NASA Astrobiology Institute (NAI)

1. Nano-structured minerals as tracers of microbial activities

   Project Investigators: Huifang Xu

   Other Project Members

   Emily Freeman (Masters Student)

   Fangfu Zhang (Doctoral Student)

   Hiromi Konishi (Research Staff)

   Jason Huberty (Undergraduate Student)

   Tianhu Chen (Collaborator)

   Summary

   Biologically produced mineral formation at the nanometer scale often produces unique morphologies and compositions relative to abiologic imineral formation. For example, calcite nano-fibers in arid and semi-arid soils are demonstrably products of microbial activities, where bioorganics derived from microorganism have controlled growth of nano-fibrous calcite. Other examples of bioorganic control on nano minerals include ttitanium-free magnetite / maghemite nano-fibers that are closely associated with Fe-bearing smectite in a weathered basaltic glass, and it is possible that these minerals were produced by bacterial dissimilatory iron reduction of ferrihydrite nano-crystals through topotactic transformation to magnetite. Both calcite nano-fibers and Ti-free maghemite nano-crystals are likely biosignatures in dry soils and weathered basalt tuff deposits, raising the possibility that such features may be found in sedimentary rocks on other planetary bodies.

   Astrobiology Roadmap Objectives:

   • Objective 7.1: Biosignatures to be sought in Solar System materials

   Project Progress

   We have studied the nano-structured calcite in modern and ancient soils (paleosols) and compared them to minerals synthesized using controlled organic components. Both modern soils and paleosols of eolian sediments (loess) that formed in arid and semi-arid environments will provide useful information about possible bacterial activities on Mars. Results from both SEM and TEM investigations show that nano-fibrous calcite in semi-arid soils of loess sediments is common. These calcite nano-fibers have unusual elogation directions in well crystallized single crystals that have smooth edges. Such kinds of calcite cannot be produced in a purely inorganic system. We suggest that bioorganics derived from microorganism controlled the growth of the nano-fibrous calcite in dry soil environment.

   We also investigated a basalt vent deposit from Snake River Plain of Idaho using X-ray diffraction and transmission electron microscopy. The layered pyroclastic and volcaniclastic vent materials (basaltic glass) was produced by phreatomagmatic eruptions in the late Miocene to early Pliocene. The maghemite nano-crystals in the samples are closely associated with Fe-bearing smectite and a zeolite of phillipsite. TEM images also show maghemite nano-fibers (arrays of nano-crystals) of maghemite with an average diameter of ~ 6 nm, in close association with smectite. The Ti-free maghemite nano-crystals and nano-fibers are interpreted to have transformed from precursor magnetite nano-crystals through topotactic oxidation. The precursor magnetite nano-crystals were products of dissimilatory iron reduction of ferrihydrite nano-crystals through topotactic transformation under anaerobic conditions. Magnetite / maghemite nano-fibers are interpreted to have formed by microbial activity through oriented attachment of magnetite nano-crystals, providing evidence that such features may serve as a biosignature.

Publications

Freeman, E.  (2008).  Characteristics and formation mechanism of nano-phase minerals in a vent deposit from the Snake River Plain: potential biosignatures of maghemite nano-crystals and nano-fibers.  Geology and Geophysics (pp. 212).  Madison: University of Wisconsin.