The Moss Back Sandstone at Temple Mountain, Utah, exhibits abundant argillic alteration in the grain interstices. Near collapse features and close to ore zones the principal clay minerals, kaolinite and illite, become more abundant. Aggregate birefringence and particle size appear to increase. Specific types of illite also develop. Mixtures of 1M and 2M1 mica polymorphs are typical. The 2M1 mica-clay polymorph also becomes a more abundant constituent of mixtures. Near collapse areas, microvermicular aggregate growths of kaolinite become common.
In unaltered and nonore-bearing sediments, the 2M1 mica-clay polymorph has not been observed. Where scattered detrital flakes of 2M1 muscovite are found along bedding planes in the unaltered sediments, they appear to be breaking down to a 1M structure.
Hydrothermal synthesis of micas by Yoder and Eugster (1955) has shown the transition of 1M to 2M1 mica polymorphs which appears to occur between 200° C. and 350° C. at 15,000 p.s.i. water pressure. The illite at Temple Mountain which contains the 2M1 polymorph is believed to have been deposited by hydrothermal solutions accompanying urano-organic ore deposition. Natural conduits for circulation appear to have been provided by the collapse features and by fractures. The source of the clay-forming elements is probably not only the original interstitial material but also includes detrital feldspar replaced during the development of ore masses.