ART. 2 PROPERTIES OF GLAUCONITE ROSS 3 



Most of the grauconite no doubt forms on the sea bottom in the 

 manner commonly accepted,® but this method of origin can not apply 

 to all occurrences. Glauconite is frequently observed that has formed 

 in the cleavage cracks of other minerals, and also where it has partly 

 or wholly replaced both silicates and carbonates. It is evident that 

 glauconite has formed from solutions by direct precipitation and by 

 replacement of preexisting minerals, and in many occurrences sub- 

 sequent to the deposition of the inclosing sediments. 



OPTICAL PROPERTIES 



The glauconite from the Bonneterre dolomite, St. Joseph Lead 

 Co. mines near Bonneterre, St. Francis County, Mo., is more com- 

 pletely crystalline and shows the optical properties more perfectly 

 than any other material that has been observed. For this reason 

 it will be described in detail and the other types of glauconite will 

 be compared with it. 



The grains of glauconite that occur as single crystals vary in 

 habit. Some have perfect, parallel cleavage and are rectangular 

 in the plane perpendicular to the cleavage and more or less rounded 

 in a plane parallel to the cleavage. A few suggest a roughly 

 hexagonal outline parallel to the cleavage but no completely euhedral 

 crystals of glauconite have been observed. Part of the grains have 

 a curved helminth-like habit and the cleavage is radial, and others 

 have a core that represents a single crystal surrounded by a fine 

 grained aggregate of crystal grains. The better crystals have a very 

 perfect cleavage, a strong pleochroism from bright green to yellow, 

 and a moderately high birefringence. Glauconite is therefore similar 

 to the micas in habit. 



The indices of refraction of glauconite from the Bonneterre dolo- 

 mite of Bonneterre, Mo., are a=1.597, y8=l,618, y=1.619, y— a=.022, 

 ±.003. Optical character negative ( — ). The acute bisectrix X 

 is nearly, but not quite normal to the cleavage. X inclined to C 

 about 3°. The absorption is Z=Y<X, Pleochroism Z and Y lemon 

 yellow, X dark bluish green (dark Russian green of Ridgeway). 

 The optical angle is nearly constant with 2 V=20°, 2 E=33°. The 

 dispersion is distinct p>v, but the absorption of the red by the blue- 

 green mineral makes it very difficult to observe. 



The other types of glauconite that have been studied show the same 

 properties in somewhat less perfection of detail. The glauconite 

 from Singleton, Tenn., contains a very large proportion of grains 

 that are individual crystals. These have the same general habit as 

 those just described, but the color is very pale yellow to pale green. 

 The largest optical angle observed (2V=30°) is that of the glaucon- 



« Challenger Rept., Deep-sea deposits, 1891, p. 383. 



