Laumontite in Soils of the San Gabriel Mountains, California
- Kelly Taylor,
- Robert C. Graham and
- Jarel O. Ervin
Laumontite, a Ca-rich zeolite, is widespread and locally abundant in the San Gabriel Mountains of southern California. It occurs in hydrothermally altered rocks of various origins and is inherited by the overlying soils. The objectives of this study were to determine the distribution and properties of laumontite in these soils. Laumontite was found in soils derived from anorthosite, arkosic sandstone, and granodiorite. All of the soils were Entisols, composed largely of colluvium, on steep, chaparral-covered slopes. Soil depths ranged from 16 to 32 cm over 25 to 40 cm of saprolite (Cr horizon). Soil pH ranged from 5.0 in the A horizon of the soil on sandstone to 7.2 in the Cr horizon of the anorthosite-derived soil. Laumontite was found in sand, silt, and clay fractions of the soils. Its particle-size distribution in the soil depended on its grain size in the parent rock and its weathering within the soil. Comparisons of laumontite grain morphology using scanning electron microscopy showed grains in the A horizons to be rounded, dissolution pitted, and crusted with weathering products, whereas those in the Cr horizons tended to have more angular, fresher crystal faces. Halloysite tubes have formed on some laumontite grain surfaces. Although the low Si/Al ratio of laumontite indicates a high cation-exchange capacity, only 17 cmolc kg−1 of native cations were extracted by shaking in 1 M NaCl solution for 17 d. Fewer cations were displaced by Ba, Sr, Cs, and NH4. Apparently, diffusion of cations from the one-dimensional channels of laumontite is very slow. Laumontite does not contribute significantly to the cation-exchange capacity of these soils, which also contain smectite and vermiculite.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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