GYPSUM REVISITED: AN ADDENDUM TO THIS INTERESTING AND COMMON MINERAL

This post is an update (with new images and new text) of one that I did five years ago, on Sept. 30, 2017. 

There are numerous varieties of the mineral gypsum (selenite), a sulfate mineral, which has the chemical formula CaSO4.2H20. A keep clue is identifying gypsum is that it does not react with hydrochloric acid. Gypsum is normally the first salt precipitated and deposited in the evaporation of sea water or salty late water. As the salinity of the water increases, the gypsum is followed by the deposition of anhydrite and halite (sodium chloride). Gypsum can also form as vein-filling cracks in rocks.

Variety 1 is colorless selenite: it is transparent, smooth, and non-fibrous. 

(a)

lmage (a) shows a transparent crystal of selenite (43 mm long and 17 mm wide).

(b)

Image (b) shows a large transparent crystal of selenite (70 mm wide) that apparently has been artificially tapered by a machine.

                   

Variety 2 is colored selenite: image (c) shows a slightly colored crystal of selenite (30 mm long) of gypsum. This specimen has been scratched by fingernails. When I was teaching, I would pass this hand specimen around the classroom, and some students would   scratch this specimen. Gypsum is very soft, with a a Mohs Scale hardness of only 2 [see my previous post about the Mohs Scale].

                                              (c)

                                               (d)

Image (d) is of another slightly colored crystal (30 mm long) of selenite. This transparent crystal of gypsum has been scratched by fingernails. When I was teaching I would pass this hand specimen around the classroom, and some students would  convince themselves that they could scratch this mineral, which is very soft and has a Mohs Scale hardness of only 2 (see my previous post on the Mohs Scale).

Variety 3 is twinned crystals of selenite: See one of my previous posts about the subject of twinning in crystals.

                                               (e)

Image (e) is of two specimens of twinned crystals (the biggest one is 37 mm in length) of selenite. 

Variety 4 of selenite is satin spar: 

                                                (f)
Image (f) is of a rectangular hand specimen (4 inches at its widest and 1.5 inches thick) is milky white, silky, and  has fibrous with a characteristic parallel structure. shown here. You can clearly see the distinctive fibrous parallel, satiny structure of this soft mineral. Surprisingly, this piece of satin spar is quite heavy. Unfortunately, the name “satin spar” has also been applied to a fibrous variety of the mineral calcite (CaCO3), but that mineral which reacts with hydrochloric acid.

                                                 (g)

Image (g) is of a machine-cut (in a spiral (corkscrew) shape) of a piece of satin spar (6.5 inches tall). This sculpture demonstrates how the softness of selenite, which allows for it to be easily manipulated into complex shapes. 

Variety 5 of selenite is "desert rose": These are rosette-shaped crystals that can resemble a flower shape because of the curved gypsum crystals. These crystals are twinned (see comment mentioned above). Desert rose crystals can be gradational with so-called, "desert-flower" gypsum crystal.

(h)

Image h is a "desert rose," (2.5 inches across) consisting of a cluster of intergrown gypsum crystals.                                          

Variety 6 of selenite is alabaster. This variety of the mineral gypsum is massive material. It looks just like white limestone or marble. Alabaster is softer than those rocks and, therefore, is commonly used for carving statues, vases, and ornaments.

                                                (i)

Image i is a closeup of the surface of a slab of alabaster. The crystal boundaries are somewhat indistinct because of weathering of the surface of the slab.