On top of one of the piles in my rumpus room is a small book labeled “Brines and Evaporites” by Peter Sonnenfeld and P.P. Perthuisot. I thought that today we might talk about one popular Evaporite: Gypsum. Would you guess that Gypsum is?
a.) A mineral
b.) A rock
c.) A Gypsy plagued by vocalized pauses
d.) A valuable construction material
e.) a, b, and d.
If you answered e., pat yourself on the back! Let’s learn more about this versatile substance!
Gypsum’s chemical makeup is calcium + sulfate. If you want to get technical about it (and I know you do), it’s really calcium sulfate dihydrate meaning that a few water molecules are thrown into the mix (CaSO4 2H20*). Gypsum is very, very soft with a hardness of 2 on the Moh’s scale, which the scale for geologists who poke and scratch minerals as part of their jobs. You can scratch Gypsum and leave a mark with your fingernail. Gypsum can grow in pretty patches of crystals. See:
What's confusing is that sometimes gypsum is a mineral and sometimes gypsum it is a rock. From my knowledge gypsum the mineral has grown crystals as seen above, and gypsum the rock is a sedimentary rock that precipitated out of a solution during evaportation. Don't forget, there’s also alabaster to continue confusing the picture. Alabaster is another name for gypsum which is often used to create vases, bowls, and priceless naked sculptures!
Calcium (Ca) and sulfate (SO4*) are elements present in seawater and tend to get together when mineral laden bodies of water evaporate. These two really like each other since calcium has a +2 charge, and sulfate has a -2 charge. Evaporation of seawater or some mineral-laden body of water can leave behind Evaporites (evaporate, Evaporite, easy to remember, yes?). Halite, also known as rock salt, is an evaporite too. If and when a large body of water evaporates, vast deposits of gypsum can be left behind. Just look at the White Sands National Monument in New Mexico, USA.
* I can’t figure out how to have blogger give me subscript numbers, so just pretend the 4 in sulfate and the 2 in water are little.
Gypsum is really quite fantastic. I didn’t even get to Plaster of Paris or drywall, but that means we’ll have something to talk about next time. See you then!!