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Ferric saponite and serpentine in the nakhlite martian meteorites

DOI: 10.1016/j.gca.2014.04.010 DOI Help

Authors: Leon Hicks (University of Leicester) , John Bridges (University of Leicester) , Steve Gurman (University of Leicester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geochimica Et Cosmochimica Acta , VOL 136 , PAGES 194 - 210

State: Published (Approved)
Published: April 2014
Diamond Proposal Number(s): 9418 , 8307

Open Access Open Access

Abstract: Transmission electron microscopy and Fe-K X-ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites, NWA 998, Lafayette, Nakhla, GV, Y 000593, Y 000749, MIL 03346, NWA 817, and NWA 5790. The secondary phases are a rapidly cooled, metastable assemblage that has preserved Mg# and Ca fractionation related to distance from the fluid source, for most of the nakhlites, though one, NWA 5790, appears not to have experienced a fluid pathway. All nine nakhlite samples have also been analysed with scanning electron microscopy, electron probe micro analysis, Bright Field high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction. By measuring the energy position of the Fe-K XANES 1s !3d pre-edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites. The crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable Fe3+/RFe values ranging from 0.4 to 0.9. In Lafayette, the central silicate gel parts of the veins are more ferric than the phyllosilicates around it, showing that the fluid became increasingly oxidised. The mesostasis of Lafayette and NWA 817 also have phyllosilicate, which have a higher ferric content than the olivine fracture deposits, with Fe3+/RFe values of up to 1.0. Further study, via TEM analyses, reveal the Lafayette and NWA 817 olivine phyllosilicates to have 2:1 T–O–T lattice structure with a the d001-spacing of 0.96 nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T–O structure with d001-spacings of 0.7 nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca0.2K0.1)P0.3(Mg2.6Fe2+ 1.3Fe3+ 1.7Mn0.1)P5.7[(Si6.7AlIV 0.9Fe3+ 0.4)P8.0O20](OH)4nH2O, and that found in the mesostasis fractures is an Fe-serpentine (Ca0.1Mg0.7Fe3+ 1.0AlVI 0.4)P2.2[Si2O5]OH4, with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.

Journal Keywords: Meteorites; nakhlite martian

Subject Areas: Environment, Materials, Physics

Instruments: I18-Microfocus Spectroscopy

Added On: 15/07/2014 13:40


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