New Mexico Mineral Symposium — Abstracts

Many varieties of evaporite sulfate and halide minerals occur in the Tularosa Basin, and other sulfates occur in the surrounding mountains. Gypsum is by far the most common and exhibits many forms, including standard euhedral monoclinic tabular clear to milky selenite crystals, swallowtail and fishtail twins. These are most common in the shallow subsurface near Lake Lucero and Alkali Flat. The largest gypsum crystals, as much as 60 cm wide, are amber-colored plates with a "habit...of a negative hemi-bypyramid and a basal pinacoid, the latter showing a vicinal formation...The crystals are tabular and have their greatest dimension along the b crystallographic axis" (Needham 1932). Sand and silt-sized gypsum takes this same disk-shaped lenticule form and makes up the bulk of gypsum in Pleistocene Lake Otero beds and in wind-blown "White Sand" dunes. This growth habit has been replicated in laboratory conditions in brines with a clay medium and organic acids (Cody 1991; Cody and Cody 1988). Both natural and lab-grown crystals fluoresce bright yellow due to included organic matter.

Other forms of gypsum include "sand crystals," in which sand grains are cemented together by euhedral or subhedral gypsum crystals, and "croute de nappe"—tabular plates of subhedral gypsum crystals that grew subvertically upward and cemented overlying sediment. Crystalline gypsum root casts 5-10 mm wide and as much as 30cm long mimic the meandering paths of roots. Satin-spar gypsum is found locally in fissures in soils. Ramshorn gypsum is found in a cave in the northern Tularosa Basin. In archaeological sites in the gypsum dunes, clumps of gypsum surrounding fire pits show that the heat turned gypsum into Plaster of Paris (bassanite), and rehydration cemented the masses together. Gypsum layers in Pleistocene Lake Otero beds exhibit crinkles from algal mats and disturbances from megafauna tracks such as camels and mammoths. The lake beds also contain many microfossils, including fish bones and scales.

Accumulations of gypsum crystals besides obvious sand dunes and lake beds include 3-m-high "mounds" of breccia of uncolored gypsum cleavage fragments with euhedral amber plates in them. These mounds are exposed where lake beds are deflated and are aligned in several directions. Scholars disagree as to how they formed, so research and arguments continue.

Finer-grained accumulations of gypsum include spring mounds, precipitated gypsum levees along streams, marshes, platform marshes, and megamounds. Gypsum springs in the northern Tularosa Basin create mounds of rock-hard gypsum as much as 6 m high and 200 m wide, with spring-orifice craters at their tops as much as 15 m wide. These mounds take thousands of years to build. Gypsum levees as much as 1.5 m high have precipitated along the meandering discharge channel from Malpais Spring. Adjacent to that channel is a marsh that has precipitated gypsum and raised itself as much as 3 m above the surrounding playa-like deflation basins and covers about 1 km'. Larger late Pleistocene–Holocene gypsum-depositing marshes (sediments 1.5-2 m thick) are evident over tens of square kilometers and include fossil aquatic gastropods, ostracodes, and fish. Megamounds are even larger accumulations of gypsum that cover tens of square kilometers and are thick enough to have developed sinkholes and caves.

Other evaporite minerals reported from the Tularosa Basin include halite, anhydrite, bassanite, mirabalite, hexahydrite, thenardite, bloedite, and possibly trona and bischofite. Other evaporite minerals are expected to be found in this complex basin. Anhydrite is most common in the subsurface as part of the Permian Yeso Formation near Carrizozo.

Acknowledgments
We thank Virgil Lueth, Paul Hlava, Ralph Tissot, Anna Szynkiewicz, David Bustos, and David Anderson for help with mineral identification, crystal morphology, and photography of landscapes and specimens.

References:

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