Abstract
The composition of analcime samples from the Green River Formation of Wyoming was determined by optical, X-ray, and electron-microprobe methods. The analcime ranges from 1.95 to 2.90 in Si/Al ratio; Ca2+ and K+ are present in only trace amounts. On the basis of silica content, most analcime falls into two groups, one generally having Si/Al ratios between 2.00 and 2.30, and the other generally ranging from 2.60 to 2.80. High-silica analcime predominates in silicic tuffs of the Laney Shale and Tipton Shale members and in that part of the Wilkins Peak Member nearest the margin of the basin.
Low-silica analcime is nearly always associated with authigenic alkali feldspars, and it is most abundant in the transitional zone between the tuffs with siliceous analcime and those lacking analcime and consisting largely of authigenic alkali feldspars. Within the transitional zone, the Si/Al ratio is inversely proportional to the amount of authigenic alkali feldspars. The Si/Al ratio in analcime also corresponds to the probable salinity of the intra-stratal solutions, with the high-silica analcimes associated with low salinities and the low-silica analcimes with high salinities. Both low- and high-silica analcimes are commonly associated with authigenic quartz.
Analcime has extensively reacted to form alkali feldspars in the Green River Formation, and new data show that siliceous analcime crystals become partly desilicated in their reaction to form feldspars. Stratigraphic position of most of the altered analcime in and near the margin of the saline facies suggests that salinity has played a major role in the reaction of analcime to form alkali feldspars. Petrographic relationships show that at least some of the widespread shortite crystallized after the analcime had formed but before it had been replaced by alkali feldspars.
