PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 8 JANUARY 15. 1922 Number 1 
THE RELATION OF CHALCEDONY TO THE OTHER FORMS OF 
SILICA 
By Edward W. Washburn and Louis Navias 
Department of Ceramic Engineering, University op Ii^linois 
Communicated by W. A. Noyes, Nov. 21, 1921 
Silica is knov^n to occur in three distinct enantiotropic forms, each 
form having two or more enantiotropic sub-forms. These forms are 
quartz {a and jS), tridymite (a, /3i, and ^2) and cristobalite {a and ^) to 
which may be added siHca glass, the amorphous or liquid form. The vari- 
ous transition and fusion temperatures and the ranges of stable existence 
of all of these forms have been accurately determined by Fenner.^ The 
position of the mineral chalcedony in this system has, however, never been 
satisfactorily established, although Fenner seemed inclined to view it as 
a form different from any of the preceding. 
The present investigation is a contribution to the elucidation of the chal- 
cedony puzzle. The method of investigation adopted was a careful de- 
termination of a number of physical properties of the raw and calcined 
chalcedony and a comparison of these properties with those of the four 
recognized forms of silica. Two minerals belonging to the chalcedony 
type were used. One of these was a very pure typical chalcedony from 
Yellowstone Park and the other was a black pebble of French flint. 
For comparison a summary of all the data obtained with these materials 
together with the corresponding data for the four recognized forms of silica 
is displayed in the accompanying table. 
The density data were determined on the finely ground powder by the 
pycnometer method, the pycnometer and powder being heated to 400° C 
and evacuated with a mercury vapor pump for several hours before ad- 
mitting the air-free distilled water. After weighing, a sample of the powder 
was removed from the pycnometer and while still under water was sent to 
the Geophysical Laboratory and subjected to a pressure of 1000 atmos- 
pheres for two hours. This treatment resulted in a density increase of 
only six units in the fourth decimal place. 
1 
