Inpact melt?

I don't think that jarosite can melt, it would rather change of chemical composition, from a sulphate to an oxyd (a common industry process). Which could, after, melt...

I'm not sure what the sulfate evaporites do when heated, but remember that underlying the evaporite is a supposed (hopeful?) basaltic unit, which likely comprises most of what remains as the surface of the ejecta apron. This is the importance of getting to Victoria, we'll get to see further down into the section than we have so far. And this is also the importance of continuing to travel further downhill once we compete the survey of Victoria, it will take us even further down in the geologic column.

--Bill

Oxide? But where would the oxygen atoms come from? Hmmm, I suppose if it was an aqueous environment...

It's an interesting question though - what, chemically speaking, would happen to evaporite subjected to a brief blip of intense heat and pressure?

No problem, the oxygen is already in there. Sulphates are compositions of the sulphate ion SO4-- with one or two metallic ions (2x M+ or M++) together with water molecules which are required to make the sulphate crystal as we find it.

When heated, (about 100°C??) usualy a sulphate first lose its crystalisation water. It is what happens when industry does plaster from gypsum.

When heated more, (about 200°C??) the sulphate ion breaks, releasing SO2. The remaining oxygen is now available to combine with the metal, it cannot do otherwise anyway because it is already reduced so it cannot escape the combinaison with the metal ion.


If we heat still more, the oxyde melts, a its own temperature, whic can vary from hundred to thousands degrees.




What happens to shocket sulphates? probably what happen to other crystals. I found an interesting link to this about heating asteroids (alternate PDF file). They explain that crystals submitted to intense shock waves undergo a variety of transformations: irregular fractures and melt pockets, together with a heating increasing with the intensity of the shock wave. For low intensities, the effect would not be very different with jarosite, but when we come to higher temperatures, the effect must be different, as jarosite cannot melt without undergoing a chemical change, involving emission of gasses.

Thanks Richard for the excellent explanation; I see that I remember even less chemistry than I thought I did

The place where I got lost in this discussion was where jarosite was assumed. Referring back to the original post, why would freshly broken jarosite be the most conservative assumption?

There has been a lot of work done on this subject in connection with the Chicxulub impact which occurred in carbonate/sulphate rock and it is well established that very large amounts of SO2/SO3 and CO2 were liberated. There is little doubt that the CO2 remained in the atmosphere for a fairly long time (millenia), while there is more doubt about the sulfur. Sulfuric acid from volcanboes remains in the stratosphere for a few years, but some people think that the sulfuric acid from Chicxulub would quickly have reacted with other material (dust) in the ejecta and fallen back to Earth. Of course things might work differently in the very thin and dry Martian atmosphere.

tty

Because it is the most common rock here. Most, if not all, jarosite blocks seen until today were old, with rounded shapes and apparent layering. These one would be fresh, with no erosion, so that they have angular shapes and the layering was not yet outlined by differential erosion. But please note that when I wrote this I did not noticed yet that these blocks are dark in visible light, so that Jarosite becomes unlikely.

As I recall, the majority of the rocks here are magnesium sulfate, keyserite. Jarosite is an iron sulfide that was discovered by the Moessbauer here at Meridiani. We'll have to see the results of the recent MB, but I don't think that these new rock types are jarosite.

--Bill

Errr? what I heard until recently was that Chicxulub impact punched throught two cristaline (granite-like) layers (which were dated with uranium clock) and that the limestones layers were deposed on it afterward, giving a flat surface obliterating the crater. So that there would be no carbonates involved.

On the other hand, the Ries crater in Germany made into a granite layer, covered by some tens of metres of limestone and shale deposits. There is a work about the granite melt, which gave a layer of about 10m of lava spread all over the crater rim, and nice glass-like light green tectites known as moldavites (because they were found in Moldavia). I know the limestone-shale layers formed large blocks (several hundred metres) which were lifted and then fell back on the crater bottom, sometimes turned upside down, but still with the layering recognizable (shattered I imagine). But I don't know how they responded to shock and heat. We can imagine that shale and clay can form a lava too, we can even try to melt some. Some carbonates too can exist in a liquid form. But sulphates, I don't know.

The light-toned outcrops vary a little in composition, but on average, the sulfate composition is somewhere around 2 parts Mg-sulfate, 1 part Ca-sulfate, and 1 part jarosite. There's also a non-trivial amount of silicates mixed in.

tglotch: Thanks for that. Those are similar to the ratios I've seen for the light-toned rocks in the Eagle and Endurance outcrops, but I didn't realize they are still about the same. As for the silicates, I think I've heard they are about 30-40%.

Is there any idea of what the silicates are? I don't think phyllosilicates (clays) or it would have been front-page news and I suspect that they are generic silicates, as in "left over from weathering processes".

--Bill

The Mini-TES data indicate that the majority of the silicate component is amorphous silica--that is a product of rather than a leftover residue of the weathering process. The rest looks like Na-rich plagioclase feldspar. Phyllosilicates are modeled, but with low confidence, and probably aren't really there. There is no evidence in the Mini-TES data for pyroxenes or olivines. Moessbauer data show a component that could be pyroxene or glass.

Tim

Tim,
Welcome to the forum! We will all look forward to posts from your unique perspective.