¹Z. W. Hu, ²R. P. Winarski
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12674]
¹XNano Sciences Inc., Huntsville, Alabama, USA
²Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, USA
Published by arrangement with John Wiley & Sons

Unlocking the 3-D structure and properties of intact chondritic porous interplanetary dust particles (IDPs) in nanoscale detail is challenging, which is also complicated by atmospheric entry heating, but is important for advancing our understanding of the formation and origins of IDPs and planetary bodies as well as dust and ice agglomeration in the outer protoplanetary disk. Here, we show that indigenous pores, pristine grains, and thermal alteration products throughout intact particles can be noninvasively visualized and distinguished morphologically and microstructurally in 3-D detail down to ~10 nm by exploiting phase contrast X-ray nanotomography. We have uncovered the surprisingly intricate, submicron, and nanoscale pore structures of a ~10-μm-long porous IDP, consisting of two types of voids that are interconnected in 3-D space. One is morphologically primitive and mostly submicron-sized intergranular voids that are ubiquitous; the other is morphologically advanced and well-defined intragranular nanoholes that run through the approximate centers of ~0.3 μm or lower submicron hollow grains. The distinct hollow grains exhibit complex 3-D morphologies but in 2-D projections resemble typical organic hollow globules observed by transmission electron microscopy. The particle, with its outer region characterized by rough vesicular structures due to thermal alteration, has turned out to be an inherently fragile and intricately submicron- and nanoporous aggregate of the sub-μm grains or grain clumps that are delicately bound together frequently with little grain-to-grain contact in 3-D space.

¹Benton C. Clark et al. (>10)*
American Mineralogist 101 (7) 1515-1526   Link to Article [DOI: 10.2138/am-2016-5575]
¹Space Science Institute, 4750 Walnut, Boulder, Colorado 80301, U.S.A.
*Find the extensive, full author and affiliation list on the publishers website
Copyright: The Mineralogical Society of America

In the search for evidence of past aqueous activity by the Mars Exploration Rover Opportunity, fracture-filling veins and rock coatings are prime candidates for exploration. At one location within a segment of remaining rim material surrounding Endeavour Crater, a set of “boxwork” fractures in an outcrop called Esperance are filled by a bright, hydrated, and highly siliceous (SiO2 ~ 66 wt%) material, which has overall a montmorillonite-like chemical composition. This material is partially covered by patches of a thin, dark coating that is sulfate-rich (SO3 ~ 21 wt%) but also contains significant levels of Si, Fe, Ca, and Mg. The simultaneous presence of abundant S, Si, and Fe indicates significant mineralogical complexity within the coating. This combination of vein and coating compositions is unlike previous analyses on Mars. Both materials are heterogeneously eroded, presumably by eolian abrasion. The evidence indicates at least two separate episodes of solute precipitation from aqueous fluids at this location, possibly widely separated in time. In addition to the implications for multiple episodes of alteration at the surface of the planet, aqueous chemical environments such as these would have been habitable at the time of their formation and are also favorable for preservation of organic material.

¹Jeffrey R. Johnson et al. (>10)*
American Mineralogist 101, 1501-1514 Link to Article [doi:10.2138/am-2016-5553]
¹Applied Physics Laboratory, Johns Hopkins University, 11101 Johns Hopkins Road 200-W230 Laurel, Maryland 20723-6005, U.S.A.
*Find the extensive, full author and affiliation list on the publishers website
Copyright: The Mineralogical Society of America

Relative reflectace point spectra (400–840 nm) were acquired by the Chemistry and Camera (ChemCam) instrument on the Mars Science Laboratory (MSL) rover Curiosity in passive mode (no laser) of drill tailings and broken rock fragments near the rover as it entered the lower reaches of Mt. Sharp and of landforms at distances of 2–8 km. Freshly disturbed surfaces are less subject to the spectral masking effects of dust, and revealed spectral features consistent with the presence of iron oxides and ferric sulfates. We present the first detection on Mars of a ~433 nm absorption band consistent with small abundances of ferric sulfates, corroborated by jarosite detections by the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument in the Mojave, Telegraph Peak, and Confidence Hills drilled samples. Disturbed materials near the Bonanza King region also exhibited strong 433 nm bands and negative near-infrared spectral slopes consistent with jarosite. ChemCam passive spectra of the Confidence Hills and Mojave drill tailings showed features suggestive of the crystalline hematite identified by CheMin analyses. The Windjana drill sample tailings exhibited flat, low relative reflectance spectra, explained by the occurrence of magnetite detected by CheMin. Passive spectra of Bonanza King were similar, suggesting the presence of spectrally dark and neutral minerals such as magnetite. Long-distance spectra of the “Hematite Ridge” feature (3–5 km from the rover) exhibited features consistent with crystalline hematite. The Bagnold dune field north of the Hematite Ridge area exhibited low relative reflectance and near-infrared features indicative of basaltic materials (olivine, pyroxene). Light-toned layers south of Hematite Ridge lacked distinct spectral features in the 400–840 nm region, and may represent portions of nearby clay minerals and sulfates mapped with orbital near-infrared observations. The presence of ferric sulfates such as jarosite in the drill tailings suggests a relatively acidic environment, likely associated with flow of iron-bearing fluids, associated oxidation, and/or hydrothermal leaching of sedimentary rocks. Combined with other remote sensing data sets, mineralogical constraints from ChemCam passive spectra will continue to play an important role in interpreting the mineralogy and composition of materials encountered as Curiosity traverses further south within the basal layers of the Mt. Sharp complex.

¹Kathleen C. Benison
American Mineralogist 101, 1499-1500 Link to Article [DOI: 10.2138/am-2016-5802]
¹Department of Geology and Geography, West Virginia University, Morgantown, West Virginia 26506, U.S.A.
Copyright: The Mineralogical Society of America

Identification of minerals on the surface of Mars is critical to understanding the geological history of our neighbor planet. In this issue of American Mineralogist, Ehlmann et al. report their discovery of alunite [KAl3(SO4)2(OH)6] in Cross Crater on Mars. Because terrestrial alunite forms from Al-rich acid sulfate waters, these results strongly suggest the past presence of Al-rich acid saline martian waters.

¹John F. Pernet-Fisher
American Mineralogist 101, 1497-1498 Link to Article [DOI: 10.2138/am-2016-5790]
¹School of Earth, Atmospheric, and Environmental Sciences, University of Manchester, Manchester M13 2PL, U.K.
Copyright: The Mineralogical Society of America

Earth-like δD values reported from lunar mare-basalt apatites have typically been interpreted to reflect the intrinsic isotopic composition of lunar-mantle water. New data indicates that some of these basalts are also characterized by having experienced a slow cooling history after their emplacement onto the lunar surface. This suggests that these basalts may have experienced metasomatism by fluxes generated during the degassing of the lunar regolith induced by the long-duration, high-temperature residence times of overlying basalts.