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Salt Mineralogy of Las Vegas Wash, Nevada

Morphology and Subsurface Evaporation

^a Dep. of Geoscience, Univ. of Nevada Las Vegas, 4505 Maryland Pkwy., Las Vegas, NV 89154
^b Nicholas School of the Environment and Earth Sciences, Duke Univ., Durham, NC 27708
^c Natural Resources Conservation Service, 5820 S. Pecos Rd. Bldg. A. Suite 400, Las Vegas NV 89120
^d Dep. of Geological Sciences, New Mexico State Univ., Las Cruces, NM 88003

View larger version (36K): [in a new window]   Fig. 1. Location of study site, Las Vegas Wash, eastern Las Vegas, Nevada.

View larger version (25K): [in a new window]   Fig. 2. Soil description of LVW1 indicating increased salt mineral concentration with depth from few snowballs near the surface to incipient Stage III pedogenic gypsum above the water table. A 2-cm-thick salt crust occurs at the surface.

View larger version (29K): [in a new window]   Fig. 3. Soil description of LVW2 indicating increased salt mineral concentration with depth. At the time of sampling all snowballs seemed to be gypsum, and only after SEM/EDS analyses were performed were the two Bz horizons identified.

View larger version (64K): [in a new window]   Fig. 4. Photograph of (left) snowball morphology in LVW1 (61–98 cm); (right) microsnowballs in LVW2 surface crust.

View larger version (14K): [in a new window]   Fig. 5. EDS results for (A) Mg-sulfate. X-ray diffraction analyses identified the Mg-sulfate phase as hexahydrite; (B) Na-Ca-sulfate. Crystal habits in SEM analyses indicate eugsterite; (C) Na-Mg-sulfate. X-ray diffraction analyses identified this phase as bloedite.

View larger version (185K): [in a new window]   Fig. 6. SEM images of (A) lenticular gypsum from LVW1 By2 horizon (61–98 cm); (B) tabular pseudo-hexagonal gypsum from LVW1, By3 horizon (98–140 cm); (C) lath gypsum from LVW1, By2 horizon (61–98 cm); (D) euhedral, tabular pseudo-hexagonal bloedite from LVW2 surface salt crust (0–1 cm); (E) euhedral bladed bloedite from LVW1 Azn horizon (0–2 cm); (F) twinned bladed bloedite from LVW1 Azn horizon (0–2 cm).

View larger version (188K): [in a new window]   Fig. 7. SEM images of (A) bloedite microsnowballs sealing the LVW2 surface salt crust (0–1 cm); (B) microsnowball of euhedral and subhedral tabular bloedite from LVW2 surface salt crust (0–1 cm); (C) euhedral tabular pseudo-hexagonal bloedite forming a series of microsnowballs and sealing LVW 2 surface salt crust (0–1 cm); (D) backscatter image of acicular eugsterite (on right) with planar bloedite base from LVW2 surface salt crust (0–1 cm); (E) acicular eugsterite from LVW1, Azn horizon (0–2 cm); (F) acicular eugsterite with tabular bloedite from LVW2 surface salt crust (0–1 cm).

View larger version (200K): [in a new window]   Fig. 8. SEM images of (A) subhedral and anhedral hexahydrite from LVW1, Azn horizon (0–2 cm); (B) backscatter image of skeletal (hopper) and anhedral halite from LVW2, 2Byzn3 horizon (62–110 cm); (C) subhedral hexahydrite from LVW2 2Byzn3 horizon (62–110 cm); (D) euhedral, skeletal (hopper), cubic halite from LVW1, Azn surface crust (0–2 cm); (E) dehydrated tabular pseudo-hexadonal hexahydrite from LVW2 2Byzn3 horizon (62–110 cm); (F) euhedral skeletal bloedite from LVW1, Azn surface crust (0–2 cm).