Arthur J. 3rd Gude 3rd
Richard A. Sheppard
1973
<p>Silicic vitric tuffs in saline, alkaline lacustrine deposits are commonly altered to a variety of zeolites and potassium feldspar. The tuffs generally show a lateral gradation, in a basinward direction, of fresh glass to zeolites and then to potassium feldspar. Zeolites were formed early in diagenesis by reaction of the glass with the interstitial water. The feldspar, however, was formed later by reaction of the zeolites with interstitial water, and its formation can be correlated with water of relatively high salinity and alkalinity. Semiquantitative spectrographic analyses for boron in the zeolites and potassium feldspar show that most of the boron resides in the relatively late feldspar. The boron content of the zeolites is commonly less than 100 ppm, whereas the boron content of the potassium feldspar is commonly greater than 1,000 ppm. Boron apparently substitutes for aluminum in the feldspar structure and causes distortion of the monoclinic unit cell such that the <i>b</i> and <i>c</i> dimensions are shortened. These boron-bearing potassium feldspars having anomalous cell parameters seem unique to saline,<br />alkaline lacustrine deposits and could serve as a prospecting aid for locating buried saline minerals.</p>
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U.S. Geological Survey
Boron-bearing potassium feldspar of authigenic origin in closed-basin deposits
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