Cape York - Northern Havens, Sol 2780 - 2947 Options

[Mongo]

I think that the importance of this discovery is not so much the discovery of a mineral probably formed in low-acidity water (that had been done first by Spirit, with Comanche) as it is the fact that the gypsum deposition was apparently done in situ. Comanche was moved to its present location from elsewhere, so there was little geological context to work with, whereas with Homestake, we have Endeavour Crater's rim and bowl to (I hope) make traverses over, in order to put this discovery into its spatial and temporal context.

[ilbasso]

Crystaline gypsum is a lot differnt from plasterboard. Think of the crystals in the Mexican cave...

Nonetheless, gypsum is relatively soft relative to other minerals. It's "2" on Moh's Scale of Mineral Hardness, roughly in between the hardness of a pencil "lead" (graphite) and a fingernail.

[marsophile]

Nonetheless, gypsum is relatively soft relative to other minerals.

It's clearly harder than the rock surrounding it, as evidenced by the drive-over. Perhaps the vein has only been exhumed relatively recently in geological terms.

[Stu]

Two frame MI mosaic...



From here, I guess...

[Marz]

Two frame MI mosaic...

Bright inclusions on the upper-right might be more gypsum? And are those vugs in the lower left?

Thanks Stu!

[Eutectic]

Nonetheless, gypsum is relatively soft relative to other minerals. It's "2" on Moh's Scale of Mineral Hardness, roughly in between the hardness of a pencil "lead" (graphite) and a fingernail.

With loss of water, gypsum can change to anhydrite, which rates 3.5 on Mohs. That's still pretty soft, especially compared to minerals in basalt (5ish-7ish). Moreover, anhydrite is about 25% denser than gypsum, so the transformation of gypsum to anhydrite would be accompanied by shrinking rather than expansion, which would have been another potential way to explain the vein's positive relief.

[Bill Harris]

Good point, Eutectic.

Mineral hardness and weatherability don't always corrrelate. Remember, in some climes limestone can be a resistant ridge-former and in others a weak valley-former.

The important thing is, is that this fracture fill is composed of CaSO4 and we need to consider the geochemical implications of that...

--Bill

[Guest_Oersted_*]

Here you are : gypsum ...and coming from our Museum of Plaster close to Paris.

Hmm, that arrowhead is made of flint if I'm not mistaken. Or did I misunderstand something?

[Stu]

Surface detail...

[Fran Ontanaya]

Maybe the surrounding terrain wasn't eroded by wind, but just dissolved in water.

[vikingmars]

Hmm, that arrowhead is made of flint if I'm not mistaken. Or did I misunderstand something?

This is the developed crystalline form of gypsum : the arrowhead (links added hereabove).
Gypsum arrowheads are very common here. We are being shown these since school in Paris and you see them oftenly when visiting the Paris underground quarries...
http://www.urban-exploration.com/index.php...;r=TOIT%20PARIS
Most of old houses in Paris and close to it (like mine) are made of rocks (taken from the quarries) and plaster made of gypsum (and also with a mix of those). Gypsum is also now transformed into construction panels easy to assemble and is considered as a very environmental-friendly material

[centsworth_II]

Gypsum arrowheads are very common here....

I found this explanation of the formation of gypsum "spearheads" in the French Wikipedia:
"Le gypse en fer de lance est le résultat de la macle de deux grands cristaux lenticulaires."

Google translation: "Gypsum as a spearhead is the result of the twinning of two large lenticular crystals."

Can you imagine if Opportunity imaged this (from vikingmars' second link) laying on Cape York?!

[CosmicRocker]

Gypsum is too soft, surely. Hardness 2. How would it stand up to erosion in this way?

Gypsum is so soft, how can it make upstanding veins?

Nonetheless, gypsum is relatively soft relative to other minerals. It's "2" on Moh's Scale of Mineral Hardness, roughly in between the hardness of a pencil "lead" (graphite) and a fingernail.

It's clearly harder than the rock surrounding it, as evidenced by the drive-over. Perhaps the vein has only been exhumed relatively recently in geological terms.

With loss of water, gypsum can change to anhydrite, which rates 3.5 on Mohs. That's still pretty soft, especially compared to minerals in basalt (5ish-7ish). Moreover, anhydrite is about 25% denser than gypsum, so the transformation of gypsum to anhydrite would be accompanied by shrinking rather than expansion, which would have been another potential way to explain the vein's positive relief.

Good question/s, ngunn. As ibasso pointed out, crystalline gypsum is quite soft as minerals go. You can scratch it with your fingernail. Marsophile hit the nail on the head. The absolute hardness of the mineral is not the key issue, it is the relative hardness compared to the rock within which the vein is enclosed.

Regarding anhydrite, that is also a good point, and one which might help explain Homestake's positive relief, but the vein has been identified as likely being gypsum (CaSO4.2H2O), and not anhydrite (CaSO4). From Salley Rayl's update, according to Ray Arvidson:

"Since Mini-TES [miniature thermal emission spectrometer] is not working, we really couldn't pin down the mineral phase based on the infrared. So the next best thing was to look at the vein with Pancam. It shows a dip at 1 micrometer, which is probably related to the presence of molecular water in the mineral, so it's calcium, and sulfur, it's water bearing. The best bet is it's gypsum, which is calcium sulfate with 2 waters in the unit cell," he explained.

Mineral (hardness) and weatherability don't always corrrelate. Remember, in some climes limestone can be a resistant ridge-former and in others a weak valley-former. ...

That's the kicker, isn't it, Bill? In a real environment where mechanical abrasion isn't the only agent responsible for the wearing down of rocks, we need to consider several physical and chemical processes capable of breaking down rocks. It's sometimes tricky enough on earth, let alone on an alien planet. If Homestake is embedded within soft sulfate sediments similar to those we have become familiar with on the Meridiani plains, perhaps this occurance resembles a gypsum crystal embedded within wallboard.

[CosmicRocker]

I found this explanation of the formation of gypsum "spearheads" ...

Fascinating...I wonder if ancient peoples actually used these twinned crystals as effective weapon points.

[Bernard]

Fascinating...I wonder if ancient peoples actually used these twinned crystals as effective weapon points.

No, it's far too soft