Calico Hills Formation Research Articles

Determination of heat capacity of Yucca Mountain stratigraphic layers

The heat generated from the radioactive waste to be placed in the proposed geologic repository at Yucca Mountain, Nevada, will affect the thermal-hydrology of the stratigraphic layers. In order to assess the effect of the movement of repository heat into the fractured rocks accurate determination of thermodynamic and hydraulic properties is important. Specific heat capacity is one of the properties that are required to evaluate energy storage in the fractured rock. Rock-grain specific heat capacity, the subject of this study, is the specific heat capacity of the solid part of the rock. Yucca Mountain consists of alternating lithostratigraphic units of welded and non-welded ash-flow tuff, mainly rhyolitic in composition and displaying varying degrees of vitrification and alteration. Different methods exist that can be used to evaluate specific heat capacity of the stratigraphic layers. In this study, the mineral summation method, which is an addition of the weighted specific heat capacity of each mineral found in a specific layer, has been used based on Kopp's rule. The method utilized a mineralogic map of the rocks at the repository site. The Calico Hills formation and adjacent bedded tuff layers display a bimodal mineral distribution of vitric and zeolitic zones with differing mineralogies. Based on this bimodal distribution in zeolite abundance, the boundary between the vitric and zeolitic zones was selected to be 15% zeolitic abundance. Specific values have been calculated for these layers both as “layer average” and “zone average”. The specific heat capacity determination method presented in this manuscript did not account for spatial variability in the horizontal direction within each layer.

Water mobility in Calico Hills tuff measured by quasielastic neutron scattering

Quasielastic neutron scattering (QNS) was used to measure the mobility of water in tuff at low saturation. The tuff sample is from a surface outcrop of the Calico Hills formation, which lies below the proposed location for a high‐level nuclear waste repository, at Yucca Mountain, NV. The sample is a devitrified tuff composed mainly of zeolites and having clinoptilolite (approximately 70%), mordenite, and opal‐CT as major constituents. The sample is an air dry, intact core sample. QNS measurements were made in the range 200 K to 423 K. Rotational motion of adsorbed water is easily observable and translational motion is observable at higher temperatures. The tuff data are analyzed to determine the rotational diffusion coefficient and the translational diffusion coefficient. The results yield a translational diffusion coefficient in the tuff that is significantly lower than that of bulk water but higher than for water in montmorillonite or vermiculite clays.

Low-Temperature Alteration in Tuffs from Yucca Mountain, Nevada

AbstractThe structure, chemistry and distribution of hydrothermal alteration and weathering products of feldspars and glass in 3 samples of Yucca Mountain tuffs (GSW G4 borehole at a depth of 1531 ft (464 m) and USW GU3 borehole at 1406 ft (426 m) from the Calico Hills Formation and USW G4 at a depth of 272 ft (82.4 m) from the Topobah Springs Member) were examined by high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM). Alteration products are of interest because they may influence the form and distribution of contaminants released from the proposed high-level nuclear waste repository. Samples from the Calico Hills Formation contain alkali-bearing aluminosilicate glass and its alteration products. Zeolites appear to have formed from compositionally similar glass by recrystallization, probably under hydrothermal conditions. Crystals are fibrous and frequently no more than a few tens of nanometers in diameter. Porous aggregates of few-nanometer-diameter, poorly crystalline silica spheres (probably opal C-T) develop adjacent to corroded glass surfaces and zeolite crystals. Finely crystalline Fe-rich smectites coat etched glass surfaces, zeolites and feldspar crystals and occur within opal-like silica aggregates. Microstructures in the clay-dominated coatings and details of smectite-glass interfaces suggest that clays grow in orientations controlled by heterogeneously retreating surfaces and from constituents released at associated glass dissolution sites. The alteration assemblage also includes finely crystalline hematite, goethite, Mn-oxide films and illite formed by alteration of muscovite. The zeolitized sample contains abundant opal-like silica whereas glass in the unzeolitized sample is weathered to smectite-like clays. These differences may be attributed to hydrological and consequent geochemical factors resulting from the higher porosity of zeolitized samples. Exsolved alkali feldspar, which occurs as micron-sized crystals in the Calico Hills Formation and as phenocrysts and in the groundmass of the devitrified Topobah Springs Member, are almost unaltered. Feldspar alteration is confined to cracks and grain boundaries, where minor, poorly crystalline, Fe-bearing aluminosilicate alteration products are developed. In these tuffs, most of the porosity, permeability, high surface area and capacity to affect solution chemistry are associated with products of glass alteration.