Cleavage, specific gravity, low hardness typify gypsum from anhydrite.

Cubic cleavage, harder than gypsum, higher specific gravity, no acid reaction are the characterstic properties of anhydrite.

One of the most distinctive properties of anhydrite is its cubic cleavage. It cleaves in three directions at right angles. This can easily be seen in coarsely crystalline specimens or with a hand lens in fine-grained specimens.

Compared to gypsum, anhydrite exhibits cleavage in three directions at right angles and has a greater hardness. I think these two can be used to distinguish anhydrite and gypsum

Dear Mr. Abdula:

It is not easy to distinguish both modification of Ca sulfate from each other, indeed. In fragments there is a trick to make use of the transparency of gypsum and the opaque behaviour of anhydrite. Use a strong lamp and illuminate the fragments, e.g., under the stereomicroscope or hand lens and  you will see a bright and translucent rim around gypsum but not in case of anhydrite. You can use it in hand specimens and smaller grains.

I wish you much success

H.G.Dill

Anhydrite closely resembles gypsum in hand

samples, but it has three cleavage planes at 90 degree

angles and since of orthorhombic nature it  does not react with hydrochloric acid. In addition, anhydrite is slightly harder and denser

compared with gypsum and may have a bluish tint.

On the other hand ,gypsum is soft and can be scratched with a fingernail. It has four different cleavage surfaces, and its crystals display a variety of forms, the most common being a granular, massive rock known as alabaster. Another type, selenite, is a transparent crystalline material that forms large, well-developed crystals. Aggregates of fibrous gypsum form a variety called satin spar.

At Moh's hardness scale minerals with a hardness up to 2 can be scratched with your finger nails. So gypsum can be scratched with your finger nail (I tried it), and anhydrite not (but I didn't try it).

Dear colleagues:

I would like to re-direct the thoughts to the question which was related to on-site well studies of drill cuttings which rarely are bigger than a finger nail and contained in drilling mud. There is often only a set of optical devices available on site, either the hand lens or the stereomicroscope for cutting examination of percussion drilling operations. Drilling muds have to be monitored on a continous basis without any interruption of the drilling processes.

Regards

Harald G. Dill

 in most case anhydrite  is snow white while, in most cases,  gypsum has grey color. the best way is to crush small pieces of the rock  and put it between two slide ( two pieces of glass) with water , then see it with polarizor microscope if the interference color is first order is gypsum while if  it was second order ( bright color of green, red and yellow) it is  the anhydrite

I got an answer and liked to share with you:

On wellsite, the Bromoform liquid may help to distinguish between Anhydrite and Gypsum. Bromoform (CHBr3) is pale yellowish liquid with a sweet odor similar to chloroform, a halomethane or haloform; the density of this liquid is 2.88 g/cc. If you take 100 ml of Bromoform in a test tube and drop a small piece of rock it should be drowned in the liquid, if the density of the rock is lesser than Bromoform. Gypsum will float whereas Anhydrite will sink. The material segregation due to the density variation is based on Law of Buoyancy. A solid will float in a liquid if its density is less than that of the liquid and solid will sink if density is more than that of the liquid. Bromoform has a high density compared to that of other liquids, and so it is ideal for this application. Regards

Dear Renas I mean distinguishing them And I know how harmful is Bromoform. However, if you know that drilling will encounter gypsum or anhydrite, define both minerals in a single term and describe them under one heading EVAPORITES  (?gypsum/?anhydrite) as the identification of both minerals usually found together.

Best regards

Hello,

the best, fastest and least ambiguous method to distingish between gypsum, anhydrite (and bassanite) is FT-spectroscopy. Nowadays, there are small, sturdy devices on the market suitable for field use. Given that the distinction of the two minerals is of vital importance, you may want to think about such an investment.

Best wishes,

Harald

Hello,

Thank you for the answer. Would you please give more detail about the small, sturdy device which suitable for field use. If you have an image or  serial number please send them to me.

Best regards,

Rzger

hello

use 10 %HCL and grid cutting and dry it in oven , if come as crystals it is anhydrite

I think, You can use your hand to see the how much is hard or soft. You can observe of smooth and rough also fibrous or translucent. there is other recommended by using microscope and coring samples.

You can use an furnace and a microbalance. It is a gypsum that gives up to 20% mass loss when it heats up to 200 degrees. In the other (anhdyrite) you only measure 1% moisture loss.