Arr, 12, SIDERITE AND ASSOCIATED MINERALS—SHANNON. vi 
white or yellowish octahedrons and spinel twins. Lacroix® found 
cristobalite with quartz and tridymite in cavities in the basaltic rock 
from Mayen, and Gaubert® found one-half mm. cristobalites in lavas 
from Mayen and Niedermendig. Lacroix’ found cristobalite octa- 
hedrons with tridymite in volcanic blocks from the St. Vincent erup- 
tion of 1902-3. 
Cohen"! has reported cristobalite with carbon and schreibersite, 
as sharp crystals 0.01 to 0.09 mm. in size, showing the cube either 
alone or with the octahedron, in the meteoric iron of Kendall County, 
Texas. Aside from this occurrence in meteoric iron, which must 
be considered as requiring confirmation, the first recognition of 
the occurrence of this mineral in the United States was by 
Rogers,’? who identified cristobalite in volcanic rocks from two 
localities in California and associated with orthoclase rods in spher- 
ulites in obsidian from Yellowstone 
Natonal Park.’ The latter cristobal- 
ite is fairly abundant in spherical ag- 
gregates 0.3 mm. in diameter and is 
subtranslucent with an enamel-like 
appearance. Under the microscope 
it is faintly birefringent, with a com- 
plicated mosaic structure, and shows 
polysynthetic twinning in spots. The 
refractive index is 1.485+0.005. 
Upon heating, the mineral became 
translucent and upon cooling reverted 
again to the subtranslucent form. 
Later Rogers has mentioned still a 
third California locality for this min- 
4 5. 
14 ‘ 
eral. Figs. 4-5.—MAGNETITE FROM BASALT CAYVI- 
While the earlier authors were in- TIES. 4, COMMON OCTAHEDRAL FORM SHOW- 
. . . ° ING DEPRESSED FACES. 5, SHOWING A NUM- 
clined to regard cristobalite as either’ spor ocranmDRONS ALIGNED IN PARALLEL 
pseudomorph after an isometric min- —P°S!TION. 
eral or as a mimeoisometric aggregate of twinned tridymite crystals, the 
mineral is now regarded as a well-substantiated species. Its synthesis 
has been effected by several investigators, and the results of Fenner, 
being the most recent and complete, may be summarized here. In 
general silica heated with a flux (either sodium tungstate or alkaline 
8 Albert Lacroix. Bull. Soc. Min., Paris, 1891, vol.14, p. 185 
9P, Gaubert. Bull. Soc. Min., Paris, 1994, vol. 27, p. 242. 
l0Albert Lacroix. Bull. Soc. Min., Paris, 1905, vol. 28, p. 59. 
u Ann. Naturhist. Hofmus. Wien., 1900, vol. 15, p. 385. 
12 Austin Flint Rogers. Amer. Journ. Sci., vol. 45, p. 222, 1918. 
13 Austin Flint Rogers. Amer. Mineral., vol. 6, p. 4, 1921. 
M4 Austin Flint Rogers. Amer. Mineral., vol. 6, p. 60, 1921. 
Clarence N. Fenner. The stability relations of the silica minerals. Amer. Journ. Sci., vol. 30, 
1913, p. 331. 
