Glauconitic Mica in the Morrison Formation in Colorado1

W. D. Keller
U. S. Geological Survey and University of Missouri, Columbia, Missouri
1 Publication authorized by the Director, U. S. Geological Survey. This report concerns work done on behalf of the Division of Raw Materials of the U. S. Atomic Energy Commission. This paper was also presented at the Twentieth International Geological Congress in Mexico City, September, 1956.

Abstract: Glauconitic mica occurs as interstitial clay in a bed of fine-grained sandstone, 15 feet thick, in the upper part of the Brushy Basin shale member of the Morrison formation of Late Jurassic age on Blue and Lone Tree Mesas near Uravan, Montrose County, Colorado. This glauconitic mica, similar to other material ordinarily called glauconite, is interesting because the Morrison formation is generally regarded as nonmarine in origin, whereas glauconite commonly forms in a marine environment.

The identifying properties of the glauconitic mica are: strong 001 d spacing, 9.97A; very weak 002, 5.01A; strong 003, 3.34A; 060, 1.51A; green color, pleochroic; X and Z indices, approximately 1.578 and 1.600; SiO2 49 percent, Al2O3 18 percent, Fe2O3 13 percent, FeO 1.3 percent, MgO 2.8 percent, K2O 7.8 percent, TiO2 1.1 percent, ignition loss 6 percent, others approximately 1 percent.

The glauconitic sandstone is underlain by variegated mudstones about 480 feet thick, which contain montmorillonite (with very sparse relicts of shards), illite, quartz, and also, in several layers, scanty analcime. The Burro Canyon formation of Early Cretaceous age overlies the glauconitic sandstone.

This paper discusses alternative origins of the glauconitic mica possible within the framework of an appropriate chemical environment other than, and apart from, one typically marine.

Clays and Clay Minerals; 1956 v. 5; no. 1; p. 120-128; DOI: 10.1346/CCMN.1956.0050109
© 1956, The Clay Minerals Society
Clay Minerals Society (www.clays.org)