New experimental results are reported on the frost weathering of olivine. After first weathering, a decrease in Fe sup 2(+)M(2) absorption bands were noted. This decrease is related to the protonation of O(+) in the mineral. It is contented that this reaction may result in the regolith storage of 100 to 1000 m of H(sub 2) over the history of Mars

Harris, S. L.

Huguenin, R. L.

English

NASA Technical Reports Server

p N89 - 10793
i' ] " _ / ,-/
CHEMICAL FROST WEATHERING OF OLIVINE: EXPER-
IMENTAL STUDY AND IMPLICATIONS; S.L. Harris, R.L. Huguenin,
Geology/Geography, University of Massachusetts, Amherst, MA 01003.
It has been proposed that chemical frost weathering of silicates may
occur in the contemporary Mars surface environment (1,2) Exposure of
olivine and other mafic silicates to Hz O at frost temperatures was proposed
to result in the incorporation of H + into the crystal, converting oxide ions
to hydroxide ions. Incorporation of electrons (derived from OH- in the
frost), at positive defects (e.g. Fe 3+ substituted for Fe z+) was proposed
to restore charge balance and convert OH- to HzOz. Production of HzOz
was observed experimentally (2,3).
A new experiment has been performed to test the proposal that H +
reacts with 0 2- to form OH- during the frost weathering of olivine. The
olivine used is the same as that in (2) and has composition Fo86. The olivine
was placed in an atmosphere and temperature controlled chamber. The
sample chamber pressure was maintained at _ 760 torr Nz, except during
initial frost formation and desorption. Frost was formed on the olivine by
cooling the sample below -10 ° C with a flow of HzO saturated N2 gas into
the sample chamber. Frost desorption was accomplished by cycling the
temperature below 0 ° C with a cold trap in the system and the vacuum
pump running. Reflectance spectra of the olivine samples were taken before,
during, and after frost formation. Band positions were extracted from
spectra using the Bands Data Analysis System described by (4).
The penetration of H + into the crystal structure during weathering
should be observable from spectra. The H + should react with 0 2- in the
crystal to produce structural hydroxyl. The structural hydroxyl absorption
bands would occur at wavenumber positions that are close to those of ab-
sorbed HzO and surface hydroxyl ions, thus they would not be uniquely
indicative of structural hydroxyl found by penetration of H + into the lat-
tice. With the H + that has been proposed to migrate to cation vacancies
(3), a more diagnostic spectral signature is a shift in the Fe z+ absorption
band positions arising from a change in the crystal field splitting parameter
Dq. Dq is defined as Dq _ Q/R 5 where Q is the charge on the ligands and
R is the metal-ligand interatomic distance (5). Since Dq is proportional to
Q, a change in Q from 2 (O z-) to 1 (OH-) should produce a decrease in
Dq.
https://ntrs.nasa.gov/search.jsp?R=19890001422 2020-03-20T06:07:10+00:00Z
48
CHEMICAL FROST WEATHERING
Harris, S. L.
Huguenin (3) found that approximately 9 x 1019 H + were incorporated
into a 1 gm sample after exposure to frost. The number of Mg 2+ + Fe 2+
cations in a 1 gm sample of olivine is approximately 4.8 x 1021. If the H +
were incorporated at or adjacent to the cation sites, then there would be a
predicted 1.9 % decrease in Dq.
Absorption bands in the olivine spectra are due to electronic transi-
tions and charge transfers by the d shell electrons in Fe 2+. The wavelength
position of the band centers depend on the charge of the ligands and on
the dimensions and symmetry of the Fe 2+ site. In a perfect octahedral
site the five d-shell orbitals are split into a triply degenerate, t2g, set and a
doubly degenerate, eg, set due to the presence of the surrounding six 0 2-
ligands. The d:___, and d_ orbitals, which point directly to coordinating
ligands, are raised in energy to the e_ set. The d_u , d:z and d w orbitals
lie between the ligands and are lowered in energy to the t_ set. Fe z+ has
six d electrons, five of which have parallel spin directions in the ground
state. The sixth electron occupies the lowest energy level with its spin in
the opposite direction. Absorption occurs when the sixth electron is moved
from the ground state energy level to one of the higher energy levels. The
energy separation between the t2g and eg orbitals is termed the crystal field
splitting and, it is denoted by Dq or Ao (5,6).
The crystal structure of olivine contains 2 six-coordinated sites; a cen-
trosymmetric M(1) site and a non-centrosymmetric M(2) site. There is no
site preference so that Mg 2+ and Fe 2+ are randomly distributed between
the sites (6,7). Thus, there are contributions from Fe z+ in both M(1) and
M(2) sites in absorption spectra. Burns (6,7) has assigned the absorption
bands occurring around 11,700 and 9,000 cm -1 to the M(1) site and the
absorption band at _ 9540 cm -1 to the M(2) site.
The experimental results showed an average decrease of 400crn-1 in the
olivine M(2) site band position upon frost weathering. Shifts in M(1) site
band position were not apparent, although a small shift may have escaped
detection. Burns (6) gives a Dq value of 8330cm -1 for Fe 2+ in the M(2)
site of an Fo88 olivine. The observed shift in the M(2) site band position
would correspond to a shift in Dq of ,_ 4.8 %. This is somewhat larger than
the predicted shift, but it generally supports the proposed model.
The observed shift in band position has two implications. First, it
supports the proposed frost weathering model and its occurrence on Mars.
It was predicted that 10o - 103 meters of H20 may have been lost to weath-
CHEMICAL FROST WEATHERING
Harris, S. L. 49
ering over the planets history, and the results here support that prediction.
Second, it suggests that frost-weathered olivine may have absorption band
positions that are lower in energy than the
Fe2+
Fe 2+ + Mg 2+
ratio would predict. This would have significant impact on interpretation of
the Fe _+ content of olivines on Mars from their absorption band positions.
Acknowledgment: This research was supported by NASA grants NSG
7405 and NAGW 570.
References
(1) Huguenin, R.L., (1986) Icarus, 28 p.203-212.
(2) Huguentn, R.L., Mtller, K.J. Harwood, W.S. (1979) J. No1. Evol.,14
p.103-132.
(3) Huguentn, R.L., (1982) J. Geophys. Res., 87 p.10,069-10,082.
(4) Huguenin, R.L., Jones, J.L., (1986) J. Geophys. Res. in press.
(5) Basolo, F., Pearson, R.G., (1967), Mechanisms of Inorganic Reactions -
A Study of Metal Complexes in Solution, J. Wiley & Sons Inc. N.Y. 701p.
(6) Burns, R.G., (1970) Mineralogical Applications of Crystal Field Theory,
Cambridge Univ. Press, 224p.
(7) Burns, R.G., (1974) Am. Min., 59 p. 625-629.


Chemical frost weathering of olivine: Experimental study and implications

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server