This mystery material on Humphrey and in Bonneville may be related to
the second class of volcanic rock observed from orbit by the TES
spectrometer aside from basalt.
The original interpretation was that it was andesitic rock. However,
another interpretation has been proposed that it may in fact be basalt
weathered by liquid water:

The Tricky Business of Identifying Rocks on Mars
--- A new analysis of thermal emission spectra suggests a new
interpretation for the composition of the Martian surface.
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
http://www.psrd.hawaii.edu/May02/MarsTES.html

As with the rover observations of the Humphrey rock, this 2nd type of
rock, referred to as Surface Type 2, contains either high silica glass
or basalt weathering products, clays for example.

These reports given at the last weeks Lunar and Planetary Science
Conference also discuss these two possibilities for this Surface Type
2 material:

VOLCANISM AND/OR AQUEOUS ALTERATION ON MARS: CONSTRAINTS ON
DISTINGUISHING
GLASS AND PHYLLOSILICATE IN THE THERMAL INFRARED. W. C. Koeppen and V.
E. Hamilton, Hawaii Institute of Geophysics and Planetology,
University of Hawaii at Manoa, 1680 East-West Rd, POST 602, Honolulu,
HI 96822 (****@higp.hawaii.edu).
Lunar and Planetary Science XXXV (2004) 1457.pdf
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1457.pdf

Emission spectroscopy of smectites: Implications for the TES
andesite-weathered basalt debate. Joseph. R. Michalski1, Michael D.
Kraft1, Thomas G. Sharp1, Lynda B. Williams 2, and Philip R.
Christensen1 1 Dept. of Geological Sciences, Arizona State University,
Tempe, AZ, 85287-1404 2 Center for Solid State Science, Arizona State
University.
Lunar and Planetary Science XXXV (2004) 1401.pdf
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1401.pdf

Interestingly the first suggests they might be distinguished by up
close examination by a lander spectrometer whereas orbital
determinations are obscured by atmospheric CO2 absorptions.
Then detailed measurements of the dark material in Bonneville by the
Spirit rover may be able to resolve this question about the origin of
the Surface Type 2 rock on Mars.
As explained in the web article by G. Jeffrey Taylor above, both the
andesitic and weathered basalt explanations have important
implications for the geologic past of Mars - the andesitic, suggesting
tectonic plates may have operated on Mars, and the weathered basalt,
suggesting liquid water oceans may have occurred on Mars.

Bob Clark

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Space Sciences
Mystery material in the Bonneville crater.
Posted by Robert Clark on 3/20/2004 4:52:48 AM

Thursday's NASA news conference is archived here:

http://mars.systemsfirst.com/news_archive/index.html

About 43 minutes in is discussed the dark deposits inside the crater. Dr. John Grant said the dark deposits in the crater were like a mystery material on the Humphrey rock. This material was either glass-like or a weathering product, presumably due to liquid water, since it was found to have been exposed to small amounts of liquid water weathering.
He said the dark material in the drift outside the crater was not like this, but more like the classic basaltic material.
If this mystery material is due to liquid water weathering, examining up close would give a chance to see a large-scale deposit of liquid water altered minerals at the Gusev site.

Bonneville in Color.
http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040315a.html

The dark mystery material is in the far side crater wall.

Bob Clark
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