364 
MINERALOGY: LARSEN AND WELLS 
Material for a chemical analysis was carefully purified by heavy 
solution (bromoform) and a careful microscopic examination showed 
only a very little kaolinite present. The analyses by R. C. Wells gave 
the following results: 
Analysis and molecular ratios of Creedite 
2. 
Al 11.58 427 210 2 X 105 11.0 
Ca 23.98 599 295 3 X 98 24.4 
SO4 18.32 191 94 1 X 94 19.5 
O* 3.97 248 122 1 X 122 3.2 
H2O- 0.72 
H20 11.08 615 303 3 X 101 11.0 
Fi* 30.35 1591 784 8 X 98 30.9 
100.00 100.00 
* Calculated on the basis of a summation of 100%. 
This leads to the formula CaSO4.2CaF2.2Al (F,OH)3.2H20 with 
F : OH 2 : 1 and the mineral has the composition of gearksutite with one 
molecule of CaS04 added. The composition corresponding to this 
formula is given in column 2. Creedite is one of the very few minerals 
described that contain both sulphate and fluoride as essential 
constituents. 
Isotropic Kaolinite-Like Mineral. — The creedite is imbedded in a small 
amount of a kaolinite-Hke mineral with rather unusual properties, which 
is believed to have been derived from some unknown mineral. In the 
hand specimen it is a white powder, in the thin sections it is clear, in large 
part sensibly isotropic, and can be distinguished from the Canada balsam ' 
in which it is imbedded, only by testing its index of refraction. Locally 
it shows birefringence in wavy streaks, probably due to strain in the soft 
mineral or to incipient crystallization. It has an index of refraction of 
1.557 ^ 0.003. When immersed in a liquid with that index of refrac- 
tion it is at first clouded but the oil rapidly penetrates its pores, the 
mineral becomes clear, and the grains can be found only by the colored 
borders formed by the Schroeder van der Kolk test for the index of 
refraction. The specific gravity of the powder measured by the pycnom- 
eter method is 2.548. 
A partial analysis by Roger C. Wells gave: 
Si02 AI2O3 CaO MgO F Ign. Total 
44.2 40.2 0.3 tr. none 15.5 100.2 
