It is frequently associated with minor quantities of Anhydrite, which is calcium sulphate without water, and under the proper natural conditions either of these materials may be changed into the other.
"The Economic Aspect of Geology"
C. K. Leith
The Anhydrite found in gypsum deposits is formed both by direct precipitation from salt water and by subsequent alteration of the gypsum.
"The Economic Aspect of Geology"
C. K. Leith