Spectral properties of Ca-sulfates: Gypsum, bassanite, and anhydrite

Abstract

This study of the spectral properties of Ca-sulfates was initiated to support remote detection of these minerals on Mars. Gypsum, bassanite, and anhydrite are the currently known forms of Ca-sulfates. They are typically found in sedimentary evaporites on Earth, but can also form via reaction of acidic fluids associated with volcanic activity. Reflectance, emission, transmittance, and Raman spectra are discussed here for various sample forms. Gypsum and bassanite spectra exhibit characteristic and distinct triplet bands near 1.4-1.5 μm, a strong band near 1.93-1.94 μm, and multiple features near 2.1-2.3 μm attributed to H 2 O. Anhydrite, bassanite, and gypsum all have SO 4 combination and overtone features from 4.2-5 μm that are present in reflectance spectra. The mid-IR region spectra exhibit strong SO 4 ν 3 and ν 4 vibrational bands near 1150-1200 and 600-680 cm -1 (~8.5 and 16 μm), respectively. Additional weaker features are observed near 1005-1015 cm -1 (~10 μm) for ν 1 and near 470-510 cm -1 (~20 μm) for ν 2 . The mid-IR H 2 O bending vibration occurs near 1623-1630 cm -1 (~6.2 μm). The visible/near-infrared region spectra are brighter for the finer-grained samples. In reflectance and emission spectra of the mid-IR region the ν 4 bands begin to invert for the finer-grained samples, and the ν 1 vibration occurs as a band instead of a peak and has the strongest intensity for the finer-grained samples. The ν 2 vibration is a sharp band for anhydrite and a broad peak for gypsum. The band center of the ν 1 vibration follows a trend of decreasing frequency (increasing wavelength) with increasing hydration of the sample in the transmittance, Raman, and reflectance spectra. Anhydrite forms at elevated temperatures compared to gypsum, and at lower temperature, salt concentration, and pH than bassanite. The relative humidity controls whether bassanite or gypsum is stable. Thus, distinguishing among gypsum, bassanite, and anhydrite via remote sensing can provide constraints on the geochemical environment.

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: II. Optical modeling

“…A similar result was found for CaSO 4 . Comparing the spectrum of the anhydrous mineral to the spectra of two hydrated forms (CaSO 4 •0.5H 2 O, bassanite and CaSO 4 •2H 2 O, gypsum), Bishop et al ., [] assigned the absorption features of the hydrated minerals in the region 1.3–2.5 μm to combinations and overtones of the fundamental vibration bands of H 2 O. For water vapor, the fundamental bands are located at 2.66 μm (asymmetric stretch), 2.73 μm (symmetric stretch), and 6.27 μm (bending) [ Eisenberg and Kauzmann , , Table 1.3].…”

Linkages between mineralogy, fluid chemistry, and microbial communities within hydrothermal chimneys from the Endeavour Segment, Juan de Fuca Ridge

“…The broader bands near 4.5 and 4.7 µm in the spectrum of Dante are consistent with a mixture of anhydrite and gypsum. A H 2 O stretch plus bend combination band near 1.94 µm in the spectra of both Dante and Hot Harold samples indicates that both samples contained H 2 O in the mineral structure [note that the anhydrite laboratory spectrum also contains this band but that is not due to anhydrite, Bishop et al ., ]. Due to the presence of this H 2 O band, Hot Harold likely contained some gypsum as well as anhydrite, although anhydrite is spectrally dominant.…”

“…(a) VNIR spectra of Bastille‐1, Bastille‐2, orthopyroxene DL064 [ Klima et al ., ], zinnwaldite mica JB729 [ Bishop et al ., ], JB205, a chlorophyll‐bearing bacterial mat from beneath Lake Hoare in the Antarctic Dry Valleys [ Bishop et al ., ], chalcopyrite HS431 [ Clark et al ., ], and pyrite GDS483 [ Clark et al ., ]. (b) VNIR spectra of Hot Harold, Dante, anhydrite GDS42 [ Clark et al ., ], gypsum JB567 [ Bishop et al ., ], chlorophyll‐bearing mat JB205, sphalerite HS136 [ Clark et al ., ], chalcopyrite HS431, and pyrite GDS483.…”

Petrography and mineralgy of gachsaran formation in west of Bandar-e-Abbas, Kuh-e-Namaki Khamir section, south of Iran

“…Bassanite mineral has been found in some sulfates sampels of Gachsaran formation. Bassanite is a metastable mineral where gypsum and anhydrite are in transition [15]. Intense solar heating in arid regions can dehydrate gypsum to form bassaniteanhydrite at the surface [2].…”