Elsevier

Geochemistry

Volume 70, Issue 1, March 2010, Pages 7-33
Geochemistry

INVITED REVIEW
Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and applications to determining the composition of planetary surfaces

https://doi.org/10.1016/j.chemer.2009.12.005Get rights and content

Abstract

The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe)2SiO4) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo70. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.

Introduction

Members of the olivine group (“olivines” hereafter) are important rock-forming minerals in terrestrial, planetary, and astronomical geomaterials. Olivines are the major component of Earth's mantle, they are common in many kinds of meteorites, and have been identified on the surfaces of planetary bodies, and in the spectra of astronomical targets. Olivine phases are indicators of low-silica environments, they crystallize at high temperatures, and they generally break down readily in the presence of weathering agents such as water. As such, their identification and characterization is a subject of considerable interest to a wide variety of researchers. Spectroscopy can be used very effectively in the laboratory to study the fine-scale structural properties of natural and experimentally produced olivines (e.g., as functions of temperature and pressure). Spectroscopy also can be used in remote-sensing applications to identify olivines in data acquired of non-terrestrial environments. Spectroscopic methods for examining olivines (and other minerals) generally utilize visible through infrared wavelengths, which are the wavelengths in which geologic materials have diagnostic spectral signatures.

Deer et al. (1992) provide detailed descriptions of the characteristics of olivine minerals, portions of which are summarized here. Olivines are silicate minerals with orthorhombic symmetry and having a general formula of X2SiO4, where X represents one or more divalent atoms in six-fold coordination with isolated silicate tetrahedra. The most common cations in olivine are Mg2+ and Fe2+, forming a solid solution between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Compositions in the Mg–Fe series commonly are identified by the molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo90Fa10), or in shortened form, by just their forsterite number, where Fo#=Mg/(Mg+Fe)×100. In this paper, the derivation of forsterite number is modified slightly (Fo#=Mg/(Mg+Fe+Ca+Mn)×100) to account for other cations that may substitute into the olivine structure in small amounts, and serves to ensure that all data being compared have similarly derived Fo#. In a few cases, this formulation results in very slight absolute differences in Fo# (<Fo2) from values published by the original authors. Olivine with up to one formula unit of calcium (CaMgSiO4) is referred to as monticellite. The iron analogue, kirschsteinite (CaFeSiO4), is rare in nature as a pure phase, but olivine in terrestrial rocks and meteorites may contain a predominance of this molecule and it is readily synthesized. There also is a manganese end member, tephroite (Mn2SiO4). The metal cations occupy two positions in the crystal structure, M1 and M2, which are defined by their symmetry; commonly, Fe2+ displays a preference for the M1 site. The thermal behavior of olivines along the Mg–Fe binary is such that the first olivines to crystallize from a liquid are Mg-rich relative to the compositions that crystallize subsequently. Among silicate minerals, olivines are relatively susceptible to mechanical and chemical breakdown, as well as low-grade metamorphism. Common products of these reactions are serpentine and mixtures of mostly phyllosilicates and oxides that are referred to as iddingsite, chlorophaeite, and bowlingite, depending on the mixture components. Serpentinization is the most common form of olivine alteration and can be thought of in simple terms as the hydration of olivine to yield serpentine (Mg3Si2O5(OH)4).

On Earth, olivine compositions in the forsterite–fayalite series are common in mafic to ultramafic rocks, in which their compositions typically range from about Fo85–95. Compositions between Fo80 and Fo50 are typical of gabbroic rocks; more Fe-rich compositions are less common, occurring in ferrodiorites, mangerites, and quartz syenites. They also are present in progressively metamorphosed serpentinites and thermally metamorphosed iron-rich sediments. In addition to terrestrial occurrences, Mg–Fe olivines are constituents of stony and stony–iron meteorites, and have been identified on the surfaces of Mars and some asteroids, in comets, in interplanetary and interstellar dusts, and in the circumstellar regions around some evolved and pre-main sequence stars. Olivines are, quite literally, universally relevant minerals, and it is their identification and discrimination in planetary materials and on planetary surfaces that will be the primary emphasis here.

In this paper, I will focus on the spectral characteristics of olivines along the Mg–Fe binary in the thermal, or middle, infrared region of the electromagnetic spectrum, from ∼1200 to 200 cm−1 (∼8.3–50 μm). Spectroscopy in the thermal infrared (TIR) region also is referred to in the literature as vibrational spectroscopy because within this range, spectral features arise from the fundamental vibrational modes of the material. More specifically, crystalline solids such as minerals are composed of a regular, repeating pattern of positive and negative ions that vibrate at quantized frequencies. When the positive and negative ions move out of phase with each other, absorption of energy becomes possible at the wavelength corresponding to the vibrational frequency of the motions, as long as there is a net dipole moment. For example, in silicate minerals the primary spectral absorptions (reststrahlen bands) are due to the stretching and bending motions in the silicon–oxygen anions. Additional absorption features result from metal–oxygen and lattice vibrations. The exact frequencies, shapes, intensities, and number of features in a mineral's spectrum are dependent on the relative masses, radii, distances, and angles between atoms and their bond strengths. These parameters are determined by the structural arrangement of the anions (i.e., their polymerization), and the location and composition of the cations associated with them. Because all minerals, by definition, have unique structures and/or compositions, virtually every mineral has a different suite of vibrational absorption characteristics and thus a unique spectrum in the thermal infrared. Several spectroscopic techniques are capable of measuring the fundamental vibrational modes of minerals, including: transmission, reflection, emission, attenuated total reflection (ATR), and Raman. Of interest here are the results from transmission, reflectance, and emission spectroscopy, where transmission data have provided much of the fundamental information on olivine spectral characteristics in a variety of pure and mixed samples, and reflectance and emission spectra are currently used for determining the composition of planetary materials and surfaces (from returned samples, meteorites, and remote sensing).

The first objective of this paper (2 Thermal infrared spectroscopy of the Mg–Fe olivine series, 3 Identification of Mg–Fe olivines in planetary materials using TIR spectroscopy, 4 Identification of Mg–Fe olivines on planetary surfaces using TIR spectroscopy) is to provide an overview of the peer-reviewed literature discussing the TIR properties of the Mg–Fe olivine series as a function of composition and physical character as measured by transmission and reflection spectroscopy, along with some new analyses of previously published data. I also will present new emission data of Mg–Fe olivine samples, where emission is the technique used for remote sensing of planetary surfaces. Additionally, I will review the literature on TIR observations of olivine in planetary materials, and the identification of Mg–Fe olivines on planetary surfaces via TIR spectroscopy. Additional discussion (Section 5) will be dedicated to describing how olivines can identified in mixture spectra and the accuracy with which their solid solution compositions and abundances can be obtained from those spectra. To my knowledge, a quantitative demonstration of the accuracy and precision with which olivine compositions and abundances can be distinguished in the emission spectra of natural mixtures has not been presented previously. The analyses of transmission, reflection, and emission data presented here complement the existing literature and will provide additional information that can aid the identification of olivines on planetary surfaces. With this application in mind, I have plotted composition and fit regressions as a function of various spectral feature parameters, rather than portraying spectral features as the dependent variable. This approach allows a consideration of how well one might expect to predict olivine composition from spectral features in remote-sensing data or when other compositional analyses are lacking for laboratory samples. Finally, I wish to point out that there are numerous un-peer-reviewed conference and workshop abstracts and papers that bear on the topic of olivine spectroscopy in the laboratory and on planetary surfaces. Because such papers do not always represent final results I have not attempted to summarize these references in this review, but I encourage interested readers to seek them out for additional information.

Section snippets

Focus of this review and definitions

The motivations for and applications of olivine spectral studies are wide-ranging, covering fields from astronomy to mantle geophysics, and as such, a wide variety of measurement techniques and samples are found in the literature. An exhaustive review of the literature on TIR spectroscopy of olivines could fill a book, so with application to planetary surfaces in mind, the discussion here will focus on the highlights of observational spectroscopy of Mg–Fe olivines as a series and in solid

Identification of Mg–Fe olivines in planetary materials using TIR spectroscopy

Samples returned from planetary bodies provide an opportunity to measure in situ materials that normally are measured remotely. Lunar samples returned by the Apollo 11, 12, 14, and 15 missions were first analyzed in the TIR by Estep et al., 1971, Estep et al., 1972, who obtained transmission spectra of mineral separates and single mineral grains. The Mg–Fe compositions of olivines in the Apollo 11 and 12 olivine separates were constrained to Fo72–66 using a cation determinative curve for Band

Identification of Mg–Fe olivines on planetary surfaces using TIR spectroscopy

Excluding Earth, where we can examine olivine-bearing rocks firsthand in most cases, the planetary surfaces in our solar system most likely to exhibit olivine signatures in the thermal infrared are Mercury, Venus, Earth's Moon, Mars, asteroids, and satellites of the outer solar system planets (comets are not considered here because most observations are of their comae, not surfaces). Although chemical evidence collected by several Soviet Venera and Vega landers in the 1970s and 1980s suggested

Linear least squares mixture analysis for determination of olivine abundance and solid solution composition

Most planetary materials are not composed of single phases, raising the question of how to distinguish various phases in mixtures and estimate their abundances. As described above in Section 3, the spectra of mixtures can be mathematically modeled relatively straightforwardly using a linear least squares approach to identify the major component phases and retrieve their abundances, typically to within about 5–10 vol% for laboratory quality (2 cm−1 sampling) data (Ramsey and Christensen, 1998;

Summary

In the study of planetary surfaces, the presence of olivine is an indicator of a low-silica environment and high temperature crystallization. Solid solution compositions, if they can be determined, are indicative of the chemistry of their parent magmas and the evolution of those magmas through processes of melting, re-equilibration, and differentiation. The literature describing the transmittance spectra of Mg–Fe olivines shows that these data can be used to determine the relative proportions

Acknowledgements

I would like to thank S.A. Morse (U. Mass.) for providing the majority of olivine samples used to measure the emissivity data presented here and for acquiring and providing the microprobe analysis of sample KI3372. Will Koeppen (U. Hawaii) undertook the challenging task of isolating several of the Kiglapait olivine samples from the mafic concentrates in which they were provided; he also acquired many of the olivine emissivity spectra used for the new analyses presented in this manuscript. I

References (153)

  • J.F. Mustard et al.

    Effects of hyperfine particles on reflectance spectra from 0.3 to 25 μm

    Icarus

    (1997)
  • J.L. Bandfield

    Global mineral distributions on Mars

    J. Geophys. Res.

    (2002)
  • J.L. Bandfield et al.

    A global view of Martian surface compositions from MGS-TES

    Science

    (2000)
  • M.A. Barucci et al.

    Asteroids 2867 Steins and 21 Lutetia: surface composition from far infrared observations with the Spitzer Space Telescope

    Astron. Astrophys.

    (2008)
  • Berkley, J.L., Keil, K., and Prinz, M., 1980. Comparative petrology and origin of Governador Valadares and other...
  • J.E. Bowey et al.

    Temperature effects on the 15–85 μm spectra of olivines and pyroxenes

    Mon. Not. R. Astron. Soc.

    (2001)
  • J.P. Bradley

    Interplanetary dust particles

  • R.G. Burns et al.

    Cation determinative curves for Mg–Fe–Mn olivines from vibrational spectra

    Am. Miner.

    (1972)
  • H. Chihara et al.

    Low-temperature optical properties of silicate particles in the far-infrared region

    Publ. Astron. Soc. Jpn.

    (2001)
  • P.R. Christensen

    Martian dust mantling and surface composition: interpretation of thermophysical properties

    J. Geophys. Res.

    (1982)
  • P.R. Christensen et al.

    The martian surface layer

  • P.R. Christensen et al.

    Identification of a basaltic component on the Martian surface from Thermal Emission Spectrometer data

    J. Geophys. Res.

    (2000)
  • P.R. Christensen et al.

    A Thermal Emission Spectral library of rock-forming minerals

    J. Geophys. Res.

    (2000)
  • P.R. Christensen et al.

    Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: evidence for near-surface water

    J. Geophys. Res.

    (2000)
  • P.R. Christensen et al.

    Mars Global Surveyor Thermal Emission Spectrometer experiment: investigation description and surface science results

    J. Geophys. Res.

    (2001)
  • P.R. Christensen et al.

    Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity rover

    Science

    (2004)
  • P.R. Christensen et al.

    Initial results from the Mini-TES experiment in Gusev crater from the Spirit rover

    Science

    (2004)
  • Clark, R.N., Swayze, G.A., Wise, R.A., Livo, K.E., Hoefen, T.M., Kokaly, R.F., and Sutley, S.J., 2007. USGS digital...
  • M. Cohen et al.

    Spectral irradiance calibration in the infrared. VIII. 5–14 micron spectroscopy of the asteroids Ceres, Vesta, and Pallas

    Astronom. J.

    (1998)
  • J.E. Conel

    Infrared emissivities of silicates: experimental results and a cloudy atmosphere model of spectral emission from condensed particulate mediums

    J. Geophys. Res.

    (1969)
  • K.L. Day

    Temperature dependence of mid-infrared silicate absorption

    Ap. J.

    (1976)
  • W.A. Deer et al.

    An Introduction to the Rock-Forming Minerals

    (1992)
  • V. Devarajan et al.

    Normal coordinate analysis of the optically active vibrations (k=0) of crystalline magnesium orthosilicate Mg2SiO3 (forsterite)

    J. Chem. Phys.

    (1975)
  • T.M. Donahue et al.

    The Venus atmosphere and ionosphere and their interaction with the solar wind: an overview

  • E. Dotto et al.

    308 Polyxo: ISO-SWS spectrum up to 26 micron

    Astron. Astrophys.

    (2004)
  • E. Dotto et al.

    ISO observations of low and moderate albedo asteroids

    Astron. Astrophys.

    (2002)
  • E. Dotto et al.

    ISO results on bright Main Belt asteroids: PHT-S observations

    Astron. Astrophys.

    (2000)
  • D.A. Duke et al.

    Infrared investigation of the olivine group minerals

    Am. Miner.

    (1964)
  • T.L. Dunn et al.

    Thermal emission spectra of terrestrial alkaline volcanic rocks: applications to Martian remote sensing

    J. Geophys. Res.

    (2007)
  • C.S. Edwards et al.

    Evidence for extensive olivine-rich basalt bedrock outcrops in Ganges and Eos chasmas, Mars

    J. Geophys. Res.

    (2008)
  • P.A. Estep et al.

    Infrared vibrational spectroscopic studies of minerals from Apollo 11 and Apollo 12 lunar samples

    Proc. Sec. Lunar Sci. Conf.

    (1971)
  • P.A. Estep et al.

    Infrared and Raman spectroscopic studies of structural variations in minerals from Apollo 11, 12, 14, and 15 samples

    Geochim. Cosmochim. Acta

    (1972)
  • D. Fabian et al.

    Steps toward interstellar silicate mineralogy: VI. Dependence of crystalline olivine IR spectra on iron content and particle shape

    Astron. Astrophys.

    (2001)
  • K.C. Feely et al.

    Quantitative compositional analysis using Thermal Emission Spectroscopy: application to igneous and metamorphic rocks

    J. Geophys. Res.

    (1999)
  • B. Fegley et al.

    Geochemistry of surface–atmosphere interactions on Venus

  • Fischer, E.M., 1995. Quantitative Compositional Analysis of the Lunar Surface from Reflectance Spectroscopy: Iron,...
  • R.D. Gehrz et al.

    Infrared observations of comets with the Spitzer Space Telescope

    Adv. Space Res.

    (2006)
  • T.D. Glotch et al.

    Determination and interpretation of surface and atmospheric Miniature Thermal Emission Spectrometer spectral end-members at the Meridiani Planum landing site

    J. Geophys. Res.

    (2006)
  • A.F.H. Goetz

    Differential infrared lunar emission spectroscopy

    J. Geophys. Res.

    (1968)
  • C.A. Goodrich

    Ureilites: a critical review

    Meteoritics

    (1992)
  • Cited by (0)

    View full text