is
also very suggestive of the color given by the sesquioxide of titanium.
The reduction test stands opposed to this idea unless the quantity in
the material used for the test was very small— or, in other words,
unless very small quantities can give distinct colors.
The
color is apparently quite stable. No evidence of fading has been
noticed in the cut stones and fragments heated to a bright red, just
short of fusion, for five or six minutes showed no change whatever
after they were again cooled.
Pleochroism
is very intense in the deep colored varieties and is probably the most
important test applicable to cut stones. In the lighter parts the
extraordinary ray is a very slightly greenish blue, inclining to indigo
as it becomes darker, and is very similar to one of the axial colors
shown by some cordierites. In the deeper colored crystals and the
thicker layers it is an intense purplish blue. The ordinary ray is
white. The color of the mineral in ordinary light is therefore merely
the color of the extraordinary ray diluted with the white of the
ordinary ray. The extraordinary ray shows strong absorption of sodium
light, and renders a determination of the refractive index for that
light difficult.
An
attempt has been made to represent the pleochroism in plate 33, figures
2 to 4, as shown in a large cut stone. Figure 2 shows the natural
color, and figures 4 and 3 the colors of the ordinary and,
extraordinary rays. The stone used is of only moderate depth of color.
Dark colored specimens, if fairly thick, give such very strong
absorption along the extraordinary ray that the depth of color is
hardly reproducible.
It fuses quietly to a transparent glass at about 3.
Chemical Characters.
In
hydrochloric acid it is practically insoluble, and this permits us to
dissolve crystals out of their natrolite matrix without injury to the
crystal faces. The natrolite dissolves, leaving the benitoite in a
matrix of hydrous silicic acid which is easily removed. Sulphuric acid
has also been used for this purpose at the mine. The mineral is
attacked by hydrofluoric acid, and dissolves readily in fused sodium
carbonate. Blasdale also finds it but slowly attacked by molten
potassium pyrosulphate.
