I decided to do a post on the mineral gypsum just after my previous post on calcite because the two minerals can resemble each other.
Gypsum is a very common mineral and is the most common sulphate mineral. It forms as an evaporite mineral and is commonly found in dry-lake beds. Gypsum consists of calcium sulphate dehydrate (CaSO4•2H20). If it becomes dehydrated it forms plaster of paris.
Gypsum is very soft and can be easily scratched by a fingernail. On the Mohs Scale of Hardness, gypsum has a hardness of 2. Diamond has a hardness of 10 on this same scale. By the way, it does not fizz in acid.
This mineral is used in making fertilizers, plaster, wallboard (drywall), blackboard chalk, and some cements.
There are several varieties of gypsum, and some are shown below.
The scale in each of the three images is the same: increments of centimeters.
This is the clear (transparent) variety called selenite. It contains no significant amount of the element selenium; rather "selenite" refers to the ancient Greek word for the Moon.
This is the tabular, massive (fibrous/silky) variety called satin spar.
These five crystals show "fishtail" twins or "swallowtail" twins of gypsum. I discussed the topic of twinning in crystals in one of recent posts.
Crystals of gypsum can be extremely large in size and are the largest crystals (39 ft. = 12 m long) of any mineral on Earth. An image of these largest crystals is shown above. Note the human for scale. This image is from Wikipedia (accessed Sept. 2017) and shows the "Cave of the Crystals in Naica, Mexico" ["Cristales Cueva de Naica, México], where these enormous crystals are found.
White Sands National Monument in southern New Mexico (USA) consists of a 270 sq. mile expanse of white gypsum sand/dunes. The gypsum was eroded by way from nearby gypsum beds and deposited in the adjacent valley. As a teenager, I visited White Sands and enjoyed sliding down the dunes. The experience is much better than sliding down normal sand dunes made of quartz sand because the gypsum crystals are soft and not abasive. This image is from Wikipedia (accessed Sept. 2017).