A Study of Glauconite

Glauconite was first named in 1828, but in spite of repeated studies and numerous analyses its exact manner of formation and composition have remained in doubt. Chemical analyses obtained from magnetically and microscopically purified samples, from widely separated localities, show that the composition of glauconite can be expressed by a definite formula which in its simplest form may be written as follows: . The principal variables in its composition are and . The ferric iron content varies from 16 to 30 per cent, and the alumina from 10 per cent down to less than 2 per cent. Glauconite is optically negative and biaxial with a small optic angle. Both the indices of refraction and the birefringence vary with the ferric iron content. Modern glauconite is essentially the same as the glauconite of the older rock formations both optically and chemically. The X-ray patterns of the samples of glauconite studied show that glauconite is a definite mineral, and not a mixture of various substances.