It looks like you can find these kind of structures in AMCSD (American Mineralogist Crystal Structure Database). If not there, then try COD (Crystal open database).

Here are the structural informations of \gamma - Ca2SiO4:

a = 11.371 b = 6.782 c = 5.091

Ca1 4a 0 0 0

Ca2 4c 0.2804 1/4 -0.0116

Si 4c 0.0985 1/4 0.4272

O1 4c 0.0867 1/4 0.7377

O2 4c 0.4579 1/4 0.1980

O3 8d 0.1633 0.0599 0.2925

There are many phases and I am not sure whether you are looking for this one.

I consulted a table which gives different phases of Ca2SiO4 but none has the orthorhombic space Pmnn you mention. Among orthorhombic phases of Ca2SiO4 I see only two space groups mentioned: Pnma and Pcmn which I do not think are same (different setting) as Pmnn or I may be mistaken. The rest of the phases are monoclinic. What are your lattice parameters?

You are right. Pnnm (Pmnn, P nmn) is IT number 58, whereas P nma (P mnb, P bnm, P cmn, P mcn, P nam) is 62.

Dear all, 

Thank you for your response.

The lattice parameters are a = 10.93 A,b = 18.41 A and c = 6.75 A. These i got from the ICDD database file No. 00-011-0585

Hi, maybe you can find information in the attached paper?

Dear Brahim Orayech,

Thanks for the response. while searching ICDD database i came across this file No. 00-011-0585 having lattice parameters a = 10.93 ,b = 18.41  and c = 6.75 A but its structure is not available. while referring some papers these parameters were given for Bregidite structure. Anyways, i will re-check my diffraction pattern for the files given by you and will get back to you.

Once again thanks to all