12 PROCEEDINGS OF THE NATIONAL MUSEUM vol.69 



PREVIOUS STUDIES OF GLAUCONITE. 



The complete crystallinity of glauconite has been widely recog- 

 nized. Lacroix ^ described crystal grains and gives the optical prop- 

 erties as follows : 



Form and mode of occurrence. — Glauconite occurs in the form of rounded 

 grains similar to grains of powder; they are rarely greater than 0.1 mm. in 

 diameter. The masses of glauconite which form true beds in certain sedimen- 

 tary horizons are formed by small granules of the nature of agglomerates with 

 or without calcite or marly material. The crystallinity of glauconite is visible 

 only under the microscope and often only with the strongest magnification; 

 when the mineral shows the form of overlapping plates. More rarely one may 

 observe true crystals of glauconite which may reach a fraction of a millimeter 

 in size and be formed entirely of plates or a piling up of lamellae which are 

 probably hexagonal ; the latter are often warped after the manner of a variety 

 of ripidolite, helminth, or of kaolinite. These are the crystals that lend them- 

 selves to a study of the optical properties. 



Cleavage. — Cleavage p. (001) analogous to that of the micas and chlorites. 



Color and luster. — Olive green, dark green. By alteration becomes more or 

 less a deep yellow. 



Appearance. — Earthy. Transparent in thin section and more or less of a 

 deep green. 



Optical properties. — The negative acute bisectrix is more or less normal to 

 the perfect cleavage. 



2E = 30° to 40° 



Sometimes glauconite is nearly uniaxial (Saint-Laurent var.). 



The birefringence is probably not determinable with great precision. How- 

 ever, in sufficiently thin plates I have been able to ascertain that it is approx- 

 imately ng-np=0.020. 



By analogy with the micas and chlorites one may suppose that glauconite is 

 monoclinic. I have obtained in a few sections which were almost perpendicu- 

 lar to the cleavage, extinction angles of 1° or 2° with the trace of the perfect 

 cleavage, but it is hardly possible to draw exact conclusions because the orien- 

 tation of the sections examined is in doubt. 



The most thorough microscopic study of glauconite ever made is 

 that by Cayeux.^" He finds that the great majority of the grains of 

 glauconite are characterized by a cryptocrystalline structure, but he 

 describes and pictures glauconite grains with strong pleochroism and 

 cleavage similar to that in micas or chlorites. 



Cayeux agrees with Lacroix as to the probable crystal habit of 

 glauconite. 



Caspari " has made a study of glauconite now forming and has 

 the following to say of the state of aggregation of glauconite : 



Owing to the birefringence and pleochroism exhibited by submarine glau- 

 conite when examined under the microscope, it has hitherto been usual to 



» Lacroix, A., Mineralogie de la Prance et de ses Colonies, vol. 1, Premier, pp. 406. 407, 

 408, 1893-1895. 



" Cayeux, Lucien, Etude Micrographique des Terrains Sedimentarles, ch. 4, pp. 163-184, 

 1897. 



" Caspari, W. A., Proc. Roy. Soc. Edinburgh, 1910, vol. 30, pp. 364-373. 



