Mineral maps based on Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used to study late Miocene advanced argillic alteration at Cuprite, Nevada. Distributions of Fe-bearing minerals, clays, micas, sulfates, and carbonates were mapped using the Tetracorder spectral-shape matching system. The Al-content of white micas increases toward altered areas and near intrusive rocks. Alunite composition varies from pure K to intimate mixtures of Na-K endmembers with subpixel occurrences of huangite, the Ca-analogue of alunite. Intimately mixed Na-K alunite marks areas of relatively lower alteration temperature, whereas co-occurring Na-alunite and dickite may delineate relict hydrothermal conduits. The presence of dickite, halloysite, and well-ordered kaolinite but absence of disordered kaolinite is consistent with acidic conditions during hydrothermal alteration. Partial lichen cover on opal spectrally mimics chalcedony, limiting its detection to lichen-free areas. Pods of buddingtonite are remnants of initial quartz-adularia-smectite alteration. Thus, spectral maps provide a synoptic view of the surface mineralogy, and define a previously unrecognized early steam-heated hydrothermal event. Faulting and episodes of hydrothermal alteration at Cuprite were intimately linked to upper plate movements above the Silver Peak-Lone Mountain detachment and growth, collapse, and resurgence of the nearby Stonewall Mountain volcanic complex between 8 and 5 Ma. Isotopic dating indicates that hydrothermal activity started at least by 7.61 Ma and ended by about 6.2 Ma. Spectral and stable isotope data suggests that Cuprite is a late Miocene lowsulfidation adulariasericite type hot spring deposit overprinted by late-stage steam-heated advanced argillic alteration formed along the margin of the Stonewall Mountain caldera.

Abstract--The highest frequency near-infrared (NIR) combination bands for specimens of four species of mica--montmorillonite-beidellite, illite, chlorite, and kaolinite--were correlated with respect to AI203 content. A direct linear correlation was found between the combination band positions and the AI203 contents of the montmorillonite-beidellite series, which may be given as: ~ cm -~ = (5.38 + 0.04) (%

Abstract-White saponite occurs in joints and open fracture zones in metamorphosed dolomitic limestone near Ballarat, California. The saponite appears to have formed by hydrothermal alteration, possibly duringPliocene times. The material shows a 06l X-ray powder diffraction peak at 1.529 ]k, A1203 and MgO contents of about 4.4 and 23.5%, respectively, and a half-cell octahedral atoms summation value of about 2.82. The saponite appears to consist of a single, uniform clay species; the main impurities are fine shards of diopside and tremolite. The infrared and thermal properties of the Ballarat saponite are similar to those of the Allt Ribhein saponite. It has a lower water-holding capacity than montmorillonite and is characterized by lower Atterberg limits and expansion pressures and higher compaction densities. The apparent density of the saponite, 2.865 g/cm 3, is greater than that of montmorillonite. This saponite is available from the Source Clays Repository of The Clay Minerals Society.

Abstract--There has been much interest in the rare specimen of beidellite from the Black Jack Mine, Florida Mountain, Idaho. A variety of aluminous clays exists along veins such as the Black Jack vein, in rhyolite and latite flows, and in near-surface ash beds, often containing less than 1.0% MgO and 0.5% Na20. Associated clays include beidellite, illite, kaolinite, 10-A halloysite, dickite, nacrite, rectorite and a tarasovite-like mineral. The predominant clay is mixed-layer illite-beidellite. The beidellites have A120 ~ contents ranging from about 28 to 33%, and predominantly Ca and K as interlayer cations. The typical beidellite dehydroxylation temperatures of about 595 ~ readily differentiate the beidellite from montmorillonite, which has a dehydroxylation temperature in the range of 735 ~ A modified differential thermal analysis (DTA) method is given for readily estimating the interlayer cation populations of smectites, including Mg ++ and A1 § § § cations. Chemical analyses and layer charges of 11 beidellites from mines around the Black Jack Mine are given. The beidellites have an American Society for Testing and Materials (ASTM) classification of CH, qb value, internal friction angle of about 8 ~ and an expansion pressure of about 9 kgf/cm 2 (88.3 kPa), similar to that of nontronite.