¹Min Tang,^1,2Anouk Ehreiser,¹Yi-Liang Li
¹Department of Earth Sciences, The University of Hong Kong, Pokfulam, Hong Kong
²Department of Physics and Astronomy, Heidelberg University, Postfach 10 57 60, 69047 Heidelberg, Germany

Gypsum is a mineral that commonly precipitates in hydrothermal environments. This study reports the electron microscopic analyses of gypsum morphologies and crystal sizes found in hot springs on the Kamchatka Peninsula, Russia, and compares these analyses with gypsum morphologies of hydrothermal genesis found in Lower Cambrian black shale. In sediments of the Kamchatka hot springs, we observed prismatic, prismatic pseudo-hexagonal, fibrous, tubular, lenticular and twinned gypsum crystals, with crystal sizes ranging from 200 μm. The coexistence of diverse crystal habits of gypsum implies a constant interaction between hot spring geochemistry and the metabolisms of the microbial community. The crystallization of Ca- and Ba-sulfates in the black shale of the Lower Cambrian, which shows similar but less varied morphology, was influenced by post-depositional hydrothermal fluids. The partial replacement of pyrite by sulfates in a situation coexisting with rich biomass deposits and animal fossils indicates limited modification of the sedimentary records by biological materials. If the gypsum precipitated on Mars underwent similar interactions between microbial communities and their geochemical environments, the resulting crystal habits could be preserved even better than those on Earth due to the weak geodynamics prevailing on Mars throughout its evolutionary history.

Reference
Tang M, Ehreiser A, Li, Y-L (2014) Gypsum in modern Kamchatka volcanic hot springs and the Lower Cambrian black shale: Applied to the microbial-mediated precipitation of sulfates on Mars. American Mineralogist 99, 2126-2137,
Link to Article: [doi:10.2138/am-2014-4754]

Copyright: The Mineralogical Society of America

¹Si Sun,¹Lung S. Chan,¹Yi-Liang Li

¹Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong

The biological cycling of phosphorus on Earth could be as early as the origin of life in early Archean. However, because of the low abundance and fine particle size, phosphate related to microbial ecophysiological activities in early sedimentary rocks, especially those deposited before the Great Oxidation Event (GOE, ca. 2.45–2.32 Ma), is still poorly addressed. It is not until recently that certain petrographic and mineralogical features of apatite in the early Precambrian sedimentary rocks were found related to microbial activities. In this study, we report high-resolution electron microscopic investigations on apatite from the Neoarchean to early Paleoproterozoic banded iron formations (BIFs), Mesoproterozoic to Lower Cambrian black shale and phosphorites, and Pliocene sediments. Apatite in BIFs occurs as 4–8 μm radial flowers with “petals” made of apatite rods. Their mineralogical and petrologic features are highly similar to those in the younger sedimentary rocks in which biomass have been confirmed to play an important role in the mineralization of phosphate. We suggest that these sedimentary rocks or sediments have experienced similar phosphogenetic processes mediated by biomass that led to the mineralization of phosphorus. The formation and preservation of phosphate (apatite) with conspicuous recognizable features in association with biological activities from Late Archean to Pliocene implies its universal significance in recording microbial processes through deep geological evolution. With mild dynamic processes, the martian (sub)surface has better preservation conditions than Earth, and the micro-structure of phosphate formed in environments mediated by microorganisms could be recognized by high-resolution observations on the surface of Mars or returned samples, if microbial life ever developed on Mars.

Reference
Sun S, Chan LS, Li Y-L (2014) Flower-like apatite recording microbial processes through deep geological time and its implication to the search for mineral records of life on Mars. American Mineralogist 99, 2116-2125
Link to Article [doi:10.2138/am-2014-4794]

Copyright: The Mineralogical Society of America

¹Janice L. Bishop,²Melissa D. Lane,¹M. Darby Dyar,¹Sara J. King, ¹Adrian J. Brown,⁴Gregg A. Swayze
¹SETI Institute, Carl Sagan Center, Mountain View, California 94043, U.S.A.
²Planetary Science Institute, 1700 E. Fort Lowell Road, Suite 106, Tucson, Arizona 85719, U.S.A.
³Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
⁴U.S. Geological Survey, Denver, Colorado 80225, U.S.A.

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 H2O. Anhydrite, bassanite, and gypsum all have SO4 combination and overtone features from 4.2–5 μm that are present in reflectance spectra. The mid-IR region spectra exhibit strong SO4 ν3 and ν4 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 ν1 and near 470–510 cm−1 (~20 μm) for ν2. The mid-IR H2O 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 ν4 bands begin to invert for the finer-grained samples, and the ν1 vibration occurs as a band instead of a peak and has the strongest intensity for the finer-grained samples. The ν2 vibration is a sharp band for anhydrite and a broad peak for gypsum. The band center of the ν1 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.

Reference
Bishop JL, Lane MD, Dyar MD, King SJ, Brown AJ, Swayze GA (2014) Spectral properties of Ca-sulfates: Gypsum, bassanite, and Anhydrite. American Mineralogist 99, 2105-2115
Link to Article [doi: 10.2138/am-2014-4756]

Copyright: The Mineralogical Society of America