[meteorite-list] New Mineral Proves an Old Idea about Space Weathering (Hapkeite)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 6 14:13:18 2004
Message-ID: <200407061805.LAA20770_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/July04/newMineral.html

New Mineral Proves an Old Idea about Space Weathering
Planetary Science Research Discoveries
July 5, 2004

--- A newly discovered vapor-deposited iron silicide in a lunar
meteorite has been named hapkeite.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

Discovered in a lunar meteorite, a new mineral named hapkeite honors the
scientist, Bruce Hapke (Emeritis Professor at University of Pittsburg),
who nearly 30 years ago predicted the importance of vaporization as one
of the processes in space weathering. The new iron silicide mineral
(Fe2Si) was announced by the research team of Mahesh Anand (formerly at
the University of Tennessee, Knoxville and now at the Natural History
Museum, London), Larry Taylor (University of Tennessee, Knoxville),
Mikhail Nazarov (Vernadsky Institute of Geochemistry and Analytical
Chemistry, Moscow), Jinfu Shu, Ho-kwang Mao, and Russell Hemley
(Carnegie Institution of Washington). This mineral likely formed by
impact vaporization of the lunar soil and subsequent condensation of the
iron and silicon into tiny metal grains. The researchers conclude that
Fe-Si phases are more common in the lunar soil than previously thought.
It is nanophase-sized Fe0, these Fe-Si phases, and other space
weathering products that profoundly affect the optical properties of the
lunar soil at visible and near infrared wavelengths and must be taken
into account when interpreting remote sensing data of the Moon.

Reference:

Anand, M., Taylor, L. A., Nazarov, M. A., Shu, J., Mao H.-K., and
Hemley, R. J. (2004) Space weathering on airless planetary bodies: clues
from the lunar mineral hapkeite. Proceedings of the National Academy of
Sciences, v. 101, no. 18, p. 6847-6851.

------------------------------------------------------------------------

Lunar Surface

whole Moon The countless craters and basins we see on the Moon attest to
a long history of meteorite bombardment. It's a bombardment rate that
reached a maximum about 3.85 billion years ago--based on the ages of
highland breccia samples returned by the Apollo astronauts. Since then,
a lesser but continuous barrage of micrometeorites and charged particles
from the Sun and stars has been generating a powdery surface, also
called regolith (or soil, though no biologic component is implied in the
name.) With nothing to stop or slow down the incoming space debris, due
to a near-complete lack of any atmosphere on the Moon, even the tiniest
grains (mostly 10s to 150 micrometers) hit the Moon's surface at full
cosmic velocity, 20 or more kilometers per second [that is, about
70,000-150,000 km/hr]. The pulverized powdery regolith is just what
you'd expect from impact debris. It has rock and mineral fragments,
impact glasses, and particles called agglutinates that are mineral and
rock fragments stuck together by impact glass. And, the high-energy
micrometeorite impacts cause flash-melting, vaporization, and
redeposition of some of the surface materials. The phenomenon
responsible for the physical and chemical makeup of the Moon's surface
is utterly unique to airless planetary bodies--space weathering.

Most of the lunar regolith is smaller than fine sand and about 20% is
smaller than 0.02 millimeters, which helps to preserve the astronaut's
bootprints. Dragging a rake through the regolith allowed the astronauts
to collect rock fragments.

------------------------------------------------------------------------

Space Weathering

Space weathering is generally defined as the processes (such as
meteorite, micrometeorite, and cosmic dust bombardment, solar wind ion
implantation, and sputtering, but also including deep vacuum and
temperatures approaching absolute zero [-273 oC]) that change the
physical structure, optical properties, and chemical and mineralogical
properties of the surface of an airless planetary body from their
original conditions. It applies to the Moon, Mercury, and asteroids, as
well as other small bodies such as Phobos and Deimos. The concept
blossomed in the early 1970s when cosmochemists studying rock and
regolith samples returned by the Apollo astronauts compared what they
were finding with what was already known of the Moon's surface through
telescopic spectral studies. There were some surprise results in the
laboratories! Apollo regolith samples were darker and spectrally redder
than the remote sensing data.

lightbulb Bruce Hapke (Emeritis Professor at University of Pittsburg)
and colleagues worked on the question of why the lunar soil becomes
darker and spectrally redder with time. In 1975, they proposed that
metallic iron would be among the elements and compounds vaporized during
space weathering and subsequently condensed in glassy coatings on the
surfaces of surrounding soil grains. These iron droplets would be
minute, only nanometers in size (billionths of a meter), but they would
dramatically alter how light interacts with the surface materials and,
hence, how it would be sensed remotely. At the time of the predictions,
however, no one could find any trace of vapor condensates on the lunar
soil particles.

Finally, technology caught up with human inspiration. In 1993, Lindsay
Keller and David McKay (Johnson Space Center) using transmission
electron microscopy (TEM) found nanophase-sized metallic iron beads in
silica-rich glassy rims on individual mineral grains in Apollo lunar
samples. In the same year, Carl? Pieters (Brown University) and
colleagues determined that the greatest spectral changes due to space
weathering were in the smallest grains of the lunar regolith and
concluded that they were caused by changes on the surfaces of grains
rather than within the grains. The general consensus reached in the past
few years is that no single space-weathering phenomenon entirely
explains the darkening and spectral-reddening characteristics of mature
lunar regolith. Rather, processes work in concert (grain fragmentation
or damage due to impact, agglutinate formation, solar-wind sputtering or
impact vaporization and condensation of nanophase-sized iron) to alter
the optical properties of the lunar regolith, with vapor-phase
deposition of nanophase-sized iron playing the most significant role.

[nanophase Fe in grain coating}

Vapor-deposited Fe metal particles (Fe0) in a SiO2-rich glassy rim of
anorthosite grain from a mature lunar soil. Virtually all grains of a
mature mare soil (long exposed to space weathering) have such rims.

------------------------------------------------------------------------

The Newest Evidence of Vapor-deposited Iron

Metallic iron particles not associated with grain coatings have now been
identified within a lunar meteorite. The host rock of the newly
discovered mineral is lunar meteorite, Dhofar 280 (pictured below),
collected in 2001 in the Dhofar region of Oman. It is classified as an
anorthositic fragmental breccia. Dhofar 280

One of the breccia clasts in the meteorite contains opaque minerals 2-30
microns in size that have a slight tarnish appearance. Upon closer
inspection with the electron microprobe, Mahesh Anand and Larry Taylor
discovered that the grains are actually compounds of iron silicides. The
largest such grain (~35 microns) turned out to be Fe2Si, and its
discovery in Dhofar 280 is the first documentation of its natural
occurrence. Similar chemical structures have been created synthetically
in the lab and other related minerals have been known to form on Earth
under unique conditions when lightening strikes sandy soil forming
glassy fulgurites. Naming the new Fe2Si mineral hapkeite seemed fitting
to Larry Taylor and his colleagues as they consider it to be a direct
product of impact-induced vapor-phase deposition in the lunar regolith.
Hapkeite is the third iron silicide identified in this lunar meteorite.
The other two minerals remain to be formerly named, FeSi and FeSi2.
Anand and colleagues consider SiO2 in the vapor phase from energetic
impact-induced melts to be an important source of SiO2+ and Si0, which
would combine in various proportions in the vapor with Fe0 to condense
out as the observed Fe-Si metal grains. Although hapkeite has not yet
been identified in Apollo regolith samples, the research team concluded
that Fe-Si phases are probably more common in the lunar regolith and may
be more closely related in origin to nanophase iron than previously
thought.

[occurrence of hapkeite]

Reflected-light image of the breccia clast in Dhofar 280 showing
hapkeite and some smaller FeNi metal grains.

------------------------------------------------------------------------

[hapkeite BSE and x-ray elemental maps]

Backscatter electron and x-ray elemental maps of hapkeite. Analyses show
that 95 wt% of hapkeite is composed of Fe and Si with spots of Ti- and
P-rich areas.

------------------------------------------------------------------------

[hapkeite formation cartoon]

This cartoon shows the researchers' interpretation of how the iron
silicides may form on the Moon.
After vaporization by micrometeorite impact, Fe and Si recombine from
the vapor phase to form various Fe-Si compounds such as those found in
Dhofar 280.

------------------------------------------------------------------------

Implications for Remote Sensing

As the Lunar Sourcebook (p.286) says, "The regolith is the source of
virtually all our information about the Moon." Covering practically the
entire surface to a depth of about 3 to 10 meters, the regolith is the
source of our Apollo samples and dominates what our remote sensing
instruments analyze. Since Hapke's predictions, cosmochemical analyses
of samples have helped to explain how the formation and accumulation of
nanophase iron and Fe-Si phases have changed the physical, optical,
chemical, and mineralogical properties of the lunar regolith.

Much of what we know today about the global properties of the Moon stems
from the early work by scientists like Hapke, Tom McCord (Emeritis
Professor at University of Hawaii), and their colleagues on the effects
of vapor-phase depositional processes and the effects on the chemical
and optical properties of the lunar regolith. Building on this knowledge
of space weathering, researchers are developing new mineral mapping
techniques in visible and near infrared wavelengths. For example, Paul
Lucey (Univeristy of Hawaii) and colleagues have used Clementine
multispectral data to determine the concentrations of FeO and TiO2 on
the lunar surface. [See PSRD articles Moonbeams and Elements
<http://www.psrd.hawaii.edu/Oct97/MoonFeO.html> and The Surprising Lunar
Maria <http://www.psrd.hawaii.edu/June00/lunarMaria.html>.]

FeO map of the Moon

Map of the FeO content on the lunar surface determined from the
intensity of light reflected in two wavelengths. The FeO technique was
invented by Paul Lucey and is based on our current understanding of
space weathering.

Experts in lunar sample analysis are collaborating with experts in
remote sensing to address outstanding questions in lunar science. One
such group, for example, the Lunar Soil Characterization Consortium, is
working to better understand the effects of space weathering on the
surfaces of airless planetary bodies. They are characterizing the
mineralogy and chemistry of the finest-sized fractions of the lunar
regolith in order to better understand remotely sensed reflectance
spectra of the Moon. (See data < http://web.utk.edu/~pgi/data.html>.)

The discovery of hapkeite in a lunar meteorite has helped improve our
understanding of space weathering on the Moon and how space weathering
plays a major role in affecting remote sensing studies of airless
planetary bodies.

ADDITIONAL RESOURCES

Anand, M., Taylor, L. A., Nazarov, M. A., Shu, J., Mao H.-K., and
Hemley, R. J. (2004) Space weathering on airless planetary bodies: clues
from the lunar mineral hapkeite. Proceedings of the National Academy of
Sciences, v. 101, no. 18, p. 6847-6851.

Chapman, C. R. (2004) Space weathering of asteroid surfaces. Annual
Review of Earth and Planetary Science, v. 32, p. 539-567.

Clark, B. E., Hapke, B., Pieters, C., and Britt, D. (2002) Asteroid
space weathering and regolith evolution in Asteroids III, W. F. Bottke
Jr., A. Cellino, P. Paolicchi, and R. P. Binzel (eds), University of
Arizona Press, Tucson, p. 585-599.

Hapke, B., Cassidy, W., Wells, E. (1975) Effects of vapor-phase
deposition processes on the optical, chemical and magnetic properties of
the lunar regolith. Moon, v. 13, p. 339-353.

Hapke, B. (2001) Space weathering from Mercury to the asteroid belt. J.
Geophys. Res., v. 106, p. 10039-10073.

Heiken, G. H., Vaniman, D. T., French, B. V. editors (1991) The Lunar
Sourcebook. Cambridge University Press, 736 p.

Keller, L. P., Wentworth, S. J., McKay, D. S., Taylor, L. A., Pieters,
C. M., and Morris, R. V. (1999) Space weathering in the fine size
fractions of lunar soils: Mare/highland differences, in Workshop on New
Views of the Moon II: Understanding the Moon Through the Integration of
Diverse Datasets, pp. 32-33. LPI Contribution No. 980, Lunar and
Planetary Institute, Houston.

Lucey, P. G., Taylor, G. J., and Malaret E. (1995) Abundance and
distribution of iron on the Moon. Science, v. 268, p. 1150-1153.

Lucey P. G., Blewett, D. T., and Jolliff, B. L. (2000) Lunar iron and
titanium abundance algorithms based on final processing of Clementine
ultraviolet-visible images. J. Geophys. Res., v. 105, No. E8, p.
20,297-20,305.

Noble, S. K., Pieters, C. M., and Keller, L. P. (2004) Quantitative
aspects of space weathering: Implications for regolith breccia
meteorites and asteroids (abstract 1301) Lunar and Planetary Science
Conference XXXV.

Pieters, C. M., Fischer, E., Rode, O., and Basu, A. (1993) Optical
effects of space weathering: The role of the finest fraction. J.
Geophys. Res., v. 98, p. 20817-20824.

Pieters, C. M., Taylor, L., Noble, S., Keller, L., Hapke, B., Morris,
R., Allen, C., McKay, D., and Wentworth, S. (2000) Space weathering on
airless bodies: Resolving a mystery with lunar samples, Meteoritics &
Planet. Sci., v. 35, p. 1101-1107.

Taylor, L. A., Pieters, C. M., Keller, L. P., Morris, R. V., and McKay,
D. S. (2001) Lunar mare soils: Space weathering and the major effects of
surface-correlated nanophase Fe. J. Geophys. Res., v. 106, p.
27,985-28,000. (Reference for The Lunar Soil Characterization Consortium.)

Taylor, G. J. (1997) Moonbeams and Elements. Planetary Science Research
Discoveries. http://www.psrd.hawaii.edu/Oct97/MoonFeO.html.

Taylor, G. J. (2000) The Surprising Lunar Maria. Planetary Science
Research Discoveries. http://www.psrd.hawaii.edu/June00/lunarMaria.html
<http://www.psrd.hawaii.edu/June00/lunarMaria.html>.

Taylor, G. J. (2001) New Data, New Ideas, and Lively Debate about
Mercury. Planetary Science Research Discoveries.
http://www.psrd.hawaii.edu/Oct01/MercuryMtg.html
<http://www.psrd.hawaii.edu/Oct01/MercuryMtg.html>
Received on Tue 06 Jul 2004 02:05:59 PM PDT