Evaporites are so soluble that they are best exposed in deserts. Despite their rariety at the surface in humid environments, they are often important below the surface, where they can constitute important natural resources in their own right or where they can host important deposits of oil or natural gas .
This gypsum outcrop in the deserts of northern Mexico is fairly non-descript. As with most outcrops, a closer look is needed to identify the rock!
Here near Medicine Bow Wyoming we have alternating layers of limestone (top), red clays, gypsum (white), and finally red clays shot through with thin white veins of gypsum.
This is part of the Triassic Chugwater Formation.
When you look closer, evaporites often show signs of dissolution etching in outcrop. Also, the rock can look a bit rotten.
To tell if this is gypsum, see if you can scratch it with your fingernail. If it's salt, it will taste salty. This is classic white gypsum.
Here is a nice sample of halite. If you click to enlarge, you will see the intergrown cubic crystals that halite forms.
To confirm your identification, just lick it. Go ahead.
One great place to see halite is in the playa lakes of the American west. The white halite deposits of Badwater in Death Valley make these interesting ridges.
