Possible solution to the mystery of Type 2 material on Mars.

Mars Global Surveyor identified two primary surface types on Mars, Type 1 and Type 2.
Type 1 material was identified as due to basaltic lava. However, the origin of Type 2 material has been a mystery. Proposed explanations include andesitic lava, or aqueously altered basaltic lava.
A report to the 7th International Conference on Mars may provide a means of resolving this mystery:

NEW CLUES TO THE ORIGIN OF TES SURFACE TYPE 2 MATERIAL.
S. W. Ruff1, P. R. Christensen1, and the Athena Science Team,
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287-6305, ****@asu.edu.
Seventh International Conference on Mars.
http://www.lpi.usra.edu/meetings/7th...7/pdf/3367.pdf

This report suggests the high silica nodules and deposits seen by Spirit may have the same spectra as the Type 2 material.
The high silica material is described here:

Mars Exploration Rovers Update:
Spirit Finds Past Water at Home, Opportunity Takes in Tierra del Fuego.
By A.J.S. Rayl
April 30, 2007
"The typical rocks that we encounter with both rovers have silica values in the mid 40% range and here all of a sudden we're seeing a value that's jumped way up to 72% SiO2," Ruff explained. In addition to that APXS-derived percentage, the Mini-TES spectra of these high silica rocks, he said, indicate they are associated with "an amorphous silica," meaning a non-crystalline-like silica. "Although we still haven't quite put that whole puzzle together, we can be pretty confident that this area of Mars went through a process that involved interaction with water." Moreover, he added, "it would take a significant amount of water to enrich this outcrop or layer to this level of silica.
"By examining Elizabeth Mahon in-depth and getting all its spectral characteristics, the MER team can then link it via morphology to a much larger region, Ruff said, suggesting that this may be part of a layer that goes all the way back to Tyrone Nodules in one direction and further to the south of Home Plate in the other direction. "It all adds up to something happening here in this area, in this particular layer, at this particular point in Mars' history," he said.
"Silica is a common hydrothermal chemical," pointed out Jack Farmer, a professor in the School of Earth and Space Exploration at ASU, and one of the long-term science planning leads on Spirit this past month. "At Yellowstone Park, most of the hot springs are delivering silica to the surface. Those big center mounds you see there are mostly made up of silica. While I personally don't think that’s what's going on at Home Plate, the point is there are a variety of origins for silica in terrestrial settings and they all, more or less, involve either enhanced water contents and magmas during explosive volcanism."
"On Earth, similarly silica-coated basaltic rocks can form, for example, as rainwater leeches away all other elements to leave behind the enriched layer or via volcanic emanations that carry and deposit water and other gaseous elements on the rocks, which then lead to the enrichment."
MER Update 043007 - Planetary News | The Planetary Society

Mars Exploration Rovers Update:
Spirit Finds "Explosive" Evidence of Past Water, Opportunity Revs Up, Revisits Cape of Good Hope.
By A.J.S. Rayl
May 31, 2007
"When the data from Gertrude Weise was in and the analysis done, the MER science team was stunned by the results. During a recent teleconference mid-month, members of the MER science team listened as a colleague told them that the APXS, a chemical analyzer at the end of Spirit's arm, had measured Gertrude's composition to be in the neighborhood of 90% pure silica. "And almost all of the rest of it is the results of contamination of nearby soil," noted Yen. Combined with mineralogical findings of the MINI-TES, its composition was unequivocally confirmed.
"You could hear people gasp in astonishment," said Squyres. Considering the staggering amount of silica, the soil seemingly had to be formed by processes that require the presence of water. "I don't know of any way in which you can concentrate that much silica without liquid water," noted Yen."
MER Update 053107 - Planetary News | The Planetary Society


This high silica content in amorphous form might be modeled by aqueously altered rhyolite. I found references for altered rhyolite containing 85% and 90% silica:

Geologic Map of the Mount Higgins 7.5-minute Quadrangle, Skagit and Snohomish Counties, Washington.
"Rhyolite—High-silica rhyolite (site 22; up to 85% SiO2) occurring as
thick flows with interbedded vitric tuff, lapilli ash-flow(?) vitric tuff, and
minor pumiceous sandstone; rhyolite is bluish to greenish gray and
weathered to shades of yellow, brown, red, or white; porphyritic rhyolite
contains scattered quartz ±plagioclase phenocrysts with local sanidine
phenocrysts in a glassy or cryptocrystalline matrix; quartz (microlitic to
0.12 in.) is smokey and varies from euhedral to anhedral with some
resorption features; plagioclase is euhedral to subhedral; glass is
regularly spherulitic and varies from clear to highly altered; lapilli tuff
clasts consist mostly of pumice; secondary minerals include chlorite,
epidote, calcite, sericite, and vesicle-filling quartz; trachytoid textures
are common; bedding is obscure except where stretched vesicles, flow
banding, or flattened pumice fiamme in welded lapilli tuff define a
primary foliation (this study; Jones, 1959; Reller, 1986; Tabor and
others, 2002). Tabor and others (2002) obtained a zircon fission-track
age of 35.2 ±3.5 Ma (site 13). Lovseth (1975) obtained a zircon fissiontrack
age of 41.5 ±3.4 Ma (late Eocene) west of the study area."
http://www.dnr.wa.gov/geology/pdf/ofr03-12.pdf

Rhyolite.
"The ground-mass of rhyolites is liable to other changes, of which the most important are silicification, kaolinization and sericitization. Among the older rocks of this group it is the exception to find that secondary quartz has not been deposited in some parts of them. Often indeed the matrix is completely replaced by silica in the form of finely crystalline quartz or chalcedony; and these rocks on analysis prove to contain over 90% of silica. In the recent rhyolites of Hungary, New Zealand, &c., the deposit of coarse opal in portions of the rock is a very common phenomenon."
Rhyolite - LoveToKnow 1911

Both mention it containing finely crystalline quartz. But the second states it may be in the form of chalcedony.
This is interesting in that the crystallinity of chalcedony may be cryptocrystalline, meaning of microscopic size:

Chalcedony.
Chalcedony - Wikipedia, the free encyclopedia

Chalcedony.
...
"By modern mineralogists the name chalcedony is restricted to those kinds of silica which occur not in distinct crystals like ordinary quartz, but in concretionary, mammillated or stalactitic forms, which break with a fine splintery fracture, and display a delicate fibrous structure. Chalcedony may be regarded as a micro-crystalline form of quartz. It is rather softer and less dense than crystallized quartz, its hardness being about 6.5 and its specific gravity 2.6, the difference being probably due to the presence of a small amount of opaline silica between the fibres."
...
Chalcedony - LoveToKnow 1911

Cryptocrystalline.
"Cryptocrystalline is a rock texture which is so finely crystalline, that is, made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Also a form of diamond, known as carbonado, is cryptocrystalline. Volcanic rocks, especially of the acidic type such as felsites and rhyolites, may have a cryptocrystalline groundmass as distinguished from pure obsidian (acidic) or tachylyte (basic), which are natural rock glasses."
Cryptocrystalline - Wikipedia, the free encyclopedia

Then depending on the method used to determine the silica seen by Spirit was non-crystalline it may be it was in this cryptocrystalline form.


Bob Clark