needles of greenish or bluish amphibole, etc., and recalling in its general nature the "Eisennatrolith" of Norway.
Following
this is the more or less altered wall-rock, which along the central
part of the deposit is made up largely of bluish or greenish amphibole
in minute prisms or needles or irregular tangled mats. It often has a
rather porous and irregular texture, as the result of considerable
leaching of the original rock substance.
THE MINERALS OF THE DEPOSIT.
BENITOITE.
Crystallography.
Benitoite
crystallizes in the trigonal division of the hexagonal system and, as
will be more fully shown below, it belongs to the twenty-second or
ditrigonal-bipyramidal group of Groth, the tri-gonotype group of
Dana—the first actual example of this type of symmetry. This is the
highest symmetry group of the trigonal division—too high to exhibit
rhombohedra which are so characteristic of this division that it is
often called the rhom-bohedral division or system.
The axial ratio c:a is 0.7344, determined as the average of 27 direct measurements of the angle between
and c
(0001) with the two-circle goniometer.
The detailed data obtained from 7 crystals are as follows:
Giving
weights of 4, 3, 2, and 1 for excellent, good, fair, and poor
reflections, respectively, the average is 40° 17:94; a simple average,
all readings being given same weight, gives 40° 18;04. The closeness to the value for apatite (Dana, 0.73465) is striking, but the symmetry is different and the relationship otherwise not apparent.
s Baumhauer in Zeit. fur Kryst., XVIII (1891), p. 40, has collected the various values for the axial ratio of apatite and gives values from 0.7294 to 0.7353.
