Fully agree with Roman Gulyaev, for XPS-analysis of this sample you need an XPS-instrument with a cooled stage and cooled sample introduction system (liquid N2). 

Hello. I suggest that you can take STXM (Scanning Transmission X-ray Microscopy) in the synchrotron facility. In this kind of beam line, the experimental pressure is about 1 atm. If you success to go there, you can take photoelectron spectroscopy or absorption spectroscopy. One thing you should take in your mind that  your sample should be very thin (about 200 micro meters) film or powder shape because this beam line used to detect the transmitted light.. 

Dear Taher,

CaSO4x2H2O (gypsum) is a stable compound, at room temperature and pressure, since the two water molecules are strongly bound to the crystal structure.

We carefully observed its growth morphology under the vacuum of SEM (Scanning Electron Microscope), along with its structure (with  the TEM vacuum) and no transformation occurred to the other hemi-hydrated phase. 

Then you must control the temperature; obviously, when working under vacuum and the temperature rises to 120°C, this may be dangerous. But, I suppose, this is not your case.

All the best

Dino Aquilano

You can find an study of the dehydratation process of vermiculites under vacuum in C. Marcos et al. Mineralogical Magazine 67 (3002) 1253. I hope this will help you.