¹Chi Ma,²Alexander N. Krot,²Kazuhide Nagashima,³Tasha Dunn
The American Mineralogist 109, 2006-2012 Link to Article [https://doi.org/10.2138/am-2023-9283]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, U.S.A.
³Department of Geology, Colby College, Waterville, Maine 04901, U.S.A.
Copyright: The Mineralogical Society of America

Louisfuchsite (IMA 2022-024), with an end-member formula Ca₂(Mg₄Ti₂)(Al₄Si₂)O₂₀, is a new refractory mineral identified in a Ca-Al-rich inclusion (CAI) from the NWA 4964 CK3.8 carbonaceous chondrite. Louisfuchsite occurs with spinel, perovskite, grossmanite, plus secondary rutile, titanite, and ilmenite in three regions in the CAI. The mean chemical composition of type louisfuchsite by electron probe microanalysis is (wt%) Al₂O₃ 25.48, SiO₂ 18.40, MgO 17.92, TiO₂ 15.36, Ti₂O₃ 3.13, CaO 14.92, FeO 3.30, V₂O₃ 0.67, Cr₂O₃ 0.08, total 99.26, giving rise to an empirical formula of Ca_2.00(Mg_3.44Ti1.494+Fe_0.36Ti0.343+Al_0.24V0.073+Ca_0.06Cr_0.01)_Σ6.01(Al_3.63Si_2.37)_Σ6.00O₂₀. Louisfuchsite has the P1 rhönite structure with a = 10.37(1) Å, b = 10.76(1) Å, c = 8.90(1) Å, α = 106.0(1)°, β = 96.0(1)°, γ = 124.7(1)°, V = 741(2) ų, and Z = 2, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.44 g/cm³. Louisfuchsite is a new refractory phase from the solar nebula, crystallized from an ¹⁶O-rich (Δ¹⁷O ~ −24 ± 2‰) refractory melt with the initial ²⁶Al/²⁷Al ratio of (5.09 ± 0.58) × 10⁻⁵ under reduced conditions. The mineral name is in honor of Louis Fuchs (1915−1991), a mineralogist at Argonne National Laboratory, for his many contributions to mineralogical research on meteorites.

¹Janice L. Bishop,²Peter Schiffman,³Enver Murad,⁴Randal J. Southard,¹Lukas Gruendler,^5,6M. Darby Dyar,⁷Melissa D. Lane
American Mineralogist 109, 1871-1887 Open Access Link to Article [https://doi.org/10.2138/am-2023-9153]
¹SETI Institute, Mountain View, California 94043, U.S.A.
²Department of Geology, University of California, Davis, California 95616, U.S.A.
³Bavarian Geologic Survey, Marktredwitz, Germany
⁴Department of Land, Air and Water Resources, University of California, Davis, California 95616, U.S.A.
⁵Planetary Science Institute, Tucson, Arizona 85719, U.S.A.
⁶Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
⁷Fibernetics, Lititz, Pennsylvania 17543, U.S.A.
Copyright The Mineralogical Society of America

Solfataric alteration at the South Sulfur Bank of the former Kilauea caldera produced opal, Mg- and Fe-rich smectites, gypsum, and jarosite through silica replacement of pyroclastic Keanakako’i ash and leaching of basaltic lavas. This site on the island of Hawaii serves as an analog for formation of several minerals found in altered deposits on Mars. Two distinct alteration environments were characterized in this study, including a light-toned, high-silica, friable outcrop adjacent to the vents and a bedded outcrop containing alternating orange/tan layers composed of smectite, gypsum, jarosite, hydrated silica, and poorly crystalline ferric oxide phases. This banded unit likely represents the deposition of pyroclastic material with variations in chemistry over time that was subsequently altered via moderate hydrothermal and pedogenic processes and leaching of basaltic caprock to enhance the Si, Al, Mg, Fe, and Ca in the altered layers. In the light-toned, friable materials closest to the vents along the base of the outcrop, glassy fragments were extensively altered to opal-A plus anatase.

Lab measurements of samples returned from the field were conducted to replicate recent instruments at Mars and provide further characterization of the samples. These include elemental analyses, sample texture, XRD, SEM, VNIR/mid-IR reflectance spectroscopy, TIR emittance spectroscopy, and Mössbauer spectroscopy. Variations in the chemistry and mineralogy of these samples are consistent with alteration through hydrothermal processes as well as brines that may have formed through rain interacting with sulfuric fumes. Silica is present in all altered samples, and the friable pyroclastic ash material with the strongest alteration contains up to 80 wt% SiO₂.

Sulfate mineralization occurred at the South Sulfur Bank through fumarolic action from vents and likely included solfataric alteration from sulfuric gases and steam, as well as oxidation of sulfides in the basaltic caprock. Gypsum and jarosite are typically present in different layers of the altered wall, likely because they require different cations and pH regimes. The presence of both jarosite and gypsum in some samples implies high-sulfate concentrations and the availability of both Ca²⁺ and Fe³⁺ cations in a brine percolating through the altered ash. Pedogenic conditions are more consistent with the observed Mg-smectites and gypsum in the tan layers, while jarosite and nontronite likely formed under more acidic conditions in the darker orange layers. Assemblages of smectite, Ca-sulfates, and jarosite similar to the banded orange/tan unit in our study are observed on Mars at Gale crater, Noctis Labyrinthus, and Mawrth Vallis, while high-silica outcrops have been identified in parts of Gusev crater, Gale crater, and Nili Patera on Mars.