What you need to know:
Carbonates (like antacids that you dissolve in water) dissolve easily in water, and are deposited easily out of water. Sulfates (which are why Yellowstone’s fantastic mud-pots smell awful) are similar. Silica-rich deposits are more commonplace – so you need more detail to tell if they’re from water or not.
Up until now you’ve taken it for granted when you read in an article that a mineral discovered on Mars indicates the prehistoric existence of water. But how can a certain mineral in a rock tell you that there was once water? How can it tell you if that water was more recent or more ancient?
There are many carbonates, but calcium carbonate is probably the most familiar to you – and one of the most common carbonates. As you know from taking Tums or Alka-Seltzer – these dissolve easily in water. Many seashells are made of carbonates as well, and when those animals die, the shells are dissolved into the ocean and eventually that calcium carbonate is redeposited as limestone. Like that, all carbonates are usually formed in “hydro” and hydrothermal situations, and are also later dissolved by even a slightly acidic situation. Although this example only represents redeposition of carbonates – they are also deposited primarily (in the first place) by water as well. Finally, in order for carbonates to last a geologically long time they must be protected from water and other acids. Carbonates are either a good indication of water activity in the geologically recent past, or there hasn’t been any water near them in a long, long time.
Sulfate minerals are similar to carbonates in terms of their indication of water – they are also water-soluble, and occur in similar places to carbonates: embedded with limestones, etc. Sulfates tend to need more “thermal” in the term “hydrothermal” than carbonates – which is why you find a lot in volcanic regions like Yellowstone. That rotten-egg smell associated with hydrothermal areas on earth is due to the sulfur in sulfates. Sulfates are a great indicator for water activity paired with heat.
Almost every rock on Earth is silica-based. The presence of silicates in and of themselves is not an indication of water, current or prehistoric. More analysis or specificity is needed. After oxygen (yes, oxygen!) silica is the most common element in the Earth’s crust, as well as the crust of the Moon, and what we know of the surface of every other terrestrial planet.
Where’d I Get My Info?
Klein, Cornelius. The 22nd Edition of the Manual of Mineral Science.
~ A l i c e !