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A Raman study of olivine alteration studies

A crucial task of Mars surface science is to determine past environmental conditions, especially aqueous environments. Olivine, a basaltic mineral, is easily altered by aqueous solutions and its alteration assemblages may be specific to conditions of alteration [1]. Raman spectra are produced by molecular vibrations and provide direct means for studying the alteration products.

Our purpose is to:

1) Find spectral indicators of olivine alteration useful for in-situ analyses on Mars.

2) Study the so called "iddingsite" mineral assemblage.

3) Determine structural and chemical variability of iddingsite components with respect to the degree of alteration.

QBV_Olivine samples from Lunar Crater volcanic field, NV

QBV is a basalt cobble from a Quaternary flow at Lunar Crater Volcanic Field, Nevada. It is vesicular and porphyritic, and its olivine grains altered to iddingsite along their edges and fractures. The data are from 3 traverses across iddingsite in one large olivine phenocryst using Raman spectroscopic point counts, supplemented with EMP analyses.

Mineral phases found in altered olivine grains:

Mineral phases identified:

1). original olivine

2). altered olivine

3). polymerized silicates

4). goethite

5). hematite

6). no saponite was found

Raman linear traverses were taken across the original and alteration zone in olivine grains

Raman spectral indication of olivine alteration:

Both Raman peaks of the olivine doublet (820 and 850 cm-1) broaden and weaken in proximity to the iddingsite while peak positions shift upwards. Good olivine spectra indicate Mg/(Mg+Fe) = 0.6 at the rim of the phenocryst and 0.55 in the unaltered core [6-8]. The 820 cm-1 peak position shifts upward more than the 850 cm-1 peak. When plotted relative to each other, the points fall outside the normal range for olivine.

Polymerized silicates as alteration products:

Comparing with silicates of different polymerization, the spectra of “polymerized” silicates  imply a structure whose degree of polymerization lies between that of a double-chain and sheet silicate. Typical  phyllosilicates have presumably not yet formed.  The breadth of the peaks imply that it is poorly crystallized. Fig.5  implies variable degrees of polymerization or a mixture of polymerized structures.

Fig.6 demonstrates that the polymerized phase is developing at the expense of the olivine structure.

Compositional variations:

Electron microprobe analyses indicate removal of Si and Mg from zones of alteration that cannot be attributed solely to igneous zonation. Although olivine compositions are consistent with incipient hydration, progressive alteration to lower SiO2 and MgO and higher FeO is not consistent with simple hydration and dilution. Iddingsite compositions lie on trajectories that depart from Fo-Fa mixing lines and the FeO trajectory projects to goethite.

Reference: Kuebler K. E., Wang A., Haskin L. A., Jolliff B. L., A Study of Olivine Alteration to Iddingsite Using Raman Spectroscopy. Abstract #1953, 34th LPSc, 2003.


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