Abstract

The Fairfield variscite deposit is located about 50 miles south of Salt Lake City. The variscite occurs as nodules in a highly brecciated zone in limestone. The nodules are veined and surrounded by several other calcium aluminum phosphates; other rarer phosphate minerals are in cavities in the nodules. Miscellaneous additional data are given on some of the minerals. Weissenberg x-ray studies were made on wardite, deltaite, and gordonite. On the basis of x-ray powder photographs, wardite is shown to be related structurally and chemically to millisite, and deltaite to pseudowavellite.

The paragenetic relations of the minerals are described in detail. Variscite was deposited as nodules, and the other phosphate minerals are alteration and replacement products of the original variscite. The alteration history can be divided into four stages: In the first stage, quantitatively the most important, the bulk of the pseudowavellite was deposited with some deltaite, then millisite and wardite with some deltaite and lehiite, and finally more pseudowavellite and deltaite. The second stage comprises the crystal-forming minerals in cavities of variscite and pseudowavellite; these are, in their probable sequence, gordonite, englishite, montgomeryite, overite, and sterrettite. The third stage represents the deposition of a small amount of pseudowavellite, and the fourth, the deposition of the members of the apatite group. Each of these stages has characteristic chemical qualities.

The variscite was formed by phosphatic groundwaters descending along open fissures; the source of the phosphate was probably a phosphorite bed, weathering at the surface. The deposition of the variscite was probably caused by the reaction of aluminous material with the phosphatic waters. Later groundwater, no longer phosphatic, reacted with the variscite to form the alteration minerals. The different chemical characteristics of each group of alteration minerals reflect the changing composition of the groundwater which in turn was due to the weathering of varying surface materials.

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