THE OPTICAL PEOPERTIES AND CHEMICAL 

 COMPOSITION OF GLAUCONITE 



By Clarence S. Ross 

 Geologist, United States Oeological Survey 



INTRODUCTION 



Glauconite has long been a subject of study by mineralogists, by 

 students of sediments, and by those interested in modern marine de- 

 posits. It is a mineral that has formed in eveiy geologic age from 

 the Cambrian to the present, and the related mineral greenalite is 

 found in pre-Cambrian rocks.^ Beds of nearly pure glauconite 

 with wide lateral extent are not uncommon and it occurs in vary- 

 ing proportions in almost all types of sediments. Glauconite is 

 one of the important materials now forming on the sea bottom, 

 and the potassium content makes it a possible source of plant fer- 

 tilizers. All this has inspired many studies of glauconite and 

 much literature has accumulated on the subject, but nevertheless 

 there has been little agreement as to the exact chemical composition 

 of glauconite and its optical properties are but imperfectly known. 

 The incomplete understanding of glauconite has led to the present 

 study which includes a determination of the optical properties of 

 material from several localities, and an investigation of the chem- 

 ical relationships which appears to satisfactorily explain the varia- 

 tion in chemical composition. 



OCCURRENCE OF GLAUCONITE STUDIED 



In 1915 L. A Myllius brought to the attention of the writer a 

 mineral from the lead mines of southeastern Missouri that had 

 long been called " chlorite " or " chloritic material,"^ and on investi- 

 gation this proved to be glauconite.^ Additional material was se- 

 cured from some of the mining companies and Doctor Buehler, the 

 State Geologist of Missouri. Diamond drill cores furnished by 

 the St. Joseph Lead Co. contained some unusually good material, 

 and this led to a thorough study of the Missouri glauconite. The 



^ Leith, C. K., U. S. Geol. Survey Mono., vol. 43, 1903, pp. 239-259. 

 2 Buckley, E. R., Mo. Bur. Geol. Mines, vol. 9, pt. 1, p. 28, 1909. 

 'Ross, C. S., Econ. Geol., vol. 11. pp. 289-290, 1916. 



No. 2628.— Proceedings U. S. National Museum, Vol. 69, Art. 2. 



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