trigonal prisms. The normal angle between
the basal plane and the pyramid is about 40° 14'. If the pyramid be
taken as a unit pyramid of the first order, this would yield an axial
ratio of .7327, if of the second order. .8460.
The
most common habit is pyramidal, one pyramid being the chief form, the
other occurring as a small but regular and brilliant truncation. One
or both prisms may be present as narrow truncations and also a small
triangular basal plane.
Occasionally
the base is developed into a broad plane, the crystals then having a
more nearly tabular habit. The outline of the base may then be
hexagonal but the edges corresponding to one pyramid will lie
considerably longer than the others.
Only
one crystal was found where the two pyramids were nearly equally
developed. The development of the faces at one end of the principal
axis always corresponds so well with those at the other, that it gives
the impression that the horizontal plane of symmetry is present. No
tendency towards a prismatic habit was observed. The angles between two
adjoining pyramid faces at one end of axis is 68° 1'. There is an
imperfect pyramidal cleavage. The fracture is conchoidal to
subconchoidal. The hardness is 6 1/4 - -61/2; distinctly above
orthoclase and labradorite and below chrysolite and quartz; density.
3.64-3.65.
The
refractive index is quite high, which adds greatly to the beauty of the
cut stone. For the ordinary ray it is about 1.77 (sodium light), for
the extraordinary, about 1.80. The double refraction is therefore very
strong and the mineral optically positive. Basal sections show a
perfect uniaxial cross which gives a distinct positive reaction with
the mica plate. The pleo-chroism has already been described and
evidently the absorption is e>o. Some difficulty was experienced in
getting a value for e, as sodium light is strongly absorbed even in
light colored specimens a eoirple of millimeters thick.
The
mineral fuses quietly to a transparent glass at about 3. It is
practically insoluble in hydrochloric acid, but it is quite easily
attacked by hydrofluoric acid, and dissolves readily in fused sodium
carbonate.
The mineral has proved to be of considerable interest from
