Gypsum and Halite: Evaporite Minerals

Gypsum is readily identified by its softness (a fingernail scratches it). Gypsum comes as clear crystals that display one perfect cleavage (selenite), as blocks of featureless white rock (alabaster), and as silky fibrous blocks (satin spar).

Halite (next page) breaks into cubes (three cleavages at right angles) and tastes like table salt. Go to Halite page

These lab specimens show both distinctive features of gypsum: softness (note the white scratches from many finger nails) and one nice cleavage plane (parallel to ground).

Clear crystals or crystal fragments of gypsum are called selenite.

These are four more cleavage fragments of selenite. The good cleavage is again parallel to the ground surface. There are also two poor cleavages that can be seen in three of the four samples shown here. These give the fragments a skewed angular look. This is also seen in the photo above.
These show the typical shape of well-formed selenite crystals. The brownish color comes from impurities incorporated by the crystals as they grew just under a soil surface.
This 3-cm disk of selenite with brownish impurities has a biconvex cross section like a lens. Note the potential cleavage planes running from the upper right to lower left.
This is the satin spar variety of gypsum. Note the long fibrous texture running the length of these pieces.
This single piece of satin spar shows some of the impurities found in typical gypsum.

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