QUOTE (ngunn @ Dec 7 2011, 05:35 PM)
Gypsum is too soft, surely. Hardness 2. How would it stand up to erosion in this way?
QUOTE (ngunn @ Dec 8 2011, 06:02 PM)
Gypsum is so soft, how can it make upstanding veins?
QUOTE (ilbasso @ Dec 8 2011, 10:49 PM)
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.
QUOTE (marsophile @ Dec 9 2011, 12:23 AM)
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.
QUOTE (Eutectic @ Dec 9 2011, 10:44 AM)
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.
QUOTE (Bill Harris @ Dec 9 2011, 11:31 AM)
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.