| Abstract: | Rates of alkali exchange between alunite and water have been measured in hydrothermal experiments of 1 hour to 259 days duration at 150 to 400??C. Examination of run products by scanning electron microscope indicates that the reaction takes place by dissolution-reprecipitation. This exchange is modeled with an empirical rate equation which assumes a linear decrease in mineral surface area with percent exchange (f) and a linear dependence of the rate on the square root of the affinity for the alkali exchange reaction. This equation provides a good fit of the experimental data for f = 17% to 90% and yields log rate constants which range from -6.25 moles alkali m-2s-1 at 400??C to - 11.7 moles alkali m-2s-1 at 200??C. The variation in these rates with temperature is given by the equation log k* = -8.17(1000/T(K)) + 5.54 (r2 = 0.987) which yields an activation energy of 37.4 ?? 1.5 kcal/mol. For comparison, data from O‘Neil and Taylor (1967) and Merigoux (1968) modeled with a pseudo-second-order rate expression give an activation energy of 36.1 ?? 2.9 kcal/mol for alkali-feldspar water Na-K exchange. In the absence of coupled alkali exchange, oxygen isotope exchange between alunite and water also occurs by dissolution-reprecipitation but rates are one to three orders of magnitude lower than those for alkali exchange. In fine-grained alunites, significant D-H exchange occurs by hydrogen diffusion at temperatures as low as 100??C. Computed hydrogen diffusion coefficients range from -15.7 to -17.3 cm2s-1 and suggest that the activation energy for hydrogen diffusion may be as low as 6 kcal/mol. These experiments indicate that rates of alkali exchange in the relatively coarse-grained alunites typical of hydrothermal ore deposits are insignificant, and support the reliability of K-Ar age data from such samples. However, the fine-grained alunites typical of low temperature settings may be susceptible to limited alkali exchange at surficial conditions which could cause alteration of their radiometric ages. Furthermore, the rapid rate of hydrogen diffusion observed at 100-150??C suggests that fine-grained alunites are susceptible to rapid D-H re-equilibration even at surficial conditions. ?? 1994. |
| Genre: | Article |
| ProdID: | 70017819 |
| Citation Author: | Stoffregen, R. E.; Rye, R. O.; Wasserman, M. D. |
| Citation Contributing Office: | |
| Citation Datum: | |
| Citation Day: | |
| Citation Edition: | |
| Citation Editor: | |
| Citation End Page: | 929 |
| Citation Issue: | 2 |
| Citation Keywords: | |
| Citation Language: | English |
| Citation Larger Work Title: | Geochimica et Cosmochimica Acta |
| Citation LatN: | |
| Citation LatS: | |
| Citation LonE: | |
| Citation LonW: | |
| Citation Month: | |
| Citation No Pagination: | |
| Citation Number Of Pages: | 13 |
| Citation Online Only Flag: | |
| Citation Phsyical Description: | |
| Citation Projection: | |
| Citation Public Comments: | |
| Citation Publisher: | |
| Citation Series: | |
| Citation Series Code: | |
| Citation Series Number: | |
| Citation Search Results Text: | Experimental studies of alunite: II. Rates of alunite-water alkali and isotope exchange; 1994; Article; Journal; Geochimica et Cosmochimica Acta; Stoffregen, R. E.; Rye, R. O.; Wasserman, M. D. |
| Citation Start Page: | 917 |
| Citation Volume: | 58 |
| Citation Year: | 1994 |
| Type: | citation/reference |
| Text: | Experimental studies of alunite: II. Rates of alunite-water alkali and isotope exchange; 1994; Article; Journal; Geochimica et Cosmochimica Acta; Stoffregen, R. E.; Rye, R. O.; Wasserman, M. D. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Sat, 1 Jan 1994 00:00 -0600 |
| Publisher: | |
