Hydrothermal Precipitation of Sanidine (Adularia) Having Full Al,Si Structural Disorder and Specular Hematite at Maunakea Volcano (Hawai'i) and at Gale Crater (Mars)

Abstract

Hydrothermal high sanidine and specular hematite are found within ferric‐rich and gray‐colored cemented basaltic breccia occurring within horizontal, weathering‐resistant strata exposed in an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The cone was extensively altered by hydrothermal, acid‐sulfate fluids at temperatures up to ~400 °C, and, within strata, plagioclase was removed by dissolution from progenitor Hawaiitic basalt, and sanidine and hematite were precipitated. Fe2O3T concentration and Fe3+/∑Fe redox state are ~12 wt.% and ~0.4 for progenitor basalt and 46–60 wt.% and ~1.0 for cemented breccias, respectively, implying open‐system alteration and oxic precipitation. Hydrothermal high sanidine (adularia) is characterized by full Al,Si structural disorder and monoclinic unit‐cell (Rietveld refinement): a = 8.563(19) Å, b = 13.040(6) Å, c = 7.169(4) Å, β = 116.02(10)°, and V = 719.4(19) Å3. Hematite (structure confirmed by Rietveld refinement) is the predominant Fe‐bearing phase detected. Coarse size fractions of powdered hematite‐rich breccia (500–1000 μm) are dark and spectrally neutral at visible wavelengths, confirming specular hematite, and SEM images show platy to polyhedral hematite morphologies with longest dimensions >10 μm. Smectite and 10‐Å phyllosilicate, both chemically dominated by Mg as octahedral cation, are additional diagenetic hydrothermal alteration products. By analogy and as a working hypothesis, high sanidine (Kimberly formation) and specular hematite (Mt. Sharp group at Hartmann's Valley and Vera Rubin ridge) at Gale crater are interpreted as diagenetic alteration products of Martian basaltic material by hydrothermal processes.

Mineralogy of Vera Rubin Ridge From the Mars Science Laboratory CheMin Instrument

“…A decrease in the a cell parameter correlates to an increase in Al, whereas a decrease in the c cell parameter correlates to an increase in OH (Stanjek & Schwertmann, 1992). The c parameters of the hematite in all four samples are relatively similar to one another and are consistent with the c parameters of unsubstituted natural and synthetic hematite (Morris et al, 2020). The a cell parameter of the hematite in Stoer is especially small compared to hematite detected in other samples in Gale crater (Morris et al, 2020), which could result from Al substitution in the structure.…”

“…Grains can be composed of one or many crystallites. Gray hematite (also called specular or microplaty hematite; Lane et al, 2002) is typically coarser‐grained than red hematite, where hematite with grain sizes %3C ~5 μm appear red to the human eye and coarser hematite grains > ~5 μm appear black or gray (e.g., Catling & Moore, 2003; Morris et al, 2020). Large particles made up of aggregates of ~10–200 nm hematite crystallites can appear gray or black (Egglseder et al, 2019; Madden et al, 2010).…”

Formation of Tridymite and Evidence for a Hydrothermal History at Gale Crater, Mars

“…Detections of gray hematite from recent samples along the Vera Rubin Ridge are consistent with hydrothermal alteration at 100-200°C [Rampe et al, 2020;Morris et al, 2020].…”

Synergistic Ground and Orbital Observations of Iron Oxides on Mt. Sharp and Vera Rubin Ridge

“…Given the evidence of hematite grain size variations on VRR by the presence of both red and gray hematite in the bedrock, the simplest explanation for the deep spectral absorptions in bedrock in VRR is that they are also related to grain size variations. Laboratory investigations have shown the depths of hematite absorption features are strongly dependent on grain size, and these absorptions increase and then decrease as grain sizes increase (e.g., Bell et al, 1990; Johnson et al, 2019; Morris et al, 1989; Morris et al, 2020). Alternative interpretations, however, cannot be ruled out.…”

“…The preceding paragraphs pertain to spectral observations of hematite‐bearing geologic materials where the oxide is distributed throughout (at least in part) as small particles of pigmentary (i.e., red) hematite. For sufficiently large hematite particles (specular hematite), the ferric absorption edge is weak or not present (Lane et al, 2002; Morris et al, 2020), meaning it is approximately spectrally neutral over visible wavelengths, imparting a black to gray color when mixed with other spectrally neutral but higher albedo materials. When present, specular hematite will reduce the contrast of spectral features from other phases compared to how they would appear if the oxide were absent as shown, for example, by Singer (1981) for mixtures of olivine and magnetite and Clark and Lucey (1984) for mixtures of ice and charcoal where magnetite and charcoal are spectral surrogates for specular hematite.…”