In which conditions gypsum is transformed to anhydrite and anhydrite to gypsum?

Hi,

Gypsum is transformed to anhydrite in depth because of the sediment load and the high temperature due to the thermal gradient. The water escapes from the evaporitic mineral which is the gypsum of formula CaSO4 · 2 H2O to give the anhydrite of formula CaSO4 formed by the dehydration of the gypsum. Do you have any other details on that? Thank you

Hafidha Khebizi

Hi,

Gypsum can be transformed to anhydrite in depth. I want to know how to calculate pressure in depth. Is there any way to calculate sedimentory load pressure for transformation in depth of gypsum to anhydrite?

Bayan Hussien

Well,

The answer can be found in the attached articles:

Article Experimental study of the dehydration reactions gypsum-bassa...

Article Assessment of the swelling potential of anhydrite in tunnell...

Harald G. Dill

Dear Colleagues:

Near surface under near-ambient conditions Ca sulfate is precipitated as anhydrite which hydrates in or near aquatic systems into gypsum, the di-hydrate, via the intermediate monohydrate. Calcium sulfate (anhydrite/gypsum) occurs from land to sea in:

1. back sabkhas (anhydrite-gypsum with gypcretes ),

2. sabkha s.str. (gypsum nodules),

3. intertidal sediments (gypsum),

4. shelf-lagoon (selenite (= fibrous gypsum)),

5. basin (laminated gypsum).

Burial diagenesis

1. gypsum -anhydrite (dehydration) ,

2. Anhydrite recrystallization ,

During uplift anhydrite may hydrate again into: 1.porphyrotpopes of gypsum 2. Fibrous vein gypsum / alabastrine gypsum.

There are deposits of anhydrite which after strong uplift well above the ground water table preserve their anhydrite structure

– anhydrite contains 23.55 wt.% S,

– bassanite 22.09 wt.% S

– gypsum 18.62 wt.% S (Selenite transparent variety of gypsum)

Gypsum-anhydrite transformation

-Gypsum looses water on burial and gypsum becomes anhydrite

-Process is accompanied by a 19% volume decrease

-Rehydrated anhydrite expands in volume that can result in brecciation

H.G.Dill

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