MINERALOGY: LARSEN AND WELLS 
363 
white enamel. It is somewhat slowly but completely soluble in acid. 
Its specific gravity as measured on the powder by the pycnometer 
method is 2.730. 
Most of the grains show no good crystal development, but a few are 
stout prismatic crystals with an acute rhombic cross section. One 
crystal showed curved and striated pyramidal faces. The faces are 
dull, curved, and imperfect, and not suitable for accurate measurements. 
The angle between the prism faces measured under the microscope on 
the cross section of a prism gave 68° and is believed to be accurate to 
within about 1°. However, a measurement on a second prism with 
distinctly curved faces gave only 59°. The mineral has a perfect cleav- 
age parallel to the elongation of the prisms and bisecting the obtuse 
angle of the prismatic faces. 
Creedite has distinctive optical properties and was first recognized 
by them. Extinction angles can not be measured accurately on ac- 
count of the imperfect development of the crystals. However, cleavage 
fragments show sensibly parallel extinction and the emergence of an 
optic axis very nearly normal to the cleavage plane. Prismatic sections 
normal to the cleavage (bisecting the acute angle of the faces) show 
extinction angles of about 41° (Z A elongation), the emergence of Y, 
sensibly normal, and irregular twinning. Crystals lying on the crystal 
face show extinction angles (Z A c) of 42|° ±1°. The mineral is there- 
fore beheved to be monoclinic with the faces (110), perfect cleavage 
(100), Y = b, and Z A c = 41°; (100) is a twinning plane. 
The mineral is optically negative. The axial angle as measured on 
the axial angle apparatus is: 
2Vii = 64° 30' =t 10', 2V„a = 64° 22' ^ 10', 2VTb 64° 20' ± 10'. 
Other less satisfactory sections gave 
2V^a = 64° 18' ± 20' and 2Vna = 64° 50' ^ 30'. 
The dispersion was perceptible only on one optic axis. 
The indices of refraction were measured by the immersion method by 
matching in a liquid and immediately measuring the index of refraction 
of the Hquid at the same temperature; the probable error should be less 
than =1= 0.001. All of the grains give the same value showing that the 
mineral is homogeneous. 
a = 1.461, /3 = 1.478, 7 = 1.485. 
The axial angle computed from the indices of refraction is 65° which 
agrees very well with the measured values. 
