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Ferric saponite and serpentine in the nakhlite martian meteorites

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journal contribution
posted on 2014-07-18, 15:35 authored by Leon J. Hicks, John C. Bridges, S. J. Gurman
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 Fe[superscript 3+]/ΣFe 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 Fe[superscript 3+]/ΣFe 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 d[subscript 001]-spacing of 0.96 nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T–O structure with d[subscript 001]-spacings of 0.7 nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca[subscript 0.2]K[subscript 0.1])[subscript ∑0.3](Mg[subscript 2.6]Fe[superscript 2+, subscript 1.3]Fe[superscript 3+, subscript 1.7]Mn[subscript 0.1])[subscript ∑5.7][(Si[subscript 6.7] Al[superscript IV, subscript 0.9] Fe[superscript 3+, subscript 0.4])[subscript ∑8.0]O[subscript 20]](OH)[subscript 4]·nH[subscript 2]O, and that found in the mesostasis fractures is an Fe-serpentine (Ca[subscript 0.1]Mg[subscript 0.7]Fe[superscript 3+, subscript 1.0]Al[superscript VI, subscript 0.4])[subscript ∑2.2][Si[subscript 2]O[subscript 5]]OH[subscript 4], with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.



Geochimica et Cosmochimica Acta, 2014, 136, pp. 194-210

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